llvm-reduce: Avoid reallocating per chunk iteration (#135029)

Author: arsenm

llvm/tools/llvm-reduce/deltas/Delta.cpp

@@ -246,16 +246,20 @@ void llvm::runDeltaPass(TestRunner &Test, const DeltaPass &Pass) {
     ChunkThreadPoolPtr =
         std::make_unique<DefaultThreadPool>(hardware_concurrency(NumJobs));
 
+  SmallString<0> OriginalBC;
+  DenseSet<Chunk> UninterestingChunks;
+  UninterestingChunks.reserve(Targets);
+
   bool FoundAtLeastOneNewUninterestingChunkWithCurrentGranularity;
   do {
     FoundAtLeastOneNewUninterestingChunkWithCurrentGranularity = false;
 
-    DenseSet<Chunk> UninterestingChunks;
+    UninterestingChunks.clear();
 
     // When running with more than one thread, serialize the original bitcode
     // to OriginalBC.
-    SmallString<0> OriginalBC;
     if (NumJobs > 1) {
+      OriginalBC.clear();
       raw_svector_ostream BCOS(OriginalBC);
       Test.getProgram().writeBitcode(BCOS);
     }