Transform (x&C)>V into (x&C)!=0 where possible

When the least bit of C is greater than V, (x&C) must be greater than V
if it is not zero, so the comparison can be simplified.

Although this was suggested in Target/X86/README.txt, it benefits any
architecture with a directly testable form of AND.

Patch by Kevin Schoedel

llvm-svn: 170576

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 1b96c3cca4e..13516d15c4d 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1226,6 +1226,16 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
         ICI.setOperand(0, NewAnd);
         return &ICI;
       }
+
+      // Replace ((X & AndCST) > RHSV) with ((X & AndCST) != 0), if any
+      // bit set in (X & AndCST) will produce a result greater than RHSV.
+      if (ICI.getPredicate() == ICmpInst::ICMP_UGT) {
+        unsigned NTZ = AndCST->getValue().countTrailingZeros();
+        if ((NTZ < AndCST->getBitWidth()) &&
+            APInt::getOneBitSet(AndCST->getBitWidth(), NTZ).ugt(RHSV))
+          return new ICmpInst(ICmpInst::ICMP_NE, LHSI,
+                              Constant::getNullValue(RHS->getType()));
+      }
     }
 
     // Try to optimize things like "A[i]&42 == 0" to index computations.