[PATCH] D49621: [libFuzzer] Initial implementation of weighted mutation leveraging during runtime.

[Jonathan Metzman via Phabricator via llvm-commits]

metzman added inline comments.


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Comment at: lib/fuzzer/FuzzerMutate.cpp:23
+const double kDefaultMutationWeight = 1;
+const double kDefaultMutationStat = 1 / (100 * 1000);
 
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Dor1s wrote:
> kodewilliams wrote:
> > Dor1s wrote:
> > > metzman wrote:
> > > > Please add a comment to explain the significance of `100` and `1000` (frankly i don't know what the purpose is of either since we don't actually round anything).
> > > +1, what is it for?
> > It is just there to represent a usefulness ratio that is near to but not entirely useless. So that it still gets weight instead of calculating to 0.
> That doesn't seem right to me. Let's say we have a tough target -- where it's hard to reach any new coverage. In fact, we have a lot of them, when we use a full corpus.
> 
> So, after running for a while, it finally finds a couple useful mutations, all of them get weights, say, 10^(-6) (again, totally possible), while all "useless" mutations get the default weight of 10^(-5). That would mean, "useless" mutations would be chosen much more often than "useful" ones.
> 
> IMO, the better approach would be something like what we've discussed in the past:
> 
> 1) make random decision whether we use weighted mutations or default selection, 80% vs 20% should be fine. Once start testing, can change to 90 vs 10 or any other proportion
> 2) if weighted mutations was chosen, call WeightedIndex(); otherwise, use default case
> 
> That approach would be universal as it doesn't use any magic numbers which correspond to a particular target, as your 100*1000 can behave very differently with targets having different speed.
I think it will be harder to determine if this technique is useful with the 80/20 strategy.
If the concern is that the weight is too high, why don't we use the smallest positive double as the default?


Repository:
  rCRT Compiler Runtime

https://reviews.llvm.org/D49621