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Auto merge of #115138 - cjgillot:dse-move-packed, r=compiler-errors

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Do not convert copies of packed projections to moves.

This code path was introduced in https://github.com/rust-lang/rust/pull/113758

After seeing https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/Packed.20fields.20and.20in-place.20function.20argument.2Freturn.20passing, this may be UB, so should be disallowed.

This should not appear in normally-built MIR, which introduces temporary copies for packed projections.
This commit is contained in:
bors 2023-08-25 13:27:21 +00:00
commit 25ed43ddf3
6 changed files with 64 additions and 2 deletions
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6
compiler/rustc_mir_transform/src/dead_store_elimination.rs
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@ -12,6 +12,7 @@
//! will still not cause any further changes.
//!
use crate::util::is_within_packed;
use rustc_index::bit_set::BitSet;
use rustc_middle::mir::visit::Visitor;
use rustc_middle::mir::*;
@ -49,6 +50,11 @@ pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, borrowed: &BitS
&& !place.is_indirect()
&& !borrowed.contains(place.local)
&& !state.contains(place.local)
// If `place` is a projection of a disaligned field in a packed ADT,
// the move may be codegened as a pointer to that field.
// Using that disaligned pointer may trigger UB in the callee,
// so do nothing.
&& is_within_packed(tcx, body, place).is_none()
{
call_operands_to_move.push((bb, index));
}