Convert a subtract into a negate and an add when it helps x86
address folding.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71446 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/X86/X86ISelDAGToDAG.cpp b/lib/Target/X86/X86ISelDAGToDAG.cpp
index f12b138..bd1fea7 100644
--- a/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -868,6 +868,76 @@
}
break;
+ case ISD::SUB: {
+ // Given A-B, if A can be completely folded into the address and
+ // the index field with the index field unused, use -B as the index.
+ // This is a win if a has multiple parts that can be folded into
+ // the address. Also, this saves a mov if the base register has
+ // other uses, since it avoids a two-address sub instruction, however
+ // it costs an additional mov if the index register has other uses.
+
+ // Test if the LHS of the sub can be folded.
+ X86ISelAddressMode Backup = AM;
+ if (MatchAddress(N.getNode()->getOperand(0), AM, Depth+1)) {
+ AM = Backup;
+ break;
+ }
+ // Test if the index field is free for use.
+ if (AM.IndexReg.getNode() || AM.isRIPRel) {
+ AM = Backup;
+ break;
+ }
+ int Cost = 0;
+ SDValue RHS = N.getNode()->getOperand(1);
+ // If the RHS involves a register with multiple uses, this
+ // transformation incurs an extra mov, due to the neg instruction
+ // clobbering its operand.
+ if (!RHS.getNode()->hasOneUse() ||
+ RHS.getNode()->getOpcode() == ISD::CopyFromReg ||
+ RHS.getNode()->getOpcode() == ISD::TRUNCATE ||
+ RHS.getNode()->getOpcode() == ISD::ANY_EXTEND ||
+ (RHS.getNode()->getOpcode() == ISD::ZERO_EXTEND &&
+ RHS.getNode()->getOperand(0).getValueType() == MVT::i32))
+ ++Cost;
+ // If the base is a register with multiple uses, this
+ // transformation may save a mov.
+ if ((AM.BaseType == X86ISelAddressMode::RegBase &&
+ AM.Base.Reg.getNode() &&
+ !AM.Base.Reg.getNode()->hasOneUse()) ||
+ AM.BaseType == X86ISelAddressMode::FrameIndexBase)
+ --Cost;
+ // If the folded LHS was interesting, this transformation saves
+ // address arithmetic.
+ if ((AM.hasSymbolicDisplacement() && !Backup.hasSymbolicDisplacement()) +
+ ((AM.Disp != 0) && (Backup.Disp == 0)) +
+ (AM.Segment.getNode() && !Backup.Segment.getNode()) >= 2)
+ --Cost;
+ // If it doesn't look like it may be an overall win, don't do it.
+ if (Cost >= 0) {
+ AM = Backup;
+ break;
+ }
+
+ // Ok, the transformation is legal and appears profitable. Go for it.
+ SDValue Zero = CurDAG->getConstant(0, N.getValueType());
+ SDValue Neg = CurDAG->getNode(ISD::SUB, dl, N.getValueType(), Zero, RHS);
+ AM.IndexReg = Neg;
+ AM.Scale = 1;
+
+ // Insert the new nodes into the topological ordering.
+ if (Zero.getNode()->getNodeId() == -1 ||
+ Zero.getNode()->getNodeId() > N.getNode()->getNodeId()) {
+ CurDAG->RepositionNode(N.getNode(), Zero.getNode());
+ Zero.getNode()->setNodeId(N.getNode()->getNodeId());
+ }
+ if (Neg.getNode()->getNodeId() == -1 ||
+ Neg.getNode()->getNodeId() > N.getNode()->getNodeId()) {
+ CurDAG->RepositionNode(N.getNode(), Neg.getNode());
+ Neg.getNode()->setNodeId(N.getNode()->getNodeId());
+ }
+ return false;
+ }
+
case ISD::ADD: {
X86ISelAddressMode Backup = AM;
if (!MatchAddress(N.getNode()->getOperand(0), AM, Depth+1) &&