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Stop using uadd.with.overflow

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This commit is contained in:
Scott McMurray
2025-08-08 21:59:28 -07:00
parent 4c7749e8c8
commit 8831c5b994
3 changed files with 41 additions and 23 deletions
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26
compiler/rustc_codegen_llvm/src/builder.rs
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@@ -557,13 +557,25 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
let (size, signed) = ty.int_size_and_signed(self.tcx);
let width = size.bits();
if oop == OverflowOp::Sub && !signed {
// Emit sub and icmp instead of llvm.usub.with.overflow. LLVM considers these
// to be the canonical form. It will attempt to reform llvm.usub.with.overflow
// in the backend if profitable.
let sub = self.sub(lhs, rhs);
let cmp = self.icmp(IntPredicate::IntULT, lhs, rhs);
return (sub, cmp);
if !signed {
match oop {
OverflowOp::Sub => {
// Emit sub and icmp instead of llvm.usub.with.overflow. LLVM considers these
// to be the canonical form. It will attempt to reform llvm.usub.with.overflow
// in the backend if profitable.
let sub = self.sub(lhs, rhs);
let cmp = self.icmp(IntPredicate::IntULT, lhs, rhs);
return (sub, cmp);
}
OverflowOp::Add => {
// Like with sub above, using icmp is the preferred form. See
// <https://rust-lang.zulipchat.com/#narrow/channel/187780-t-compiler.2Fllvm/topic/.60uadd.2Ewith.2Eoverflow.60.20.28again.29/near/533041085>
let add = self.add(lhs, rhs);
let cmp = self.icmp(IntPredicate::IntULT, add, lhs);
return (add, cmp);
}
OverflowOp::Mul => {}
}
}
let oop_str = match oop {