[llvm] [SLP][NFCI] Address issues seen in downstream Coverity scan. (PR #93757)

[Tyler Lanphear via llvm-commits]

https://github.com/tylanphear created https://github.com/llvm/llvm-project/pull/93757

- 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)

>From 5c14ed1f229ef5a6827a64107b0119c9d1a07b2a Mon Sep 17 00:00:00 2001
From: Tyler Lanphear <tyler.lanphear at intel.com>
Date: Wed, 29 May 2024 17:34:41 -0700
Subject: [PATCH] [SLP][NFCI] Address issues seen in downstream Coverity scan.

- 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)
---
 llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

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(