Rollup merge of #139261 - RalfJung:msvc-align-mitigation, r=oli-obk

mitigate MSVC alignment issue on x86-32

This implements mitigation for https://github.com/rust-lang/rust/issues/112480 by stopping to emit `align` attributes on loads and function arguments when building for a win32 MSVC target. MSVC is known to not properly align `u64` and similar types, and claiming to LLVM that everything is properly aligned increases the chance that this will cause problems.

Of course, the misalignment is still a bug, but we can't fix that bug, only MSVC can.

Also add an errata note to the platform support page warning users about this known problem.

try-job: `i686-msvc*`

1 files changed, 7 insertions, 5 deletions

diff --git a/src/doc/rustc/src/platform-support.md b/src/doc/rustc/src/platform-support.md
index 9870e5011eb..50c7ae3ef8d 100644
--- a/src/doc/rustc/src/platform-support.md
+++ b/src/doc/rustc/src/platform-support.md
@@ -34,7 +34,7 @@ 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-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`](platform-support/windows-gnu.md) | 64-bit MinGW (Windows 10+, Windows Server 2016+)
@@ -43,6 +43,8 @@ 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
 
@@ -95,7 +97,7 @@ target | notes
 [`armv7-unknown-linux-ohos`](platform-support/openharmony.md) | Armv7-A OpenHarmony
 [`loongarch64-unknown-linux-gnu`](platform-support/loongarch-linux.md) | LoongArch64 Linux, LP64D ABI (kernel 5.19, glibc 2.36)
 [`loongarch64-unknown-linux-musl`](platform-support/loongarch-linux.md) | LoongArch64 Linux, LP64D ABI (kernel 5.19, musl 1.2.5)
-[`i686-pc-windows-gnu`](platform-support/windows-gnu.md) | 32-bit MinGW (Windows 10+, Windows Server 2016+, Pentium 4) [^x86_32-floats-return-ABI]
+[`i686-pc-windows-gnu`](platform-support/windows-gnu.md) | 32-bit MinGW (Windows 10+, Windows Server 2016+, Pentium 4) [^x86_32-floats-return-ABI] [^win32-msvc-alignment]
 `powerpc-unknown-linux-gnu` | PowerPC Linux (kernel 3.2, glibc 2.17)
 `powerpc64-unknown-linux-gnu` | PPC64 Linux (kernel 3.2, glibc 2.17)
 [`powerpc64le-unknown-linux-gnu`](platform-support/powerpc64le-unknown-linux-gnu.md) | PPC64LE Linux (kernel 3.10, glibc 2.17)
@@ -169,7 +171,7 @@ target | std | notes
 [`i686-pc-windows-gnullvm`](platform-support/windows-gnullvm.md) | ✓ | 32-bit x86 MinGW (Windows 10+, Pentium 4), LLVM ABI [^x86_32-floats-return-ABI]
 [`i686-unknown-freebsd`](platform-support/freebsd.md) | ✓ | 32-bit x86 FreeBSD (Pentium 4) [^x86_32-floats-return-ABI]
 `i686-unknown-linux-musl` | ✓ | 32-bit Linux with musl 1.2.3 (Pentium 4) [^x86_32-floats-return-ABI]
-[`i686-unknown-uefi`](platform-support/unknown-uefi.md) | ? | 32-bit UEFI (Pentium 4, softfloat)
+[`i686-unknown-uefi`](platform-support/unknown-uefi.md) | ? | 32-bit UEFI (Pentium 4, softfloat) [^win32-msvc-alignment]
 [`loongarch64-unknown-none`](platform-support/loongarch-none.md) | * | LoongArch64 Bare-metal (LP64D ABI)
 [`loongarch64-unknown-none-softfloat`](platform-support/loongarch-none.md) | * | LoongArch64 Bare-metal (LP64S ABI)
 [`nvptx64-nvidia-cuda`](platform-support/nvptx64-nvidia-cuda.md) | * | --emit=asm generates PTX code that [runs on NVIDIA GPUs]
@@ -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