Emit comparisons against the sign bit better. Codegen this:
bool %test1(long %X) {
%A = setlt long %X, 0
ret bool %A
}
like this:
test1:
cmpl $0, 8(%esp)
setl %al
movzbl %al, %eax
ret
instead of:
test1:
movl 8(%esp), %ecx
cmpl $0, %ecx
setl %al
movzbw %al, %ax
cmpl $0, 4(%esp)
setb %dl
movzbw %dl, %dx
cmpl $0, %ecx
cmove %dx, %ax
movzbl %al, %eax
ret
llvm-svn: 21243
diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
index 3bd2494..7965e14 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
@@ -768,6 +768,16 @@
DAG.getConstant(0, Tmp1.getValueType()));
break;
default:
+ // If this is a comparison of the sign bit, just look at the top part.
+ // X > -1, x < 0
+ if (ConstantSDNode *CST = dyn_cast<ConstantSDNode>(Node->getOperand(1)))
+ if ((cast<SetCCSDNode>(Node)->getCondition() == ISD::SETLT &&
+ CST->getValue() == 0) || // X < 0
+ (cast<SetCCSDNode>(Node)->getCondition() == ISD::SETGT &&
+ (CST->isAllOnesValue()))) // X > -1
+ return DAG.getSetCC(cast<SetCCSDNode>(Node)->getCondition(),
+ Node->getValueType(0), LHSHi, RHSHi);
+
// FIXME: This generated code sucks.
ISD::CondCode LowCC;
switch (cast<SetCCSDNode>(Node)->getCondition()) {