[InstCombine] Fix regression introduced at r227197.

This patch fixes a problem I accidentally introduced in an instruction combine
on select instructions added at r227197. That revision taught the instruction
combiner how to fold a cttz/ctlz followed by a icmp plus select into a single
cttz/ctlz with flag 'is_zero_undef' cleared.

However, the new rule added at r227197 would have produced wrong results in the
case where a cttz/ctlz with flag 'is_zero_undef' cleared was follwed by a
zero-extend or truncate. In that case, the folded instruction would have
been inserted in a wrong location thus leaving the CFG in an inconsistent
state.

This patch fixes the problem and add two reproducible test cases to
existing test 'InstCombine/select-cmp-cttz-ctlz.ll'.

llvm-svn: 229124

1 files changed, 6 insertions, 10 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 0debe63..b92d90d 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -482,16 +482,12 @@ static Value *foldSelectCttzCtlz(ICmpInst *ICI, Value *TrueVal, Value *FalseVal,
       match(Count, m_Intrinsic<Intrinsic::ctlz>(m_Specific(CmpLHS)))) {
     IntrinsicInst *II = cast<IntrinsicInst>(Count);
     IRBuilder<> Builder(II);
-    if (cast<ConstantInt>(II->getArgOperand(1))->isOne()) {
-      // Explicitly clear the 'undef_on_zero' flag.
-      IntrinsicInst *NewI = cast<IntrinsicInst>(II->clone());
-      Type *Ty = NewI->getArgOperand(1)->getType();
-      NewI->setArgOperand(1, Constant::getNullValue(Ty));
-      Builder.Insert(NewI);
-      Count = NewI;
-    }
-
-    return Builder.CreateZExtOrTrunc(Count, ValueOnZero->getType());
+    // Explicitly clear the 'undef_on_zero' flag.
+    IntrinsicInst *NewI = cast<IntrinsicInst>(II->clone());
+    Type *Ty = NewI->getArgOperand(1)->getType();
+    NewI->setArgOperand(1, Constant::getNullValue(Ty));
+    Builder.Insert(NewI);
+    return Builder.CreateZExtOrTrunc(NewI, ValueOnZero->getType());
   }
 
   return nullptr;