[SDAG] Fix pow2 assumption when splitting vectors

[critson]

Summary

When reducing vector builds to shuffles it possible that
the DAG combiner may try to extract invalid subvectors.

This happens as the existing code assumes vectors will be power
of 2 sizes, which is already untrue, but becomes more noticable
with v6 and v7 types.
Specifically the existing code assumes that half PowerOf2Ceil of
a given vector index will fit twice into a given vector.

[RKSimon]

test case?

[critson]

In D103880#2807899, @RKSimon wrote:

test case?

I am not sure if I can trigger this without v6 types, as the current non-pow2 types v3 and v5 are not divisible by 2.

Without this change simply adding v6i32 types will cause an assertion failure while compiling: llvm/test/CodeGen/X86/pr44976.ll

[craig.topper]

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

19366

I think you can use getVectorNumElements(). These should be fixed vectors I think.

[critson]

• Replace getVectorMinNumElements with getVectorNumElements

[RKSimon]

In D103880#2809550, @critson wrote:

In D103880#2807899, @RKSimon wrote:

test case?

I am not sure if I can trigger this without v6 types, as the current non-pow2 types v3 and v5 are not divisible by 2.

Without this change simply adding v6i32 types will cause an assertion failure while compiling: llvm/test/CodeGen/X86/pr44976.ll

OK - if this per-emptively fixes an existing test regression then that's fine.

[RKSimon]

LGTM

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if (VecIn.size() == 2) {
     }
 
     NearestPow2 = PowerOf2Ceil(MaxIndex);
     if (InVT.isSimple() && NearestPow2 > 2 && MaxIndex < NearestPow2 &&
         NumElems * 2 < NearestPow2) {
       unsigned SplitSize = NearestPow2 / 2;
       EVT SplitVT = EVT::getVectorVT(*DAG.getContext(),
                                      InVT.getVectorElementType(), SplitSize);
-      if (TLI.isTypeLegal(SplitVT)) {
+      if (TLI.isTypeLegal(SplitVT) &&
+          SplitSize + SplitVT.getVectorMinNumElements() <=
+              InVT.getVectorMinNumElements()) {
         SDValue VecIn2 = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, SplitVT, Vec,
                                      DAG.getVectorIdxConstant(SplitSize, DL));
         SDValue VecIn1 = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, SplitVT, Vec,
                                      DAG.getVectorIdxConstant(0, DL));
         VecIn.pop_back();
         VecIn.push_back(VecIn1);
         VecIn.push_back(VecIn2);
         DidSplitVec = true;