Fuzzer: Improve handling of small inputs and their debugging#7832
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tlively
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It would be useful to document the env var somewhere, too.
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Good idea, added to the wiki, https://github.com/WebAssembly/binaryen/wiki/Fuzzing#advanced-topics |
kripken
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Apr 28, 2026
This was added to simplify our output when the random data is small - it makes us pick simpler options, which helps a little there - but the downsides are large, so revert this part. Specifically, we consume all the random input to generate functions. We then do a small amount of random usage at the end, things like export mutation. To handle that, we keep returning random values even after the random input is consumed (using some xor-ing to try to keep things random). But if we just return 0 at that point, we are missing out on a lot of variety. This was added in #7832. All the rest of that PR makes sense - we can check for the end of the random data and do simpler things - but we should not do the simplest possible thing in the generic `upTo` method which would simplify everything, even things we don't want to.
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Add a new env var to control the fuzzer input size,
When set, we will truncate the random bytes we receive as
input in translate-to-fuzz. This makes it simple to bisect on the
input, for example.
Also make the fuzzer notice that the input has run out in more
cases, and stop emitting things. This makes small inputs to
the fuzzer smaller in output.