[InstCombine] Add trivial folding (bitreverse (bitreverse x)) -> x

There are plenty more instcombines we could probably do with bitreverse, but this seems like a very obvious and trivial starting point and was brought up by Hal in his review.

llvm-svn: 252879

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index c3fa7be3272..cde26cc24c2 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -788,6 +788,16 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
     break;
   }
 
+  case Intrinsic::bitreverse: {
+    Value *IIOperand = II->getArgOperand(0);
+    Value *X = nullptr;
+
+    // bitreverse(bitreverse(x)) -> x
+    if (match(IIOperand, m_Intrinsic<Intrinsic::bitreverse>(m_Value(X))))
+      return ReplaceInstUsesWith(CI, X);
+    break;
+  }
+
   case Intrinsic::powi:
     if (ConstantInt *Power = dyn_cast<ConstantInt>(II->getArgOperand(1))) {
       // powi(x, 0) -> 1.0