[SLP][NFCI] Address issues seen in downstream Coverity scan.

[tylanphear]

• Prevent null dereference: if the Mask given to
  ShuffleInstructionBuilder::adjustExtracts() is empty or all-poison,
  then VecBase will be nullptr and the call to
  castToScalarTyElem(VecBase) will dereference it. Add an assert
  to guard against this.

• Prevent use of uninitialized scalar: in the unlikely event that
  CandidateVFs is empty, then AnyProfitableGraph will be
  uninitialized in if condition following the loop. (This seems like a
  false-positive, but I submitted this change anyways as initializing
  bools costs nothing and is generally good practice)

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Tyler Lanphear (tylanphear)

Changes

• Prevent null dereference: if the Mask given to
  ShuffleInstructionBuilder::adjustExtracts() is empty or all-poison,
  then VecBase will be nullptr and the call to
  castToScalarTyElem(VecBase) will dereference it. Add an explicit
  null check.

• Prevent use of uninitialized scalar: in the unlikely event that
  CandidateVFs is empty, then AnyProfitableGraph will be
  uninitialized in if condition following the loop. (This seems like a
  false-positive, but I submitted this change anyways as initializing
  bools costs nothing and is generally good practice)

---

Full diff: https://github.com/llvm/llvm-project/pull/93757.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp (+2-3)

diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index 2e0a39c4b4fdc..e8db98a991845 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -11700,8 +11700,7 @@ class BoUpSLP::ShuffleInstructionBuilder final : public BaseShuffleAnalysis {
       R.eraseInstruction(EI);
     }
     if (NumParts == 1 || UniqueBases.size() == 1) {
-      VecBase = castToScalarTyElem(VecBase);
-      return VecBase;
+      return VecBase ? castToScalarTyElem(VecBase) : nullptr;
     }
     UseVecBaseAsInput = true;
     auto TransformToIdentity = [](MutableArrayRef<int> Mask) {
@@ -15837,7 +15836,7 @@ bool SLPVectorizerPass::vectorizeStores(
       while (true) {
         ++Repeat;
         bool RepeatChanged = false;
-        bool AnyProfitableGraph;
+        bool AnyProfitableGraph = false;
         for (unsigned Size : CandidateVFs) {
           AnyProfitableGraph = false;
           unsigned StartIdx = std::distance(

[alexey-bataev]

LG