[llvm-commits] [llvm] r147256 - in /llvm/trunk: lib/Transforms/InstCombine/InstCombineAddSub.cpp test/Transforms/InstCombine/sub-xor.ll

[Benjamin Kramer]

Author: d0k
Date: Sat Dec 24 11:31:53 2011
New Revision: 147256

URL: http://llvm.org/viewvc/llvm-project?rev=147256&view=rev
Log:
InstCombine: Add a combine that turns (2^n)-1 ^ x back into (2^n)-1 - x iff x is smaller than 2^n and it fuses with a following add.

This was intended to undo the sub canonicalization in cases where it's not profitable, but it also
finds some cases on it's own.

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

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp?rev=147256&r1=147255&r2=147256&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp Sat Dec 24 11:31:53 2011
@@ -136,6 +136,19 @@
         Value *NewShl = Builder->CreateShl(XorLHS, ShAmt, "sext");
         return BinaryOperator::CreateAShr(NewShl, ShAmt);
       }
+
+      // If this is a xor that was canonicalized from a sub, turn it back into
+      // a sub and fuse this add with it.
+      if (LHS->hasOneUse() && (XorRHS->getValue()+1).isPowerOf2()) {
+        IntegerType *IT = cast<IntegerType>(I.getType());
+        APInt Mask = APInt::getAllOnesValue(IT->getBitWidth());
+        APInt LHSKnownOne(IT->getBitWidth(), 0);
+        APInt LHSKnownZero(IT->getBitWidth(), 0);
+        ComputeMaskedBits(XorLHS, Mask, LHSKnownZero, LHSKnownOne);
+        if ((XorRHS->getValue() | LHSKnownZero).isAllOnesValue())
+          return BinaryOperator::CreateSub(ConstantExpr::getAdd(XorRHS, CI),
+                                           XorLHS);
+      }
     }
   }
 

Modified: llvm/trunk/test/Transforms/InstCombine/sub-xor.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/sub-xor.ll?rev=147256&r1=147255&r2=147256&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/sub-xor.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/sub-xor.ll Sat Dec 24 11:31:53 2011
@@ -23,3 +23,15 @@
 ; CHECK-NEXT: xor i32 %count, 31
 ; CHECK-NEXT: ret
 }
+
+define i32 @test3(i32 %x) nounwind {
+  %and = and i32 %x, 31
+  %sub = xor i32 31, %and
+  %add = add i32 %sub, 42
+  ret i32 %add
+
+; CHECK: @test3
+; CHECK-NEXT: and i32 %x, 31
+; CHECK-NEXT: sub i32 73, %and
+; CHECK-NEXT: ret
+}