[llvm] [DAGCombine] Correctly extend the constant RHS in `TargetLowering::SimplifySetCC` (PR #152862)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

In https://github.com/llvm/llvm-project/pull/150270, when the predicate is eq/ne and the trunc has only an nsw flag, the RHS is incorrectly zero-extended.

Closes https://github.com/llvm/llvm-project/issues/152630.


---
Full diff: https://github.com/llvm/llvm-project/pull/152862.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp (+5-4) 
- (added) llvm/test/CodeGen/X86/pr152630.ll (+34) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index e235d144e85ff..8fbabfa55e93f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -5125,10 +5125,11 @@ SDValue TargetLowering::SimplifySetCC(EVT VT, SDValue N0, SDValue N1,
           !ISD::isUnsignedIntSetCC(Cond))) &&
         isTypeDesirableForOp(ISD::SETCC, N0.getOperand(0).getValueType())) {
       EVT NewVT = N0.getOperand(0).getValueType();
-      SDValue NewConst = DAG.getConstant(ISD::isSignedIntSetCC(Cond)
-                                             ? C1.sext(NewVT.getSizeInBits())
-                                             : C1.zext(NewVT.getSizeInBits()),
-                                         dl, NewVT);
+      SDValue NewConst = DAG.getConstant(
+          (N0->getFlags().hasNoSignedWrap() && !ISD::isUnsignedIntSetCC(Cond))
+              ? C1.sext(NewVT.getSizeInBits())
+              : C1.zext(NewVT.getSizeInBits()),
+          dl, NewVT);
       return DAG.getSetCC(dl, VT, N0.getOperand(0), NewConst, Cond);
     }
 
diff --git a/llvm/test/CodeGen/X86/pr152630.ll b/llvm/test/CodeGen/X86/pr152630.ll
new file mode 100644
index 0000000000000..8fa98831c37b0
--- /dev/null
+++ b/llvm/test/CodeGen/X86/pr152630.ll
@@ -0,0 +1,34 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
+
+define i32 @pr152630(i1 %cond) nounwind {
+; CHECK-LABEL: pr152630:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    andl $1, %edi
+; CHECK-NEXT:    decl %edi
+; CHECK-NEXT:    cmpl $-1, %edi
+; CHECK-NEXT:    je .LBB0_2
+; CHECK-NEXT:  # %bb.1: # %entry
+; CHECK-NEXT:    movzbl %dil, %eax
+; CHECK-NEXT:    testl %eax, %eax
+; CHECK-NEXT:    jne .LBB0_3
+; CHECK-NEXT:  .LBB0_2: # %if.then
+; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    retq
+; CHECK-NEXT:  .LBB0_3: # %if.else
+; CHECK-NEXT:    movl $1, %eax
+; CHECK-NEXT:    retq
+entry:
+  %sel = select i1 %cond, i32 0, i32 -1
+  %conv = trunc nsw i32 %sel to i8
+  switch i8 %conv, label %if.else [
+    i8 -1, label %if.then
+    i8 0, label %if.then
+  ]
+
+if.then:
+  ret i32 0
+
+if.else:
+  ret i32 1
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/152862