mitigate MSVC unsoundness by not emitting alignment attributes on win32-msvc targets

also mention the MSVC alignment issue in platform-support.md

Author: RalfJung

compiler/rustc_codegen_llvm/src/builder.rs

@@ -594,6 +594,7 @@ 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);
+            let align = align.min(self.cx().tcx.sess.target.max_reliable_alignment());
             llvm::LLVMSetAlignment(load, align.bytes() as c_uint);
             load
         }
@@ -807,6 +808,7 @@ 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 };
             llvm::LLVMSetAlignment(store, align);

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;
@@ -11,8 +12,7 @@ pub(super) struct CheckAlignment;
 
 impl<'tcx> crate::MirPass<'tcx> for CheckAlignment {
     fn is_enabled(&self, sess: &Session) -> bool {
-        // FIXME(#112480) MSVC and rustc disagree on minimum stack alignment on x86 Windows
-        if sess.target.llvm_target == "i686-pc-windows-msvc" {
+        if sess.target.max_reliable_alignment() < Align::MAX {
             return false;
         }
         sess.ub_checks()

compiler/rustc_target/src/callconv/mod.rs

@@ -144,6 +144,7 @@ pub struct ArgAttributes {
     /// (corresponding to LLVM's dereferenceable_or_null attributes, i.e., it is okay for this to be
     /// set on a null pointer, but all non-null pointers must be dereferenceable).
     pub pointee_size: Size,
+    /// The minimum alignment of the pointee, if any.
     pub pointee_align: Option<Align>,
 }
 

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};
@@ -3598,6 +3600,25 @@ impl Target {
             _ => return None,
         })
     }
+
+    /// Returns whether this target is known to have unreliable alignment:
+    /// native C code for the target fails to align some data to the degree
+    /// required by the C standard. We can't *really* do anything about that
+    /// since unsafe Rust code may assume alignment any time, but we can at least
+    /// inhibit some optimizations, and we suppress the alignment checks that
+    /// would detect this unsoundness.
+    ///
+    /// 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. This makes alignments larger than 4 generally
+        // unreliable on 32bit Windows.
+        if self.is_like_windows && self.arch == "x86" {
+            Align::from_bytes(4).unwrap()
+        } else {
+            Align::MAX
+        }
+    }
 }
 
 /// Either a target tuple string or a path to a JSON file.

compiler/rustc_ty_utils/src/abi.rs

@@ -347,7 +347,8 @@ fn adjust_for_rust_scalar<'tcx>(
             None
         };
         if let Some(kind) = kind {
-            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

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

tests/codegen/align-struct.rs

@@ -1,5 +1,7 @@
 //@ 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-i686-pc-windows-msvc
+//@ ignore-i686-pc-windows-gnu
 
 #![crate_type = "lib"]
 

tests/codegen/issues/issue-56927.rs

@@ -1,4 +1,7 @@
 //@ compile-flags: -C no-prepopulate-passes
+// 32bit MSVC does not align things properly so we suppress high alignment annotations (#112480)
+//@ ignore-i686-pc-windows-msvc
+//@ ignore-i686-pc-windows-gnu
 
 #![crate_type = "rlib"]