[llvm] r185915 - InstCombine: variations on 0xffffffff - x >= 4

[David Majnemer]

Author: majnemer
Date: Tue Jul  9 04:20:58 2013
New Revision: 185915

URL: http://llvm.org/viewvc/llvm-project?rev=185915&view=rev
Log:
InstCombine: variations on 0xffffffff - x >= 4

The following transforms are valid if -C is a power of 2:
(icmp ugt (xor X, C), ~C) -> (icmp ult X, C)
(icmp ult (xor X, C), -C) -> (icmp uge X, C)

These are nice, they get rid of the xor.

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
    llvm/trunk/test/Transforms/InstCombine/icmp.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp?rev=185915&r1=185914&r2=185915&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp Tue Jul  9 04:20:58 2013
@@ -1126,6 +1126,18 @@ Instruction *InstCombiner::visitICmpInst
                               Builder->getInt(RHSV ^ NotSignBit));
         }
       }
+
+      // (icmp ugt (xor X, C), ~C) -> (icmp ult X, C)
+      //   iff -C is a power of 2
+      if (ICI.getPredicate() == ICmpInst::ICMP_UGT &&
+          XorCST->getValue() == ~RHSV && (RHSV + 1).isPowerOf2())
+        return new ICmpInst(ICmpInst::ICMP_ULT, LHSI->getOperand(0), XorCST);
+
+      // (icmp ult (xor X, C), -C) -> (icmp uge X, C)
+      //   iff -C is a power of 2
+      if (ICI.getPredicate() == ICmpInst::ICMP_ULT &&
+          XorCST->getValue() == -RHSV && RHSV.isPowerOf2())
+        return new ICmpInst(ICmpInst::ICMP_UGE, LHSI->getOperand(0), XorCST);
     }
     break;
   case Instruction::And:         // (icmp pred (and X, AndCST), RHS)

Modified: llvm/trunk/test/Transforms/InstCombine/icmp.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/icmp.ll?rev=185915&r1=185914&r2=185915&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/icmp.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/icmp.ll Tue Jul  9 04:20:58 2013
@@ -1181,3 +1181,21 @@ define i1 @icmp_and_X_-16_ne-16(i32 %X)
   %cmp = icmp ne i32 %and, -16
   ret i1 %cmp
 }
+
+; CHECK: @icmp_sub_-1_X_ult_4
+; CHECK-NEXT: [[CMP:%[a-z0-9]+]] = icmp ugt i32 %X, -5
+; CHECK-NEXT: ret i1 [[CMP]]
+define i1 @icmp_sub_-1_X_ult_4(i32 %X) {
+  %sub = sub i32 -1, %X
+  %cmp = icmp ult i32 %sub, 4
+  ret i1 %cmp
+}
+
+; CHECK: @icmp_sub_-1_X_uge_4
+; CHECK-NEXT: [[CMP:%[a-z0-9]+]] = icmp ult i32 %X, -4
+; CHECK-NEXT: ret i1 [[CMP]]
+define i1 @icmp_sub_-1_X_uge_4(i32 %X) {
+  %sub = sub i32 -1, %X
+  %cmp = icmp uge i32 %sub, 4
+  ret i1 %cmp
+}