[InstCombine] Drop noundef attributes in foldCttzCtlz (#116718)

Closes #112068. (cherry picked from commit a59976b)
llvm · Nov 19, 2024 · 5151407 · 5151407
1 parent 51dee6b
commit 5151407
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Showing 2 changed files with 19 additions and 1 deletion.
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -506,8 +506,10 @@ static Instruction *foldCttzCtlz(IntrinsicInst &II, InstCombinerImpl &IC) {
 
   // If ctlz/cttz is only used as a shift amount, set is_zero_poison to true.
   if (II.hasOneUse() && match(Op1, m_Zero()) &&
-      match(II.user_back(), m_Shift(m_Value(), m_Specific(&II))))
+      match(II.user_back(), m_Shift(m_Value(), m_Specific(&II)))) {
+    II.dropUBImplyingAttrsAndMetadata();
     return IC.replaceOperand(II, 1, IC.Builder.getTrue());
+  }
 
   Constant *C;
 

diff --git a/llvm/test/Transforms/InstCombine/shift-cttz-ctlz.ll b/llvm/test/Transforms/InstCombine/shift-cttz-ctlz.ll
@@ -15,6 +15,22 @@ entry:
   ret i32 %res
 }
 
+; Make sure that noundef is dropped.
+
+define i32 @shl_cttz_false_noundef(i32 %x, i32 %y) {
+; CHECK-LABEL: define i32 @shl_cttz_false_noundef(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CTTZ:%.*]] = call range(i32 0, 33) i32 @llvm.cttz.i32(i32 [[Y]], i1 true)
+; CHECK-NEXT:    [[RES:%.*]] = shl i32 [[X]], [[CTTZ]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+entry:
+  %cttz = call noundef i32 @llvm.cttz.i32(i32 %y, i1 false)
+  %res = shl i32 %x, %cttz
+  ret i32 %res
+}
+
 define i32 @shl_ctlz_false(i32 %x, i32 %y) {
 ; CHECK-LABEL: define i32 @shl_ctlz_false(
 ; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {