Revert "Revert '[DAGCombiner] Split up an indexed load if only the base pointer value is live'"
I reverted r208640 in r209747 because r208640 broke self-hosting on PPC64. The
underlying cause of the failure is that pre-inc loads with increments
represented by ISD::TargetConstants were being transformed into ISD:::ADDs with
ISD::TargetConstant operands. PPC doesn't have a pattern for those, and so they
were selected as invalid r+r adds.
This recommits r208640, rebased and with an exclusion for ISD::TargetConstant
increments. This behavior seems correct, although in the future we might want
to ask the target to split out the indexing that uses ISD::TargetConstants.
Unfortunately, I don't yet have small test case where the relevant invalid
'add' instruction is not itself dead (and thus eliminated by
DeadMachineInstructionElim -- sometimes bugpoint is too good at removing things)
Original commit message (by Adam Nemet):
Right now the load may not get DCE'd because of the side-effect of updating
the base pointer.
This can happen if we lower a read-modify-write of an illegal larger type
(e.g. i48) such that the modification only affects one of the subparts (the
lower i32 part but not the higher i16 part). See the testcase.
In order to spot the dead load we need to revisit it when SimplifyDemandedBits
decided that the value of the load is masked off. This is the
CommitTargetLoweringOpt piece.
I checked compile time with ARM64 by sending SPEC bitcode files through llc.
No measurable change.
Fixes <rdar://problem/16031651>
llvm-svn: 216898
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 3259904..92ee5c9 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -77,6 +77,10 @@
"slicing"),
cl::init(false));
+ static cl::opt<bool>
+ MaySplitLoadIndex("combiner-split-load-index", cl::Hidden, cl::init(true),
+ cl::desc("DAG combiner may split indexing from loads"));
+
//------------------------------ DAGCombiner ---------------------------------//
class DAGCombiner {
@@ -182,6 +186,7 @@
bool CombineToPreIndexedLoadStore(SDNode *N);
bool CombineToPostIndexedLoadStore(SDNode *N);
+ SDValue SplitIndexingFromLoad(LoadSDNode *LD);
bool SliceUpLoad(SDNode *N);
/// \brief Replace an ISD::EXTRACT_VECTOR_ELT of a load with a narrowed
@@ -462,7 +467,10 @@
// If the operands of this node are only used by the node, they will now be
// dead. Make sure to re-visit them and recursively delete dead nodes.
for (const SDValue &Op : N->ops())
- if (Op->hasOneUse())
+ // For an operand generating multiple values, one of the values may
+ // become dead allowing further simplification (e.g. split index
+ // arithmetic from an indexed load).
+ if (Op->hasOneUse() || Op->getNumValues() > 1)
AddToWorklist(Op.getNode());
DAG.DeleteNode(N);
@@ -8015,6 +8023,19 @@
return false;
}
+/// \brief Return the base-pointer arithmetic from an indexed \p LD.
+SDValue DAGCombiner::SplitIndexingFromLoad(LoadSDNode *LD) {
+ ISD::MemIndexedMode AM = LD->getAddressingMode();
+ assert(AM != ISD::UNINDEXED);
+ SDValue BP = LD->getOperand(1);
+ SDValue Inc = LD->getOperand(2);
+ assert(Inc.getOpcode() != ISD::TargetConstant &&
+ "Cannot split out indexing using target constants");
+ unsigned Opc =
+ (AM == ISD::PRE_INC || AM == ISD::POST_INC ? ISD::ADD : ISD::SUB);
+ return DAG.getNode(Opc, SDLoc(LD), BP.getSimpleValueType(), BP, Inc);
+}
+
SDValue DAGCombiner::visitLOAD(SDNode *N) {
LoadSDNode *LD = cast<LoadSDNode>(N);
SDValue Chain = LD->getChain();
@@ -8049,8 +8070,23 @@
} else {
// Indexed loads.
assert(N->getValueType(2) == MVT::Other && "Malformed indexed loads?");
- if (!N->hasAnyUseOfValue(0) && !N->hasAnyUseOfValue(1)) {
+
+ // If this load has an TargetConstant offset, then we cannot split the
+ // indexing into an add/sub directly (that TargetConstant may not be
+ // valid for a different type of node).
+ bool HasTCInc = LD->getOperand(2).getOpcode() == ISD::TargetConstant;
+
+ if (!N->hasAnyUseOfValue(0) &&
+ ((MaySplitLoadIndex && !HasTCInc) || !N->hasAnyUseOfValue(1))) {
SDValue Undef = DAG.getUNDEF(N->getValueType(0));
+ SDValue Index;
+ if (N->hasAnyUseOfValue(1) && MaySplitLoadIndex && !HasTCInc) {
+ Index = SplitIndexingFromLoad(LD);
+ // Try to fold the base pointer arithmetic into subsequent loads and
+ // stores.
+ AddUsersToWorklist(N);
+ } else
+ Index = DAG.getUNDEF(N->getValueType(1));
DEBUG(dbgs() << "\nReplacing.7 ";
N->dump(&DAG);
dbgs() << "\nWith: ";
@@ -8058,8 +8094,7 @@
dbgs() << " and 2 other values\n");
WorklistRemover DeadNodes(*this);
DAG.ReplaceAllUsesOfValueWith(SDValue(N, 0), Undef);
- DAG.ReplaceAllUsesOfValueWith(SDValue(N, 1),
- DAG.getUNDEF(N->getValueType(1)));
+ DAG.ReplaceAllUsesOfValueWith(SDValue(N, 1), Index);
DAG.ReplaceAllUsesOfValueWith(SDValue(N, 2), Chain);
deleteAndRecombine(N);
return SDValue(N, 0); // Return N so it doesn't get rechecked!