[libFuzzer] Use ChooseUnitIdxToMutate to pick item for crossover.

[kcc]

don't forget to CC llvm-commits

lib/Fuzzer/FuzzerMutate.h

91

No.
Instead, please make ChooseUnitIdxToMutate const.

[kcc]

LGTM
Ah, I see.
piecewise_constant_distribution::operator() is not const.

[vitalybuka]

lib/Fuzzer/FuzzerMutate.h

91

ChooseUnitIdxToMutate needs mutable CorpusDistribution

lib/Fuzzer/FuzzerMutate.h

size_t CrossOver(const uint8_t *Data1, size_t Size1, const uint8_t *Data2,
                    size_t Size2, uint8_t *Out, size_t MaxOutSize);
 
   void AddWordToManualDictionary(const Word &W);
 
   void AddWordToAutoDictionary(DictionaryEntry DE);
   void ClearAutoDictionary();
   void PrintRecommendedDictionary();
 
-  void SetCorpus(const InputCorpus *Corpus) { this->Corpus = Corpus; }
+  void SetCorpus(InputCorpus *Corpus) { this->Corpus = Corpus; }
 
   Random &GetRand() { return Rand; }
 
 private:
 
   struct Mutator {
     size_t (MutationDispatcher::*Fn)(uint8_t *Data, size_t Size, size_t Max);
     const char *Name;
private:
 
   std::vector<Mutator> CurrentMutatorSequence;
   std::vector<DictionaryEntry *> CurrentDictionaryEntrySequence;
 
   static const size_t kCmpDictionaryEntriesDequeSize = 16;
   DictionaryEntry CmpDictionaryEntriesDeque[kCmpDictionaryEntriesDequeSize];
   size_t CmpDictionaryEntriesDequeIdx = 0;
 
-  const InputCorpus *Corpus = nullptr;
+  InputCorpus *Corpus = nullptr;
   std::vector<uint8_t> MutateInPlaceHere;
 
   std::vector<Mutator> Mutators;
   std::vector<Mutator> DefaultMutators;
 };
 
 }  // namespace fuzzer
 
 #endif  // LLVM_FUZZER_MUTATE_H

lib/Fuzzer/FuzzerMutate.cpp

size_t MutationDispatcher::Mutate_Custom(uint8_t *Data, size_t Size,
                                          size_t MaxSize) {
   return EF->LLVMFuzzerCustomMutator(Data, Size, MaxSize, Rand.Rand());
 }
 
 size_t MutationDispatcher::Mutate_CustomCrossOver(uint8_t *Data, size_t Size,
                                                   size_t MaxSize) {
   if (!Corpus || Corpus->size() < 2 || Size == 0)
     return 0;
-  size_t Idx = Rand(Corpus->size());
+  size_t Idx = Corpus->ChooseUnitIdxToMutate(Rand);
   const Unit &Other = (*Corpus)[Idx];
   if (Other.empty())
     return 0;
   MutateInPlaceHere.resize(MaxSize);
   auto &U = MutateInPlaceHere;
   size_t NewSize = EF->LLVMFuzzerCustomCrossOver(
       Data, Size, Other.data(), Other.size(), U.data(), U.size(), Rand.Rand());
   if (!NewSize)