[LegalizeVectorTypes] Improve SplitVecRes_INSERT_SUBVECTOR to handle subvector being in the high half of the split or not at element 0 of the low half.

[craig.topper]

Summary

This function isn't exercised in lit tests today today according to
the code coverage report. But will be after the tests in D97543 and
D97559.

Posting this patch to help a crash that Fraser hit.

[frasercrmck]

Hah, I was going to make the same change on Monday.

LGTM but the other reviewers know the code here better, and I'll wait for the tests before accepting.

[RKSimon]

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

1265

typo /

1269

Should we be using LoVT.getVectorNumElements() to compute the boundary?

[craig.topper]

Address review comments.

[RKSimon]

Are tests available now?

[craig.topper]

Add test case that previously crashed

[craig.topper]

Run update_llc_test_checks.py which updates some other tests that were falling back to memory before.

[RKSimon]

LGTM @frasercrmck Are you happy with this?

[frasercrmck]

LGTM other than my final nit.

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

1271

While we're changing these lines, I'm not sure this style of parentheses around subexpressions is common?

[uabelho]

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

1271

Since IdxVal is unsigned the condition

IdxVal >= 0

is always true. Gcc warns about this.

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

 //===------- LegalizeVectorTypes.cpp - Legalization of vector types -------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
void DAGTypeLegalizer::SplitVecRes_INSERT_SUBVECTOR(SDNode *N, SDValue &Lo,
   SDValue Idx = N->getOperand(2);
   SDLoc dl(N);
   GetSplitVector(Vec, Lo, Hi);
 
   EVT VecVT = Vec.getValueType();
   unsigned VecElems = VecVT.getVectorNumElements();
   unsigned SubElems = SubVec.getValueType().getVectorNumElements();
 
-  // If we know the index is 0, and we know the subvector doesn't cross the
-  // boundary between the halves, we can avoid spilling the vector, and insert
-  // into the lower half of the split vector directly.
-  // TODO: The IdxVal == 0 constraint is artificial, we could do this whenever
-  // there is no boundary crossing. But those cases don't seem to get hit in
-  // practice.
+  // If we know the index is in the first half, and we know the subvector
+  // doesn't cross the/ boundary between the halves, we can avoid spilling the
+  // vector, and insert into the lower half of the split vector directly.
+  // Similarly if the subvector is fully in the high half.
   unsigned IdxVal = cast<ConstantSDNode>(Idx)->getZExtValue();
-  if ((IdxVal == 0) && (IdxVal + SubElems <= VecElems / 2)) {
+  if (IdxVal >= 0 && (IdxVal + SubElems <= VecElems / 2)) {
     EVT LoVT, HiVT;
     std::tie(LoVT, HiVT) = DAG.GetSplitDestVTs(N->getValueType(0));
     Lo = DAG.getNode(ISD::INSERT_SUBVECTOR, dl, LoVT, Lo, SubVec, Idx);
     return;
   }
+  if ((IdxVal >= VecElems / 2) && (IdxVal + SubElems <= VecElems)) {
+    EVT LoVT, HiVT;
+    std::tie(LoVT, HiVT) = DAG.GetSplitDestVTs(N->getValueType(0));
+    Hi = DAG.getNode(ISD::INSERT_SUBVECTOR, dl, HiVT, Hi, SubVec,
+                     DAG.getVectorIdxConstant(IdxVal - (VecElems / 2), dl));
+    return;
+  }
 
   // Spill the vector to the stack.
   // In cases where the vector is illegal it will be broken down into parts
   // and stored in parts - we should use the alignment for the smallest part.
   Align SmallestAlign = DAG.getReducedAlign(VecVT, /*UseABI=*/false);
   SDValue StackPtr =
       DAG.CreateStackTemporary(VecVT.getStoreSize(), SmallestAlign);
   auto &MF = DAG.getMachineFunction();