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Rollup merge of #143718 - scottmcm:ub-transmute-is-ub, r=WaffleLapkin

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Make UB transmutes really UB in LLVM

Ralf suggested in <https://github.com/rust-lang/rust/pull/143410#discussion_r2184928123> that UB transmutes shouldn't be trapping, which happened for the one path *that* PR was changing, but there's another path as well, so *this* PR changes that other path to match.

r? codegen
This commit is contained in:
Matthias Krüger
2025-07-11 07:35:22 +02:00
committed by GitHub
parent dbd2f30395 f5fc8727db
commit e43481e362
3 changed files with 34 additions and 27 deletions
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14
compiler/rustc_codegen_ssa/src/mir/rvalue.rs
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@@ -207,9 +207,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
{
// These cases are all UB to actually hit, so don't emit code for them.
// (The size mismatches are reachable via `transmute_unchecked`.)
// We can't use unreachable because that's a terminator, and we
// need something that can be in the middle of a basic block.
bx.assume(bx.cx().const_bool(false))
bx.unreachable_nonterminator();
} else {
// Since in this path we have a place anyway, we can store or copy to it,
// making sure we use the destination place's alignment even if the
@@ -236,14 +234,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
|| operand.layout.is_uninhabited()
|| cast.is_uninhabited()
{
if !operand.layout.is_uninhabited() {
// Since this is known statically and the input could have existed
// without already having hit UB, might as well trap for it.
bx.abort();
}
bx.unreachable_nonterminator();
// Because this transmute is UB, return something easy to generate,
// since it's fine that later uses of the value are probably UB.
// We still need to return a value of the appropriate type, but
// it's already UB so do the easiest thing available.
return OperandValue::poison(bx, cast);
}

10
compiler/rustc_codegen_ssa/src/traits/builder.rs
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@@ -136,6 +136,16 @@ pub trait BuilderMethods<'a, 'tcx>:
) -> Self::Value;
fn unreachable(&mut self);
/// Like [`Self::unreachable`], but for use in the middle of a basic block.
fn unreachable_nonterminator(&mut self) {
// This is the preferred LLVM incantation for this per
// https://llvm.org/docs/Frontend/PerformanceTips.html#other-things-to-consider
// Other backends may override if they have a better way.
let const_true = self.cx().const_bool(true);
let poison_ptr = self.const_poison(self.cx().type_ptr());
self.store(const_true, poison_ptr, Align::ONE);
}
fn add(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
fn fadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
fn fadd_fast(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;