[DAGCombine] Fix splitting indexed loads in ForwardStoreValueToDirectLoad()

In DAGCombiner::visitLOAD() we perform some checks before breaking up an indexed
load. However, we don't do the same checking in ForwardStoreValueToDirectLoad()
which can lead to failures later during combining
(see: https://bugs.llvm.org/show_bug.cgi?id=45301).

This patch just adds the same checks to this function as well.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=45301

Differential revision: https://reviews.llvm.org/D76778

(cherry picked from commit 482141134729237072cb94248381dab96ce34374)

1 files changed, 15 insertions, 10 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 8ff04797c8d..2476fd26f25 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -886,6 +886,13 @@ static bool isAnyConstantBuildVector(SDValue V, bool NoOpaques = false) {
          ISD::isBuildVectorOfConstantFPSDNodes(V.getNode());
 }
 
+// Determine if this an indexed load with an opaque target constant index.
+static bool canSplitIdx(LoadSDNode *LD) {
+  return MaySplitLoadIndex &&
+         (LD->getOperand(2).getOpcode() != ISD::TargetConstant ||
+          !cast<ConstantSDNode>(LD->getOperand(2))->isOpaque());
+}
+
 bool DAGCombiner::reassociationCanBreakAddressingModePattern(unsigned Opc,
                                                              const SDLoc &DL,
                                                              SDValue N0,
@@ -14222,11 +14229,11 @@ SDValue DAGCombiner::ForwardStoreValueToDirectLoad(LoadSDNode *LD) {
 
   auto ReplaceLd = [&](LoadSDNode *LD, SDValue Val, SDValue Chain) -> SDValue {
     if (LD->isIndexed()) {
-      bool IsSub = (LD->getAddressingMode() == ISD::PRE_DEC ||
-                    LD->getAddressingMode() == ISD::POST_DEC);
-      unsigned Opc = IsSub ? ISD::SUB : ISD::ADD;
-      SDValue Idx = DAG.getNode(Opc, SDLoc(LD), LD->getOperand(1).getValueType(),
-                             LD->getOperand(1), LD->getOperand(2));
+      // Cannot handle opaque target constants and we must respect the user's
+      // request not to split indexes from loads.
+      if (!canSplitIdx(LD))
+        return SDValue();
+      SDValue Idx = SplitIndexingFromLoad(LD);
       SDValue Ops[] = {Val, Idx, Chain};
       return CombineTo(LD, Ops, 3);
     }
@@ -14322,14 +14329,12 @@ SDValue DAGCombiner::visitLOAD(SDNode *N) {
       // the indexing into an add/sub directly (that TargetConstant may not be
       // valid for a different type of node, and we cannot convert an opaque
       // target constant into a regular constant).
-      bool HasOTCInc = LD->getOperand(2).getOpcode() == ISD::TargetConstant &&
-                       cast<ConstantSDNode>(LD->getOperand(2))->isOpaque();
+      bool CanSplitIdx = canSplitIdx(LD);
 
-      if (!N->hasAnyUseOfValue(0) &&
-          ((MaySplitLoadIndex && !HasOTCInc) || !N->hasAnyUseOfValue(1))) {
+      if (!N->hasAnyUseOfValue(0) && (CanSplitIdx || !N->hasAnyUseOfValue(1))) {
         SDValue Undef = DAG.getUNDEF(N->getValueType(0));
         SDValue Index;
-        if (N->hasAnyUseOfValue(1) && MaySplitLoadIndex && !HasOTCInc) {
+        if (N->hasAnyUseOfValue(1) && CanSplitIdx) {
           Index = SplitIndexingFromLoad(LD);
           // Try to fold the base pointer arithmetic into subsequent loads and
           // stores.