const checking: properly compute the set of transient locals #129508
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Cc @rust-lang/wg-const-eval |
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Now I just hope this is okay from a perf perspective... we don't seem to have a cache for these analyses so unfortunately it seems like we have to re-do them here. @bors try |
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const checking: properly compute the set of transient locals For const-checking the MIR of a const/static initializer, we have to know the set of "transient" locals. The reason for this is that creating a mutable (or interior mutable) reference to a transient local is "safe" in the sense that this reference cannot possibly end up in the final value of the const -- even if it is turned into a raw pointer and stored in a union, we will see that pointer during interning and reliably reject it as dangling. So far, we determined the set of transient locals as "locals that have a `StorageDead` somewhere". But that's not quite right -- if we had MIR like ```rust StorageLive(_5); StorageDead(_5); StorageLive(_5); // no further storage annotations for _5 ``` Then we'd consider `_5` to be "transient", but it is not actually transient. We do not currently generate such MIR, but I feel uneasy relying on subtle invariants like this. So this PR implements a proper analysis for computing the set of "transient" locals: a local is "transient" if it is guaranteed dead at all `Return` terminators. Cc `@cjgillot`
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Finished benchmarking commit (eb67cda): comparison URL. Overall result: no relevant changes - no action neededBenchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf. @bors rollup=never Instruction countThis benchmark run did not return any relevant results for this metric. Max RSS (memory usage)Results (secondary 2.3%)This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
CyclesThis benchmark run did not return any relevant results for this metric. Binary sizeThis benchmark run did not return any relevant results for this metric. Bootstrap: 750.904s -> 751.176s (0.04%) |
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Awesome, perf is happy :)
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const checking: properly compute the set of transient locals For const-checking the MIR of a const/static initializer, we have to know the set of "transient" locals. The reason for this is that creating a mutable (or interior mutable) reference to a transient local is "safe" in the sense that this reference cannot possibly end up in the final value of the const -- even if it is turned into a raw pointer and stored in a union, we will see that pointer during interning and reliably reject it as dangling. So far, we determined the set of transient locals as "locals that have a `StorageDead` somewhere". But that's not quite right -- if we had MIR like ```rust StorageLive(_5); StorageDead(_5); StorageLive(_5); // no further storage annotations for _5 ``` Then we'd consider `_5` to be "transient", but it is not actually transient. We do not currently generate such MIR, but I feel uneasy relying on subtle invariants like this. So this PR implements a proper analysis for computing the set of "transient" locals: a local is "transient" if it is guaranteed dead at all `Return` terminators. Cc `@cjgillot`
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Finished benchmarking commit (22572d0): comparison URL. Overall result: no relevant changes - no action needed@rustbot label: -perf-regression Instruction countThis benchmark run did not return any relevant results for this metric. Max RSS (memory usage)Results (primary 2.2%, secondary -2.1%)This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
CyclesThis benchmark run did not return any relevant results for this metric. Binary sizeThis benchmark run did not return any relevant results for this metric. Bootstrap: 751.886s -> 752.172s (0.04%) |
For const-checking the MIR of a const/static initializer, we have to know the set of "transient" locals. The reason for this is that creating a mutable (or interior mutable) reference to a transient local is "safe" in the sense that this reference cannot possibly end up in the final value of the const -- even if it is turned into a raw pointer and stored in a union, we will see that pointer during interning and reliably reject it as dangling.
So far, we determined the set of transient locals as "locals that have a
StorageDeadsomewhere". But that's not quite right -- if we had MIR likeThen we'd consider
_5to be "transient", but it is not actually transient.We do not currently generate such MIR, but I feel uneasy relying on subtle invariants like this. So this PR implements a proper analysis for computing the set of "transient" locals: a local is "transient" if it is guaranteed dead at all
Returnterminators.Cc @cjgillot