[llvm] [NVPTX] fold movs into loads and stores (PR #144581)

[Princeton Ferro via llvm-commits]

================
@@ -5047,26 +5044,237 @@ PerformFADDCombineWithOperands(SDNode *N, SDValue N0, SDValue N1,
   return SDValue();
 }
 
-static SDValue PerformStoreCombineHelper(SDNode *N, std::size_t Front,
-                                         std::size_t Back) {
+/// Combine extractelts into a load by increasing the number of return values.
+static SDValue
+combineUnpackingMovIntoLoad(SDNode *N, TargetLowering::DAGCombinerInfo &DCI) {
+  // Don't run this optimization before the legalizer
+  if (!DCI.isAfterLegalizeDAG())
+    return SDValue();
+
+  EVT ElemVT = N->getValueType(0);
+  if (!Isv2x16VT(ElemVT))
+    return SDValue();
+
+  // Check whether all outputs are either used by an extractelt or are
+  // glue/chain nodes
+  if (!all_of(N->uses(), [&](SDUse &U) {
+        // Skip glue, chain nodes
+        if (U.getValueType() == MVT::Glue || U.getValueType() == MVT::Other)
+          return true;
+        if (U.getUser()->getOpcode() == ISD::EXTRACT_VECTOR_ELT) {
+          if (N->getOpcode() != ISD::LOAD)
+            return true;
+          // Since this is an ISD::LOAD, check all extractelts are used. If
+          // any are not used, we don't want to defeat another optimization that
+          // will narrow the load.
+          //
+          // For example:
+          //
+          // L: v2f16,ch = load <p>
+          // e0: f16 = extractelt L:0, 0
+          // e1: f16 = extractelt L:0, 1        <-- unused
+          // store e0
+          //
+          // Can be optimized by DAGCombiner to:
+          //
+          // L: f16,ch = load <p>
+          // store L:0
+          return !U.getUser()->use_empty();
+        }
+
+        // Otherwise, this use prevents us from splitting a value.
+        return false;
+      }))
+    return SDValue();
+
+  auto *LD = cast<MemSDNode>(N);
+  EVT MemVT = LD->getMemoryVT();
+  SDLoc DL(LD);
+
+  // the new opcode after we double the number of operands
+  NVPTXISD::NodeType Opcode;
+  SmallVector<SDValue> Operands(LD->ops());
+  unsigned OldNumValues; // non-glue, non-chain outputs
+  switch (LD->getOpcode()) {
+  case ISD::LOAD:
+    OldNumValues = 1;
+    // Any packed type is legal, so the legalizer will not have lowered
+    // ISD::LOAD -> NVPTXISD::Load (unless it's under-aligned). We have to do it
+    // here.
+    Opcode = NVPTXISD::LoadV2;
+    Operands.push_back(DCI.DAG.getIntPtrConstant(
+        cast<LoadSDNode>(LD)->getExtensionType(), DL));
+    break;
+  case NVPTXISD::LoadParamV2:
+    OldNumValues = 2;
+    Opcode = NVPTXISD::LoadParamV4;
+    break;
+  case NVPTXISD::LoadV2:
+    OldNumValues = 2;
+    Opcode = NVPTXISD::LoadV4;
+    break;
+  case NVPTXISD::LoadV4:
+    // PTX doesn't support v8 for 16-bit values
+  case NVPTXISD::LoadV8:
+    // PTX doesn't support the next doubling of outputs
+    return SDValue();
+  }
+
+  SmallVector<EVT> NewVTs(OldNumValues * 2, ElemVT.getVectorElementType());
+  // add remaining chain and glue values
+  NewVTs.append(LD->value_begin() + OldNumValues, LD->value_end());
+
+  // Create the new load
+  SDValue NewLoad =
+      DCI.DAG.getMemIntrinsicNode(Opcode, DL, DCI.DAG.getVTList(NewVTs),
+                                  Operands, MemVT, LD->getMemOperand());
+
+  // Now we use a combination of BUILD_VECTORs and a MERGE_VALUES node to keep
+  // the outputs the same. These nodes will be optimized away in later
+  // DAGCombiner iterations.
+  SmallVector<SDValue> Results;
+  for (unsigned I = 0; I < NewLoad->getNumValues();) {
+    if (I < OldNumValues * 2) {
+      Results.push_back(DCI.DAG.getBuildVector(
+          ElemVT, DL, {NewLoad.getValue(I), NewLoad.getValue(I + 1)}));
+      I += 2;
+    } else {
+      Results.push_back(NewLoad.getValue(I));
+      I += 1;
----------------
Prince781 wrote:

Is the idea to do something like `Results.append(NewLoad->value_begin() + OldNumValues *2, NewLoad->value_end())` or did you have something else in mind? This doesn't work because the iterator element (`EVT`) isn't convertible to `SDValue`. So I chose to use `push_back()` here.

https://github.com/llvm/llvm-project/pull/144581