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Rollup merge of #118540 - RalfJung:unsized-packed-offset, r=TaKO8Ki

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codegen, miri: fix computing the offset of an unsized field in a packed struct

`#[repr(packed)]`  strikes again.

Fixes https://github.com/rust-lang/rust/issues/118537
Fixes https://github.com/rust-lang/miri/issues/3200

`@bjorn3` I assume cranelift needs the same fix.
This commit is contained in:
Takayuki Maeda 2023-12-04 21:19:44 +09:00 • committed by GitHub
commit 87625dbf2b
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6 changed files with 171 additions and 11 deletions
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22
compiler/rustc_codegen_ssa/src/mir/place.rs
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@ -143,7 +143,8 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
// Simple cases, which don't need DST adjustment:
// * no metadata available - just log the case
// * known alignment - sized types, `[T]`, `str` or a foreign type
// * packed struct - there is no alignment padding
// Note that looking at `field.align` is incorrect since that is not necessarily equal
// to the dynamic alignment of the type.
match field.ty.kind() {
_ if self.llextra.is_none() => {
debug!(
@ -154,14 +155,6 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
}
_ if field.is_sized() => return simple(),
ty::Slice(..) | ty::Str | ty::Foreign(..) => return simple(),
ty::Adt(def, _) => {
if def.repr().packed() {
// FIXME(eddyb) generalize the adjustment when we
// start supporting packing to larger alignments.
assert_eq!(self.layout.align.abi.bytes(), 1);
return simple();
}
}
_ => {}
}
@ -186,7 +179,16 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
let unaligned_offset = bx.cx().const_usize(offset.bytes());
// Get the alignment of the field
let (_, unsized_align) = glue::size_and_align_of_dst(bx, field.ty, meta);
let (_, mut unsized_align) = glue::size_and_align_of_dst(bx, field.ty, meta);
// For packed types, we need to cap alignment.
if let ty::Adt(def, _) = self.layout.ty.kind()
&& let Some(packed) = def.repr().pack
{
let packed = bx.const_usize(packed.bytes());
let cmp = bx.icmp(IntPredicate::IntULT, unsized_align, packed);
unsized_align = bx.select(cmp, unsized_align, packed)
}
// Bump the unaligned offset up to the appropriate alignment
let offset = round_up_const_value_to_alignment(bx, unaligned_offset, unsized_align);