[llvm] [CodeGenPrepare] Bail out of usubo creation if sub's parent is not the same as the comparison (PR #160358)

[via llvm-commits]

https://github.com/AZero13 created https://github.com/llvm/llvm-project/pull/160358

We match uadd's behavior here.

Codegen comparison: https://godbolt.org/z/x8j4EhGno

>From 607185edc5384973793dc345a5b95bae325abaf3 Mon Sep 17 00:00:00 2001
From: AZero13 <gfunni234 at gmail.com>
Date: Tue, 23 Sep 2025 14:27:07 -0400
Subject: [PATCH] [CodeGenPrepare] Bail out of usubo creation if sub's parent
 is not the same as the comparison

We match uadd's behavior here.

Codegen comparison: https://godbolt.org/z/x8j4EhGno
---
 llvm/lib/CodeGen/CodeGenPrepare.cpp  |  6 ++++++
 llvm/test/CodeGen/X86/usub_inc_iv.ll | 10 +++++-----
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index d290f202f3cca..eb73d01b3558c 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -1749,6 +1749,12 @@ bool CodeGenPrepare::combineToUSubWithOverflow(CmpInst *Cmp,
                                  Sub->hasNUsesOrMore(1)))
     return false;
 
+  // We don't want to move around uses of condition values this late, so we
+  // check if it is legal to create the call to the intrinsic in the basic
+  // block containing the icmp.
+  if (Sub->getParent() != Cmp->getParent() && !Sub->hasOneUse())
+    return false;
+
   if (!replaceMathCmpWithIntrinsic(Sub, Sub->getOperand(0), Sub->getOperand(1),
                                    Cmp, Intrinsic::usub_with_overflow))
     return false;
diff --git a/llvm/test/CodeGen/X86/usub_inc_iv.ll b/llvm/test/CodeGen/X86/usub_inc_iv.ll
index 88bfddb51f2d4..ff06aaabd1b0c 100644
--- a/llvm/test/CodeGen/X86/usub_inc_iv.ll
+++ b/llvm/test/CodeGen/X86/usub_inc_iv.ll
@@ -303,14 +303,14 @@ define i32 @test_06(ptr %p, i64 %len, i32 %x) {
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ [[MATH:%.*]], [[BACKEDGE:%.*]] ], [ [[LEN:%.*]], [[ENTRY:%.*]] ]
-; CHECK-NEXT:    [[TMP0:%.*]] = call { i64, i1 } @llvm.usub.with.overflow.i64(i64 [[IV]], i64 1)
-; CHECK-NEXT:    [[MATH]] = extractvalue { i64, i1 } [[TMP0]], 0
-; CHECK-NEXT:    [[OV:%.*]] = extractvalue { i64, i1 } [[TMP0]], 1
+; CHECK-NEXT:    [[OV:%.*]] = icmp eq i64 [[IV]], 0
 ; CHECK-NEXT:    br i1 [[OV]], label [[EXIT:%.*]], label [[BACKEDGE]]
 ; CHECK:       backedge:
-; CHECK-NEXT:    [[SUNKADDR:%.*]] = mul i64 [[MATH]], 4
+; CHECK-NEXT:    [[SUNKADDR:%.*]] = mul i64 [[IV]], 4
 ; CHECK-NEXT:    [[SUNKADDR1:%.*]] = getelementptr i8, ptr [[P:%.*]], i64 [[SUNKADDR]]
-; CHECK-NEXT:    [[LOADED:%.*]] = load atomic i32, ptr [[SUNKADDR1]] unordered, align 4
+; CHECK-NEXT:    [[SUNKADDR2:%.*]] = getelementptr i8, ptr [[SUNKADDR1]], i64 -4
+; CHECK-NEXT:    [[LOADED:%.*]] = load atomic i32, ptr [[SUNKADDR2]] unordered, align 4
+; CHECK-NEXT:    [[MATH]] = add i64 [[IV]], -1
 ; CHECK-NEXT:    [[COND_2:%.*]] = icmp eq i32 [[LOADED]], [[X:%.*]]
 ; CHECK-NEXT:    br i1 [[COND_2]], label [[FAILURE:%.*]], label [[LOOP]]
 ; CHECK:       exit: