platform-support: mention the MSVC alignment issue

Author: RalfJung

Diff for: compiler/rustc_codegen_llvm/src/builder.rs

@@ -594,9 +594,8 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
     fn load(&mut self, ty: &'ll Type, ptr: &'ll Value, align: Align) -> &'ll Value {
         unsafe {
             let load = llvm::LLVMBuildLoad2(self.llbuilder, ty, ptr, UNNAMED);
-            if !self.cx().tcx.sess.target.has_unreliable_alignment() {
-                llvm::LLVMSetAlignment(load, align.bytes() as c_uint);
-            }
+            let align = align.min(self.cx().tcx.sess.target.max_reliable_alignment());
+            llvm::LLVMSetAlignment(load, align.bytes() as c_uint);
             load
         }
     }
@@ -809,11 +808,10 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         assert_eq!(self.cx.type_kind(self.cx.val_ty(ptr)), TypeKind::Pointer);
         unsafe {
             let store = llvm::LLVMBuildStore(self.llbuilder, val, ptr);
+            let align = align.min(self.cx().tcx.sess.target.max_reliable_alignment());
             let align =
                 if flags.contains(MemFlags::UNALIGNED) { 1 } else { align.bytes() as c_uint };
-            if !self.cx().tcx.sess.target.has_unreliable_alignment() {
-                llvm::LLVMSetAlignment(store, align);
-            }
+            llvm::LLVMSetAlignment(store, align);
             if flags.contains(MemFlags::VOLATILE) {
                 llvm::LLVMSetVolatile(store, llvm::True);
             }

Diff for: compiler/rustc_mir_transform/src/check_alignment.rs

@@ -1,3 +1,4 @@
+use rustc_abi::Align;
 use rustc_index::IndexVec;
 use rustc_middle::mir::interpret::Scalar;
 use rustc_middle::mir::visit::PlaceContext;
@@ -11,7 +12,7 @@ pub(super) struct CheckAlignment;
 
 impl<'tcx> crate::MirPass<'tcx> for CheckAlignment {
     fn is_enabled(&self, sess: &Session) -> bool {
-        if sess.target.has_unreliable_alignment() {
+        if sess.target.max_reliable_alignment() < Align::MAX {
             return false;
         }
         sess.ub_checks()

Diff for: compiler/rustc_target/src/spec/mod.rs

@@ -42,7 +42,9 @@ use std::path::{Path, PathBuf};
 use std::str::FromStr;
 use std::{fmt, io};
 
-use rustc_abi::{Endian, ExternAbi, Integer, Size, TargetDataLayout, TargetDataLayoutErrors};
+use rustc_abi::{
+    Align, Endian, ExternAbi, Integer, Size, TargetDataLayout, TargetDataLayoutErrors,
+};
 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
 use rustc_fs_util::try_canonicalize;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
@@ -3610,11 +3612,15 @@ impl Target {
     /// inhibit some optimizations, and we suppress the alignment checks that
     /// would detect this unsoundness.
     ///
-    /// Every target that returns `true` here is still has a soundness bug.
-    pub fn has_unreliable_alignment(&self) -> bool {
+    /// Every target that returns less than `Align::MAX` here is still has a soundness bug.
+    pub fn max_reliable_alignment(&self) -> Align {
         // FIXME(#112480) MSVC on x86-32 is unsound and fails to properly align many types with
         // more-than-4-byte-alignment on the stack.
-        self.is_like_msvc && self.arch == "x86"
+        if self.is_like_msvc && self.arch == "x86" {
+            Align::from_bytes(4).unwrap()
+        } else {
+            Align::MAX
+        }
     }
 }
 

Diff for: compiler/rustc_ty_utils/src/abi.rs

@@ -347,9 +347,8 @@ fn adjust_for_rust_scalar<'tcx>(
             None
         };
         if let Some(kind) = kind {
-            if !cx.tcx().sess.target.has_unreliable_alignment() {
-                attrs.pointee_align = Some(pointee.align);
-            }
+            attrs.pointee_align =
+                Some(pointee.align.min(cx.tcx().sess.target.max_reliable_alignment()));
 
             // `Box` are not necessarily dereferenceable for the entire duration of the function as
             // they can be deallocated at any time. Same for non-frozen shared references (see

Diff for: src/doc/rustc/src/platform-support.md

@@ -35,7 +35,7 @@ target | notes
 [`aarch64-apple-darwin`](platform-support/apple-darwin.md) | ARM64 macOS (11.0+, Big Sur+)
 `aarch64-unknown-linux-gnu` | ARM64 Linux (kernel 4.1, glibc 2.17+)
 `i686-pc-windows-gnu` | 32-bit MinGW (Windows 10+, Windows Server 2016+, Pentium 4) [^x86_32-floats-return-ABI]
-`i686-pc-windows-msvc` | 32-bit MSVC (Windows 10+, Windows Server 2016+, Pentium 4) [^x86_32-floats-return-ABI]
+`i686-pc-windows-msvc` | 32-bit MSVC (Windows 10+, Windows Server 2016+, Pentium 4) [^x86_32-floats-return-ABI] [^win32-msvc-alignment]
 `i686-unknown-linux-gnu` | 32-bit Linux (kernel 3.2+, glibc 2.17+, Pentium 4) [^x86_32-floats-return-ABI]
 [`x86_64-apple-darwin`](platform-support/apple-darwin.md) | 64-bit macOS (10.12+, Sierra+)
 `x86_64-pc-windows-gnu` | 64-bit MinGW (Windows 10+, Windows Server 2016+)
@@ -44,6 +44,8 @@ target | notes
 
 [^x86_32-floats-return-ABI]: Due to limitations of the C ABI, floating-point support on `i686` targets is non-compliant: floating-point return values are passed via an x87 register, so NaN payload bits can be lost. Functions with the default Rust ABI are not affected. See [issue #115567][x86-32-float-return-issue].
 
+[^win32-msvc-alignment]: Due to non-standard behavior of MSVC, native C code on this target can cause types with an alignment of more than 4 bytes to be incorrectly aligned to only 4 bytes (this affects, e.g., `u64` and `i64`). Rust applies some mitigations to reduce the impact of this issue, but this can still cause unsoundness due to unsafe code that (correctly) assumes that references are always properly aligned. See [issue #112480](https://github.com/rust-lang/rust/issues/112480).
+
 [77071]: https://github.com/rust-lang/rust/issues/77071
 [x86-32-float-return-issue]: https://github.com/rust-lang/rust/issues/115567
 
@@ -317,9 +319,9 @@ target | std | host | notes
 [`i686-unknown-netbsd`](platform-support/netbsd.md) | ✓ | ✓ | NetBSD/i386 (Pentium 4) [^x86_32-floats-return-ABI]
 [`i686-unknown-openbsd`](platform-support/openbsd.md) | ✓ | ✓ | 32-bit OpenBSD (Pentium 4) [^x86_32-floats-return-ABI]
 `i686-uwp-windows-gnu` | ✓ |  | [^x86_32-floats-return-ABI]
-[`i686-uwp-windows-msvc`](platform-support/uwp-windows-msvc.md) | ✓ |  | [^x86_32-floats-return-ABI]
+[`i686-uwp-windows-msvc`](platform-support/uwp-windows-msvc.md) | ✓ |  | [^x86_32-floats-return-ABI] [^win32-msvc-alignment]
 [`i686-win7-windows-gnu`](platform-support/win7-windows-gnu.md) | ✓ |   | 32-bit Windows 7 support [^x86_32-floats-return-ABI]
-[`i686-win7-windows-msvc`](platform-support/win7-windows-msvc.md) | ✓ |   | 32-bit Windows 7 support [^x86_32-floats-return-ABI]
+[`i686-win7-windows-msvc`](platform-support/win7-windows-msvc.md) | ✓ |   | 32-bit Windows 7 support [^x86_32-floats-return-ABI] [^win32-msvc-alignment]
 [`i686-wrs-vxworks`](platform-support/vxworks.md) | ✓ |  | [^x86_32-floats-return-ABI]
 [`loongarch64-unknown-linux-ohos`](platform-support/openharmony.md) | ✓ |   | LoongArch64 OpenHarmony
 [`m68k-unknown-linux-gnu`](platform-support/m68k-unknown-linux-gnu.md) | ? |  | Motorola 680x0 Linux

Diff for: tests/codegen/align-struct.rs

@@ -1,5 +1,6 @@
 //@ compile-flags: -C no-prepopulate-passes -Z mir-opt-level=0
-//
+// 32bit MSVC does not align things properly so we suppress high alignment annotations (#112480)
+//@ ignore-msvc
 
 #![crate_type = "lib"]
 

Diff for: tests/codegen/issues/issue-56927.rs

@@ -1,4 +1,6 @@
 //@ compile-flags: -C no-prepopulate-passes
+// 32bit MSVC does not align things properly so we suppress high alignment annotations (#112480)
+//@ ignore-msvc
 
 #![crate_type = "rlib"]