[LSV] Use a set rather than an ArraySlice at the end of getVectorizablePrefix. NFC

[jlebar]

Summary

This avoids a small O(n^2) loop.

[arsenm]

LGTM

llvm/lib/Transforms/Vectorize/LoadStoreVectorizer.cpp

520

16 seems high. 8?

[jlebar]

Thank you for the review, Matt. Will submit with s/16/8/

llvm/lib/Transforms/Vectorize/LoadStoreVectorizer.cpp

for (unsigned E = ChainInstrs.size(); ChainInstrIdx < E; ++ChainInstrIdx) {
     if (AliasFound)
       break;
   }
 
   // Find the largest prefix of Chain whose elements are all in
   // ChainInstrs[0, ChainInstrIdx).  This is the largest vectorizable prefix of
   // Chain.  (Recall that Chain is in address order, but ChainInstrs is in BB
   // order.)
-  auto VectorizableChainInstrs =
-      makeArrayRef(ChainInstrs.data(), ChainInstrIdx);
-  unsigned ChainIdx, ChainLen;
-  for (ChainIdx = 0, ChainLen = Chain.size(); ChainIdx < ChainLen; ++ChainIdx) {
-    Instruction *I = Chain[ChainIdx];
-    if (!is_contained(VectorizableChainInstrs, I))
+  SmallPtrSet<Instruction *, 16> VectorizableChainInstrs(
+      ChainInstrs.begin(), ChainInstrs.begin() + ChainInstrIdx);
+  unsigned ChainIdx = 0;
+  for (unsigned ChainLen = Chain.size(); ChainIdx < ChainLen; ++ChainIdx) {
+    if (!VectorizableChainInstrs.count(Chain[ChainIdx]))
       break;
   }
   return Chain.slice(0, ChainIdx);
 }
 
 std::pair<InstrListMap, InstrListMap>
 Vectorizer::collectInstructions(BasicBlock *BB) {
   InstrListMap LoadRefs;