[InstCombine] Fix worklist management in select fold

[ParkHanbum]

InstCombine uses Worklist to manage change history. setOperand, which was previously used to change the Select Instruction, does not, so it is run twice, which causes an LLVM ERROR.

This problem is resolved by changing setOperand to replaceOperand as the change history will be registered in the Worklist.

Fixes #77553.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: hanbeom (ParkHanbum)

Changes

InstCombine uses Worklist to manage change history. setOperand, which was previously used to change the Select Instruction, does not, so it is run twice, which causes an LLVM ERROR.

This problem is resolved by changing setOperand to replaceOperand as the change history will be registered in the Worklist.

---

Full diff: https://github.com/llvm/llvm-project/pull/77738.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+2-2)
• (modified) llvm/test/Transforms/InstCombine/select.ll (+14)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 2dda46986f0fd0..1aa47b2ccfd82b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1703,11 +1703,11 @@ Instruction *InstCombinerImpl::foldSelectInstWithICmp(SelectInst &SI,
   if (CmpRHS != CmpLHS && isa<Constant>(CmpRHS) && !isa<Constant>(CmpLHS)) {
     if (CmpLHS == TrueVal && Pred == ICmpInst::ICMP_EQ) {
       // Transform (X == C) ? X : Y -> (X == C) ? C : Y
-      SI.setOperand(1, CmpRHS);
+      replaceOperand(SI, 1, CmpRHS);
       Changed = true;
     } else if (CmpLHS == FalseVal && Pred == ICmpInst::ICMP_NE) {
       // Transform (X != C) ? Y : X -> (X != C) ? Y : C
-      SI.setOperand(2, CmpRHS);
+      replaceOperand(SI, 2, CmpRHS);
       Changed = true;
     }
   }
diff --git a/llvm/test/Transforms/InstCombine/select.ll b/llvm/test/Transforms/InstCombine/select.ll
index 7583a75385a769..5687371b4e1dd2 100644
--- a/llvm/test/Transforms/InstCombine/select.ll
+++ b/llvm/test/Transforms/InstCombine/select.ll
@@ -3646,3 +3646,17 @@ loop:
 exit:
   ret i32 %rem
 }
+
+; (X == C) ? X : Y -> (X == C) ? C : Y
+; Fixed #77553
+define i32 @src_select_xxory_eq0_xorxy_y(i32 %x, i32 %y) {
+; CHECK-LABEL: @src_select_xxory_eq0_xorxy_y(
+; CHECK-NEXT:    [[XOR0:%.*]] = icmp eq i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[COND:%.*]] = select i1 [[XOR0]], i32 0, i32 [[Y]]
+; CHECK-NEXT:    ret i32 [[COND]]
+;
+  %xor = xor i32 %x, %y
+  %xor0 = icmp eq i32 %xor, 0
+  %cond = select i1 %xor0, i32 %xor, i32 %y
+  ret i32 %cond
+}

[dtcxzyw]

LGTM. I tested it on my local machine.