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Auto merge of rust-lang#103299 - nikic:usub-overflow, r=wesleywiser
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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 rust-lang#103285.
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bors committed Oct 30, 2022
2 parents 5ab7445 + 7833012 commit f42b6fa
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Showing 2 changed files with 24 additions and 5 deletions.
13 changes: 8 additions & 5 deletions compiler/rustc_codegen_llvm/src/builder.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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!(),
},
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16 changes: 16 additions & 0 deletions src/test/codegen/issue-103285-ptr-addr-overflow-check.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,16 @@
// compile-flags: -O -C debug-assertions=yes

#![crate_type = "lib"]
#![feature(strict_provenance)]

#[no_mangle]
pub fn test(src: *const u8, dst: *const u8) -> usize {
// CHECK-LABEL: @test(
// CHECK-NOT: panic
let src_usize = src.addr();
let dst_usize = dst.addr();
if src_usize > dst_usize {
return src_usize - dst_usize;
}
return 0;
}

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