release/19.x: [InstCombine] Drop noundef attributes in foldCttzCtlz (#116718)

[AZero13]

Closes #112068.

(cherry picked from commit a59976b)

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Rose (AreaZR)

Changes

Closes #112068.

(cherry picked from commit a59976b)

---

Full diff: https://github.com/llvm/llvm-project/pull/116865.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp (+3-1)
• (modified) llvm/test/Transforms/InstCombine/shift-cttz-ctlz.ll (+16)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 9d2990c98ce275..3223fccbcf49a9 100644
--- 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
index 1c381d08390715..63caec95013254 100644
--- 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:%.*]]) {

[AZero13]

I am done finding things to backport to 19.x. Hopefully we do like one more round and release 19.1.6 but yeah.

[AZero13]

@nikic can you please add the 19.x milestone tag thing?

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