Comptime dereference bugs tracking issue #19452
Labels
bug
Observed behavior contradicts documented or intended behavior
error message
This issue points out an error message that is unhelpful and should be improved.
frontend
Tokenization, parsing, AstGen, Sema, and Liveness.
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This pull request is not ready for review yet.
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mlugg
added
bug
This was referenced Mar 28, 2024
|
Here is a simplified version of something I hit while trying to get bun to compile on the latest version of zig. pub const hi = brk: {
var buf: [4][]const u8 = undefined;
for (0..2) |i| {
buf[i] = "hello";
}
// error: comptime dereference requires '[2][]const u8' to have a well-defined layout, but it does not.
const names = buf[0..2].*;
break :brk names;
};
test {
_ = &hi;
}I think I can get a workaround by generating this code during zig build, like inlined actually, while typing this, someone told me that the workaround is to ensure the definition Besides this one, I've been pretty happy about the recent changes, as they've caught a lot of const/non-const issues across the codebase. |
|
regarding above post: i've hit this in my build.zig where you can't move the logic to the build system. buffer size needs to be computed in my case. it was previously set to the known upper bound. |
|
I too faced similar issue while building enum from type information that slices 2 enum fields from an array: const std = @import("std");
const Type = std.builtin.Type;
pub fn main() !void {
const enum_fields: [3]Type.EnumField = .{ .{ .name = "field1", .value = 1 }, .{ .name = "field2", .value = 2 }, .{ .name = "field3", .value = 3 } };
const custom_enum = @Type(Type{ .Enum = .{ .tag_type = u8, .is_exhaustive = true, .decls = &.{}, .fields = enum_fields[0..2] } });
std.debug.print("{}", .{custom_enum.field1});
} |
|
Thanks for the examples, everyone. I'm getting my branch into a working state now, and will be testing the given examples shortly. |
mlugg
added a commit
to mlugg/zig
that referenced
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Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 13, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
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this issue
Apr 13, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 13, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 13, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 13, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 14, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 14, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this issue
Apr 14, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
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Apr 15, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
const std = @import("std");
const HashTag = enum {
sha1,
pub const map = std.ComptimeStringMap(@This(), .{
.{ encode("1.3.14.3.2.26"), .sha1 },
});
};
pub inline fn encode(comptime oid: []const u8) []const u8 {
var res: [256]u8 = undefined;
res[0] = @intCast(oid.len);
@memcpy(res[1..][0..oid.len], oid);
return res[0..oid.len + 1];
}
pub fn main() !void {
std.debug.print("{?}\n", .{ HashTag.map.get("1.3.14.3.2.26asdf") });
}Error in #19666, which I posted before finding this issue. |
mlugg
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this issue
Apr 17, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
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May 9, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
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The Sema logic for accessing pointers at comptime is incredibly flawed and riddled with bugs. Recent changes in the frontend have exposed more of these bugs, such as those encountered in #19092.
I am actively working on a rewrite of this logic which will hopefully fix all of these issues, but in the meantime, workarounds are necessary.
This issue serves to track these bugs. Feel free to share workarounds here, and please link any PRs/commits - in the Zig project or otherwise - which work around the issue. When my rewrite is complete, I will test it on everything linked here to ensure there are no failing edge cases.
Resolution of this issue should likely block 0.12.0.
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