[SelectionDAG] Use UnknownSize for VP memory ops

[frasercrmck]

Summary

In the style of D113888, this patch updates the various VP memory
operations (load, store, gather, scatter) to use UnknownSize. This is
for the same reason as for masked loads and stores: the number of
elements accessed is not generally known at compile time.

This is somewhat pessimistic in the sense that we may still find
un-canonicalized intrinsics featuring both an all-true mask and an EVL
equal to the vector size. Arguably those should be canonicalized before
the SelectionDAG, so those have been left for future work.

[simoll]

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

7332

Not intimately familiar with MemoryLocations, but conceptually, for fixed size, we should be able to use an imprecise MemoryLocation here, quoting from the MemLoc header:

// precisely N bytes. An imprecise value is formed as the union of two or more
// precise values, and can conservatively represent all of the values unioned
// into it. Importantly, imprecise values are an *upper-bound* on the size of a
// MemoryLocation.

[craig.topper]

LGTM

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

7332

We're re-using MemoryLocation::UnknownSize, but I don't think MachineMemOperand uses MemoryLocation or LocationSize in its underlying implementation.

[dmgreen]

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

7332

Yeah, it's really stored as a LLT, where the size is passed along to AA. MemoryLocation::UnknownSize creates an invalid LLT.

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

void SelectionDAGBuilder::visitVPLoadGather(const VPIntrinsic &VPIntrin, EVT VT,
   MaybeAlign Alignment = DAG.getEVTAlign(VT);
   AAMDNodes AAInfo = VPIntrin.getAAMetadata();
   const MDNode *Ranges = VPIntrin.getMetadata(LLVMContext::MD_range);
   SDValue LD;
   bool AddToChain = true;
   if (!isGather) {
     // Do not serialize variable-length loads of constant memory with
     // anything.
-    MemoryLocation ML;
-    if (VT.isScalableVector())
-      ML = MemoryLocation::getAfter(PtrOperand);
-    else
-      ML = MemoryLocation(
-          PtrOperand,
-          LocationSize::precise(
-              DAG.getDataLayout().getTypeStoreSize(VPIntrin.getType())),
-          AAInfo);
+    MemoryLocation ML = MemoryLocation::getAfter(PtrOperand, AAInfo);
     AddToChain = !AA || !AA->pointsToConstantMemory(ML);
     SDValue InChain = AddToChain ? DAG.getRoot() : DAG.getEntryNode();
     MachineMemOperand *MMO = DAG.getMachineFunction().getMachineMemOperand(
         MachinePointerInfo(PtrOperand), MachineMemOperand::MOLoad,
-        VT.getStoreSize().getKnownMinSize(), *Alignment, AAInfo, Ranges);
+        MemoryLocation::UnknownSize, *Alignment, AAInfo, Ranges);
     LD = DAG.getLoadVP(VT, DL, InChain, OpValues[0], OpValues[1], OpValues[2],
                        MMO, false /*IsExpanding */);
   } else {
     unsigned AS =
         PtrOperand->getType()->getScalarType()->getPointerAddressSpace();
     MachineMemOperand *MMO = DAG.getMachineFunction().getMachineMemOperand(
         MachinePointerInfo(AS), MachineMemOperand::MOLoad,
         MemoryLocation::UnknownSize, *Alignment, AAInfo, Ranges);
void SelectionDAGBuilder::visitVPStoreScatter(const VPIntrinsic &VPIntrin,
   Value *PtrOperand = VPIntrin.getArgOperand(1);
   EVT VT = OpValues[0].getValueType();
   MaybeAlign Alignment = DAG.getEVTAlign(VT);
   AAMDNodes AAInfo = VPIntrin.getAAMetadata();
   SDValue ST;
   if (!isScatter) {
     MachineMemOperand *MMO = DAG.getMachineFunction().getMachineMemOperand(
         MachinePointerInfo(PtrOperand), MachineMemOperand::MOStore,
-        VT.getStoreSize().getKnownMinSize(), *Alignment, AAInfo);
+        MemoryLocation::UnknownSize, *Alignment, AAInfo);
     ST =
         DAG.getStoreVP(getMemoryRoot(), DL, OpValues[0], OpValues[1],
                        OpValues[2], OpValues[3], MMO, false /* IsTruncating */);
   } else {
     unsigned AS =
         PtrOperand->getType()->getScalarType()->getPointerAddressSpace();
     MachineMemOperand *MMO = DAG.getMachineFunction().getMachineMemOperand(
         MachinePointerInfo(AS), MachineMemOperand::MOStore,