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, 2 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index 27f7f95f100..04c8118b616 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/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);