[llvm-commits] CVS: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

[Chris Lattner]

Changes in directory llvm/lib/CodeGen/SelectionDAG:

DAGCombiner.cpp updated: 1.20 -> 1.21
---
Log message:

implement CodeGen/PowerPC/div-2.ll:test2-4 by propagating zero bits through
C-X's


---
Diffs of the changes:  (+18 -0)

 DAGCombiner.cpp |   18 ++++++++++++++++++
 1 files changed, 18 insertions(+)


Index: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
diff -u llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:1.20 llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:1.21
--- llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:1.20	Fri Oct  7 01:10:46 2005
+++ llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp	Fri Oct  7 10:30:32 2005
@@ -205,6 +205,24 @@
       return MaskedValueIsZero(Op.getOperand(0), NewVal, TLI);
     }
     return false;
+  case ISD::SUB:
+    if (ConstantSDNode *CLHS = dyn_cast<ConstantSDNode>(Op.getOperand(0))) {
+      // We know that the top bits of C-X are clear if X contains less bits
+      // than C (i.e. no wrap-around can happen).  For example, 20-X is
+      // positive if we can prove that X is >= 0 and < 16.
+      unsigned Bits = MVT::getSizeInBits(CLHS->getValueType(0));
+      if ((CLHS->getValue() & (1 << (Bits-1))) == 0) {  // sign bit clear
+        unsigned NLZ = CountLeadingZeros_64(CLHS->getValue()+1);
+        uint64_t MaskV = (1ULL << (63-NLZ))-1;
+        if (MaskedValueIsZero(Op.getOperand(1), ~MaskV, TLI)) {
+          // High bits are clear this value is known to be >= C.
+          unsigned NLZ2 = CountLeadingZeros_64(CLHS->getValue());
+          if ((Mask & ((1ULL << (64-NLZ2))-1)) == 0)
+            return true;
+        }
+      }
+    }
+    break;
   case ISD::CTTZ:
   case ISD::CTLZ:
   case ISD::CTPOP: