lower "X = seteq Y, Z" to '(shr (ctlz (xor Y, Z)), 5)' instead of

'(shr (ctlz (sub Y, Z)), 5)'.

The use of xor better exposes the operation to bit-twiddling logic in the
dag combiner.  For example, this:

typedef struct {
  unsigned prefix : 4;
  unsigned code : 4;
  unsigned unsigned_p : 4;
} tree_common;

int foo(tree_common *a, tree_common *b) {
  return a->code == b->code;
}

Now compiles to:

_foo:
        lwz r2, 0(r4)
        lwz r3, 0(r3)
        xor r2, r3, r2
        rlwinm r2, r2, 28, 28, 31
        cntlzw r2, r2
        srwi r3, r2, 5
        blr

instead of:

_foo:
        lbz r2, 3(r4)
        lbz r3, 3(r3)
        srwi r2, r2, 4
        srwi r3, r3, 4
        subf r2, r2, r3
        cntlzw r2, r2
        srwi r3, r2, 5
        blr

saving a cycle.

llvm-svn: 31725

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

@@ -1038,12 +1038,14 @@ static SDOperand LowerSETCC(SDOperand Op, SelectionDAG &DAG) {
   }
   
   // If we have an integer seteq/setne, turn it into a compare against zero
-  // by subtracting the rhs from the lhs, which is faster than setting a
-  // condition register, reading it back out, and masking the correct bit.
+  // by xor'ing the rhs with the lhs, which is faster than setting a
+  // condition register, reading it back out, and masking the correct bit.  The
+  // normal approach here uses sub to do this instead of xor.  Using xor exposes
+  // the result to other bit-twiddling opportunities.
   MVT::ValueType LHSVT = Op.getOperand(0).getValueType();
   if (MVT::isInteger(LHSVT) && (CC == ISD::SETEQ || CC == ISD::SETNE)) {
     MVT::ValueType VT = Op.getValueType();
-    SDOperand Sub = DAG.getNode(ISD::SUB, LHSVT, Op.getOperand(0), 
+    SDOperand Sub = DAG.getNode(ISD::XOR, LHSVT, Op.getOperand(0), 
                                 Op.getOperand(1));
     return DAG.getSetCC(VT, Sub, DAG.getConstant(0, LHSVT), CC);
   }