DAGCombine: fold "(zext x) == C" into "x == (trunc C)" if the trunc is lossless.
On x86 this allows to fold a load into the cmp, greatly reducing register pressure.
movzbl (%rdi), %eax
cmpl $47, %eax
->
cmpb $47, (%rdi)
This shaves 8k off gcc.o on i386. I'll leave applying the patch in README.txt to Chris :)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130005 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 843ed96..15606af 100644
--- a/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1916,6 +1916,42 @@
// TODO: (ctpop x) == 1 -> x && (x & x-1) == 0 iff ctpop is illegal.
}
+ // (zext x) == C --> x == (trunc C)
+ if (DCI.isBeforeLegalize() && N0->hasOneUse() &&
+ (Cond == ISD::SETEQ || Cond == ISD::SETNE)) {
+ unsigned MinBits = N0.getValueSizeInBits();
+ SDValue PreZExt;
+ if (N0->getOpcode() == ISD::ZERO_EXTEND) {
+ // ZExt
+ MinBits = N0->getOperand(0).getValueSizeInBits();
+ PreZExt = N0->getOperand(0);
+ } else if (N0->getOpcode() == ISD::AND) {
+ // DAGCombine turns costly ZExts into ANDs
+ if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(N0->getOperand(1)))
+ if ((C->getAPIntValue()+1).isPowerOf2()) {
+ MinBits = C->getAPIntValue().countTrailingOnes();
+ PreZExt = N0->getOperand(0);
+ }
+ } else if (LoadSDNode *LN0 = dyn_cast<LoadSDNode>(N0)) {
+ // ZEXTLOAD
+ if (LN0->getExtensionType() == ISD::ZEXTLOAD) {
+ MinBits = LN0->getMemoryVT().getSizeInBits();
+ PreZExt = N0;
+ }
+ }
+
+ // Make sure we're not loosing bits from the constant.
+ if (MinBits < C1.getBitWidth() && MinBits > C1.getActiveBits()) {
+ EVT MinVT = EVT::getIntegerVT(*DAG.getContext(), MinBits);
+ if (isTypeDesirableForOp(ISD::SETCC, MinVT)) {
+ // Will get folded away.
+ SDValue Trunc = DAG.getNode(ISD::TRUNCATE, dl, MinVT, PreZExt);
+ SDValue C = DAG.getConstant(C1.trunc(MinBits), MinVT);
+ return DAG.getSetCC(dl, VT, Trunc, C, Cond);
+ }
+ }
+ }
+
// If the LHS is '(and load, const)', the RHS is 0,
// the test is for equality or unsigned, and all 1 bits of the const are
// in the same partial word, see if we can shorten the load.