[InstCombine] Simplify ctlz/cttz with bitreverse

Summary: Fixes PR41273

Reviewers: spatel

Reviewed By: spatel

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60096

llvm-svn: 357521

1 files changed, 9 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index e6f7e623067..c2506798660 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1324,12 +1324,20 @@ static Instruction *foldCttzCtlz(IntrinsicInst &II, InstCombiner &IC) {
   assert((II.getIntrinsicID() == Intrinsic::cttz ||
           II.getIntrinsicID() == Intrinsic::ctlz) &&
          "Expected cttz or ctlz intrinsic");
+  bool IsTZ = II.getIntrinsicID() == Intrinsic::cttz;
   Value *Op0 = II.getArgOperand(0);
+  Value *X;
+  // ctlz(bitreverse(x)) -> cttz(x)
+  // cttz(bitreverse(x)) -> ctlz(x)
+  if (match(Op0, m_BitReverse(m_Value(X)))) {
+    Intrinsic::ID ID = IsTZ ? Intrinsic::ctlz : Intrinsic::cttz;
+    Function *F = Intrinsic::getDeclaration(II.getModule(), ID, II.getType());
+    return CallInst::Create(F, {X, II.getArgOperand(1)});
+  }
 
   KnownBits Known = IC.computeKnownBits(Op0, 0, &II);
 
   // Create a mask for bits above (ctlz) or below (cttz) the first known one.
-  bool IsTZ = II.getIntrinsicID() == Intrinsic::cttz;
   unsigned PossibleZeros = IsTZ ? Known.countMaxTrailingZeros()
                                 : Known.countMaxLeadingZeros();
   unsigned DefiniteZeros = IsTZ ? Known.countMinTrailingZeros()