[llvm-commits] [llvm] r147254 - in /llvm/trunk: lib/Transforms/InstCombine/InstCombineAddSub.cpp lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp test/Transforms/InstCombine/sub-xor.ll

[Benjamin Kramer]

Author: d0k
Date: Sat Dec 24 11:31:38 2011
New Revision: 147254

URL: http://llvm.org/viewvc/llvm-project?rev=147254&view=rev
Log:
InstCombine: Canonicalize (2^n)-1 - x into (2^n)-1 ^ x iff x is known to be smaller than 2^n.

This has the obvious advantage of being commutable and is always a win on x86 because
const - x wastes a register there. On less weird architectures this may lead to
a regression because other arithmetic doesn't fuse with it anymore. I'll address that
problem in a followup.

Added:
    llvm/trunk/test/Transforms/InstCombine/sub-xor.ll
Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
    llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp?rev=147254&r1=147253&r2=147254&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp Sat Dec 24 11:31:38 2011
@@ -587,6 +587,9 @@
     ConstantInt *C2;
     if (match(Op1, m_Add(m_Value(X), m_ConstantInt(C2))))
       return BinaryOperator::CreateSub(ConstantExpr::getSub(C, C2), X);
+
+    if (SimplifyDemandedInstructionBits(I))
+      return &I;
   }
 
   

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp?rev=147254&r1=147253&r2=147254&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp Sat Dec 24 11:31:38 2011
@@ -567,9 +567,20 @@
                                LHSKnownZero, LHSKnownOne, Depth+1))
         return I;
     }
+
     // Otherwise just hand the sub off to ComputeMaskedBits to fill in
     // the known zeros and ones.
     ComputeMaskedBits(V, DemandedMask, KnownZero, KnownOne, Depth);
+
+    // Turn this into a xor if LHS is 2^n-1 and the remaining bits are known
+    // zero.
+    if (ConstantInt *C0 = dyn_cast<ConstantInt>(I->getOperand(0))) {
+      APInt I0 = C0->getValue();
+      if ((I0 + 1).isPowerOf2() && (I0 | KnownZero).isAllOnesValue()) {
+        Instruction *Xor = BinaryOperator::CreateXor(I->getOperand(1), C0);
+        return InsertNewInstWith(Xor, *I);
+      }
+    }
     break;
   case Instruction::Shl:
     if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {

Added: llvm/trunk/test/Transforms/InstCombine/sub-xor.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/sub-xor.ll?rev=147254&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/sub-xor.ll (added)
+++ llvm/trunk/test/Transforms/InstCombine/sub-xor.ll Sat Dec 24 11:31:38 2011
@@ -0,0 +1,12 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+define i32 @test1(i32 %x) nounwind {
+  %and = and i32 %x, 31
+  %sub = sub i32 63, %and
+  ret i32 %sub
+
+; CHECK: @test1
+; CHECK-NEXT: and i32 %x, 31
+; CHECK-NEXT: xor i32 %and, 63
+; CHECK-NEXT: ret
+}