From 8831c5b9946561a37dc7ee6fd826ef1470e8f380 Mon Sep 17 00:00:00 2001
From: Scott McMurray <scottmcm@users.noreply.github.com>
Date: Fri, 8 Aug 2025 21:59:28 -0700
Subject: Stop using uadd.with.overflow

---
 compiler/rustc_codegen_llvm/src/builder.rs | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

(limited to 'compiler/rustc_codegen_llvm/src')

diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index 32cdef075e7..917d07e3c61 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -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 {
-- 
cgit 1.4.1-3-g733a5