If the false value for a select_cc is really simple (has no inputs), evaluate

it in the block.  This codegens:

int %test(bool %c) {
        %retval = select bool %c, int 17, int 1
        ret int %retval
}

as:

_test:
        rlwinm r2, r3, 0, 31, 31
        li r2, 17
        cmpwi cr0, r3, 0
        bne .LBB_test_2 ;
.LBB_test_1:    ;
        li r2, 1
.LBB_test_2:    ;
        or r3, r2, r2
        blr

instead of:

_test:
        rlwinm r2, r3, 0, 31, 31
        li r2, 17
        li r4, 1
        cmpwi cr0, r3, 0
        bne .LBB_test_2 ;
.LBB_test_1:    ;
        or r2, r4, r4
.LBB_test_2:    ;
        or r3, r2, r2
        blr

... which is one fewer instruction.  The savings are more significant for
global address and constantfp nodes.

llvm-svn: 22946

llvm/lib/Target/PowerPC/PPC32ISelPattern.cpp

@@ -1675,7 +1675,16 @@ unsigned ISel::SelectExpr(SDOperand N, bool Recording) {
     // selecting it in the fallthrough basic block rather than here, which
     // increases register pressure.
     unsigned TrueValue = SelectExpr(N.getOperand(2));
-    unsigned FalseValue = SelectExpr(N.getOperand(3));
+    unsigned FalseValue;
+
+    // If the false value is simple enough, evaluate it inline in the false
+    // block.
+    if (isa<ConstantSDNode>(N.getOperand(3)) ||
+        isa<ConstantFPSDNode>(N.getOperand(3)) ||
+        isa<GlobalAddressSDNode>(N.getOperand(3)))
+      FalseValue = 0;
+    else
+      FalseValue = SelectExpr(N.getOperand(3));
     unsigned CCReg = SelectCC(N.getOperand(0), N.getOperand(1), CC);
     Opc = getBCCForSetCC(CC);
     
@@ -1706,6 +1715,12 @@ unsigned ISel::SelectExpr(SDOperand N, bool Recording) {
     //   %FalseValue = ...
     //   # fallthrough to sinkMBB
     BB = copy0MBB;
+
+    // If the false value is simple enough, evaluate it here, to avoid it being
+    // evaluated on the true edge.
+    if (FalseValue == 0)
+      FalseValue = SelectExpr(N.getOperand(3));
+
     // Update machine-CFG edges
     BB->addSuccessor(sinkMBB);