[AIX] Lint on structs that have a different alignment in AIX's C ABI

compiler/rustc_lint/messages.ftl

@@ -970,6 +970,8 @@ lint_unused_result = unused result of type `{$ty}`
 
 lint_use_let_underscore_ignore_suggestion = use `let _ = ...` to ignore the expression or result
 
+lint_uses_power_alignment = repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+
 lint_variant_size_differences =
     enum variant is more than three times larger ({$largest} bytes) than the next largest
 

compiler/rustc_lint/src/lints.rs

@@ -1692,6 +1692,10 @@ pub(crate) struct OverflowingLiteral<'a> {
     pub lit: String,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_uses_power_alignment)]
+pub(crate) struct UsesPowerAlignment;
+
 #[derive(LintDiagnostic)]
 #[diag(lint_unused_comparisons)]
 pub(crate) struct UnusedComparisons;

compiler/rustc_lint/src/types.rs

@@ -1,14 +1,15 @@
 use std::iter;
 use std::ops::ControlFlow;
 
-use rustc_abi::{BackendRepr, ExternAbi, TagEncoding, Variants, WrappingRange};
+use rustc_abi::{BackendRepr, ExternAbi, TagEncoding, VariantIdx, Variants, WrappingRange};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::DiagMessage;
 use rustc_hir::{Expr, ExprKind, LangItem};
 use rustc_middle::bug;
 use rustc_middle::ty::layout::{LayoutOf, SizeSkeleton};
 use rustc_middle::ty::{
-    self, AdtKind, GenericArgsRef, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
+    self, Adt, AdtKind, GenericArgsRef, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable,
+    TypeVisitableExt,
 };
 use rustc_session::{declare_lint, declare_lint_pass, impl_lint_pass};
 use rustc_span::def_id::LocalDefId;
@@ -23,7 +24,7 @@ use crate::lints::{
     AmbiguousWidePointerComparisonsAddrSuggestion, AtomicOrderingFence, AtomicOrderingLoad,
     AtomicOrderingStore, ImproperCTypes, InvalidAtomicOrderingDiag, InvalidNanComparisons,
     InvalidNanComparisonsSuggestion, UnpredictableFunctionPointerComparisons,
-    UnpredictableFunctionPointerComparisonsSuggestion, UnusedComparisons,
+    UnpredictableFunctionPointerComparisonsSuggestion, UnusedComparisons, UsesPowerAlignment,
     VariantSizeDifferencesDiag,
 };
 use crate::{LateContext, LateLintPass, LintContext, fluent_generated as fluent};
@@ -727,7 +728,60 @@ declare_lint! {
     "proper use of libc types in foreign item definitions"
 }
 
-declare_lint_pass!(ImproperCTypesDefinitions => [IMPROPER_CTYPES_DEFINITIONS]);
+declare_lint! {
+    /// The `uses_power_alignment` lint detects specific `repr(C)`
+    /// aggregates on AIX.
+    /// In its platform C ABI, AIX uses the "power" (as in PowerPC) alignment
+    /// rule (detailed in https://www.ibm.com/docs/en/xl-c-and-cpp-aix/16.1?topic=data-using-alignment-modes#alignment),
+    /// which can also be set for XLC by `#pragma align(power)` or
+    /// `-qalign=power`. Aggregates with a floating-point type as the
+    /// recursively first field (as in "at offset 0") modify the layout of
+    /// *subsequent* fields of the associated structs to use an alignment value
+    /// where the floating-point type is aligned on a 4-byte boundary.
+    ///
+    /// The power alignment rule for structs needed for C compatibility is
+    /// unimplementable within `repr(C)` in the compiler without building in
+    /// handling of references to packed fields and infectious nested layouts,
+    /// so a warning is produced in these situations.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,ignore
+    /// #[repr(C)]
+    /// pub struct Floats {
+    ///     a: f64,
+    ///     b: u8,
+    ///     c: f64,
+    /// }
+    /// ```
+    ///
+    /// This will produce:
+    ///
+    /// ```text
+    /// warning: repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    ///  --> <source>:5:3
+    ///   |
+    /// 5 |   c: f64,
+    ///   |   ^^^^^^
+    ///   |
+    ///   = note: `#[warn(uses_power_alignment)]` on by default
+    /// ```
+    ///
+    /// ### Explanation
+    ///
+    /// The power alignment rule specifies that the above struct has the
+    /// following alignment:
+    ///  - offset_of!(Floats, a) == 0
+    ///  - offset_of!(Floats, b) == 8
+    ///  - offset_of!(Floats, c) == 12
+    /// However, rust currently aligns `c` at offset_of!(Floats, c) == 16.
+    /// Thus, a warning should be produced for the above struct in this case.
+    USES_POWER_ALIGNMENT,
+    Warn,
+    "Structs do not follow the power alignment rule under repr(C)"
+}
+
+declare_lint_pass!(ImproperCTypesDefinitions => [IMPROPER_CTYPES_DEFINITIONS, USES_POWER_ALIGNMENT]);
 
 #[derive(Clone, Copy)]
 pub(crate) enum CItemKind {
@@ -1539,6 +1593,71 @@ impl ImproperCTypesDefinitions {
             vis.check_type_for_ffi_and_report_errors(span, fn_ptr_ty, true, false);
         }
     }
+
+    fn check_arg_for_power_alignment<'tcx>(
+        &mut self,
+        cx: &LateContext<'tcx>,
+        ty: Ty<'tcx>,
+    ) -> bool {
+        // Structs (under repr(C)) follow the power alignment rule if:
+        //   - the first field of the struct is a floating-point type that
+        //     is greater than 4-bytes, or
+        //   - the first field of the struct is an aggregate whose
+        //     recursively first field is a floating-point type greater than
+        //     4 bytes.
+        if cx.tcx.sess.target.os != "aix" {
+            return false;
+        }
+        if ty.is_floating_point() && ty.primitive_size(cx.tcx).bytes() > 4 {
+            return true;
+        } else if let Adt(adt_def, _) = ty.kind()
+            && adt_def.is_struct()
+        {
+            let struct_variant = adt_def.variant(VariantIdx::ZERO);
+            // Within a nested struct, all fields are examined to correctly
+            // report if any fields after the nested struct within the
+            // original struct are misaligned.
+            for struct_field in &struct_variant.fields {
+                let field_ty = cx.tcx.type_of(struct_field.did).instantiate_identity();
+                if self.check_arg_for_power_alignment(cx, field_ty) {
+                    return true;
+                }
+            }
+        }
+        return false;
+    }
+
+    fn check_struct_for_power_alignment<'tcx>(
+        &mut self,
+        cx: &LateContext<'tcx>,
+        item: &'tcx hir::Item<'tcx>,
+    ) {
+        let adt_def = cx.tcx.adt_def(item.owner_id.to_def_id());
+        if adt_def.repr().c()
+            && !adt_def.repr().packed()
+            && cx.tcx.sess.target.os == "aix"
+            && !adt_def.all_fields().next().is_none()
+        {
+            let struct_variant_data = item.expect_struct().0;
+            for (index, ..) in struct_variant_data.fields().iter().enumerate() {
+                // Struct fields (after the first field) are checked for the
+                // power alignment rule, as fields after the first are likely
+                // to be the fields that are misaligned.
+                if index != 0 {
+                    let first_field_def = struct_variant_data.fields()[index];
+                    let def_id = first_field_def.def_id;
+                    let ty = cx.tcx.type_of(def_id).instantiate_identity();
+                    if self.check_arg_for_power_alignment(cx, ty) {
+                        cx.emit_span_lint(
+                            USES_POWER_ALIGNMENT,
+                            first_field_def.span,
+                            UsesPowerAlignment,
+                        );
+                    }
+                }
+            }
+        }
+    }
 }
 
 /// `ImproperCTypesDefinitions` checks items outside of foreign items (e.g. stuff that isn't in
@@ -1562,8 +1681,13 @@ impl<'tcx> LateLintPass<'tcx> for ImproperCTypesDefinitions {
             }
             // See `check_fn`..
             hir::ItemKind::Fn { .. } => {}
+            // Structs are checked based on if they follow the power alignment
+            // rule (under repr(C)).
+            hir::ItemKind::Struct(..) => {
+                self.check_struct_for_power_alignment(cx, item);
+            }
             // See `check_field_def`..
-            hir::ItemKind::Union(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Enum(..) => {}
+            hir::ItemKind::Union(..) | hir::ItemKind::Enum(..) => {}
             // Doesn't define something that can contain a external type to be checked.
             hir::ItemKind::Impl(..)
             | hir::ItemKind::TraitAlias(..)

tests/ui/layout/reprc-power-alignment.rs

@@ -0,0 +1,152 @@
+//@ check-pass
+//@ compile-flags: --target powerpc64-ibm-aix
+//@ needs-llvm-components: powerpc
+//@ add-core-stubs
+#![feature(no_core)]
+#![no_core]
+#![no_std]
+
+extern crate minicore;
+use minicore::*;
+
+#[warn(uses_power_alignment)]
+
+#[repr(C)]
+pub struct Floats {
+    a: f64,
+    b: u8,
+    c: f64, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    d: f32,
+}
+
+pub struct Floats2 {
+    a: f64,
+    b: u32,
+    c: f64,
+}
+
+#[repr(C)]
+pub struct Floats3 {
+    a: f32,
+    b: f32,
+    c: i64,
+}
+
+#[repr(C)]
+pub struct Floats4 {
+    a: u64,
+    b: u32,
+    c: f32,
+}
+
+#[repr(C)]
+pub struct Floats5 {
+    a: f32,
+    b: f64, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    c: f32,
+}
+
+#[repr(C)]
+pub struct FloatAgg1 {
+    x: Floats,
+    y: f64, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+}
+
+#[repr(C)]
+pub struct FloatAgg2 {
+    x: i64,
+    y: Floats, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+}
+
+#[repr(C)]
+pub struct FloatAgg3 {
+    x: FloatAgg1,
+    // NOTE: the "power" alignment rule is infectious to nested struct fields.
+    y: FloatAgg2, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    z: FloatAgg2, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+}
+
+#[repr(C)]
+pub struct FloatAgg4 {
+    x: FloatAgg1,
+    y: FloatAgg2, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+}
+
+#[repr(C)]
+pub struct FloatAgg5 {
+    x: FloatAgg1,
+    y: FloatAgg2, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    z: FloatAgg3, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+}
+
+#[repr(C)]
+pub struct FloatAgg6 {
+    x: i64,
+    y: Floats, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    z: u8,
+}
+
+#[repr(C)]
+pub struct FloatAgg7 {
+    x: i64,
+    y: Floats, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    z: u8,
+    zz: f32,
+}
+
+#[repr(C)]
+pub struct A {
+  d: f64,
+}
+#[repr(C)]
+pub struct B {
+  a: A,
+  f: f32,
+  d: f64, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+}
+#[repr(C)]
+pub struct C {
+  c: u8,
+  b: B, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+}
+#[repr(C)]
+pub struct D {
+  x: f64,
+}
+#[repr(C)]
+pub struct E {
+  x: i32,
+  d: D, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+}
+#[repr(C)]
+pub struct F {
+  a: u8,
+  b: f64, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+}
+#[repr(C)]
+pub struct G {
+    a: u8,
+    b: u8,
+    c: f64, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    d: f32,
+    e: f64, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+}
+// Should not warn on #[repr(packed)].
+#[repr(packed)]
+pub struct H {
+    a: u8,
+    b: u8,
+    c: f64,
+    d: f32,
+    e: f64,
+}
+#[repr(C, packed)]
+pub struct I {
+    a: u8,
+    b: u8,
+    c: f64,
+    d: f32,
+    e: f64,
+}
+
+fn main() { }