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[LibFuzzer] Improve comments on CounterToFeature() function.

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This is based on discussion in https://reviews.llvm.org/D40376 .

The comments try to explain the reason for the current implementation
and note that it might change in the future, so clients should not
rely on this particular implementation.

Differential Revision: https://reviews.llvm.org/D40565

llvm-svn: 319190
This commit is contained in:
Dan Liew 2017-11-28 17:41:58 +00:00
parent 4e9e9bf0cf
commit 37516b527b
1 changed files with 12 additions and 1 deletions
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13
compiler-rt/lib/fuzzer/FuzzerTracePC.h
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@ -196,9 +196,20 @@ void ForEachNonZeroByte(const uint8_t *Begin, const uint8_t *End,
Handle8bitCounter(FirstFeature, P - Begin, V);
}
// Given a non-zero Counters returns a number in [0,7].
// Given a non-zero Counter returns a number in the range [0,7].
template<class T>
unsigned CounterToFeature(T Counter) {
// Returns a feature number by placing Counters into buckets as illustrated
// below.
//
// Counter bucket: [1] [2] [3] [4-7] [8-15] [16-31] [32-127] [128+]
// Feature number: 0 1 2 3 4 5 6 7
//
// This is a heuristic taken from AFL (see
// http://lcamtuf.coredump.cx/afl/technical_details.txt).
//
// This implementation may change in the future so clients should
// not rely on it.
assert(Counter);
unsigned Bit = 0;
/**/ if (Counter >= 128) Bit = 7;