From 783301298f1e151b82c1be21bc6a214b9ef525bd Mon Sep 17 00:00:00 2001 From: Nikita Popov Date: Thu, 20 Oct 2022 12:28:31 +0200 Subject: Don't use usub.with.overflow intrinsic The canonical form of a usub.with.overflow check in LLVM are separate sub + icmp instructions, rather than a usub.with.overflow intrinsic. Using usub.with.overflow will generally result in worse optimization potential. The backend will attempt to form usub.with.overflow when it comes to actual instruction selection. This is not fully reliable, but I believe this is a better tradeoff than using the intrinsic in IR. Fixes #103285. --- compiler/rustc_codegen_llvm/src/builder.rs | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) (limited to 'compiler/rustc_codegen_llvm/src/builder.rs') diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs index fca43a0d86d..9cb36ce7f18 100644 --- a/compiler/rustc_codegen_llvm/src/builder.rs +++ b/compiler/rustc_codegen_llvm/src/builder.rs @@ -365,11 +365,14 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> { Int(I64) => "llvm.ssub.with.overflow.i64", Int(I128) => "llvm.ssub.with.overflow.i128", - Uint(U8) => "llvm.usub.with.overflow.i8", - Uint(U16) => "llvm.usub.with.overflow.i16", - Uint(U32) => "llvm.usub.with.overflow.i32", - Uint(U64) => "llvm.usub.with.overflow.i64", - Uint(U128) => "llvm.usub.with.overflow.i128", + Uint(_) => { + // 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); + } _ => unreachable!(), }, -- cgit 1.4.1-3-g733a5