[llvm] r252879 - [InstCombine] Add trivial folding (bitreverse (bitreverse x)) -> x

[James Molloy via llvm-commits]

Author: jamesm
Date: Thu Nov 12 06:39:41 2015
New Revision: 252879

URL: http://llvm.org/viewvc/llvm-project?rev=252879&view=rev
Log:
[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.

Added:
    llvm/trunk/test/Transforms/InstCombine/bitreverse-fold.ll
Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp?rev=252879&r1=252878&r2=252879&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp Thu Nov 12 06:39:41 2015
@@ -788,6 +788,16 @@ Instruction *InstCombiner::visitCallInst
     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

Added: llvm/trunk/test/Transforms/InstCombine/bitreverse-fold.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/bitreverse-fold.ll?rev=252879&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/bitreverse-fold.ll (added)
+++ llvm/trunk/test/Transforms/InstCombine/bitreverse-fold.ll Thu Nov 12 06:39:41 2015
@@ -0,0 +1,11 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+define i32 @test1(i32 %p) {
+; CHECK-LABEL: @test1
+; CHECK-NEXT: ret i32 %p
+  %a = call i32 @llvm.bitreverse.i32(i32 %p)
+  %b = call i32 @llvm.bitreverse.i32(i32 %a)
+  ret i32 %b
+}
+
+declare i32 @llvm.bitreverse.i32(i32) readnone