Interpreter: Don't change NaN bits when multiplying by 1 #3096
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tlively
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Sep 8, 2020
This was referenced Sep 17, 2020
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kripken
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Sep 30, 2020
Similar to clang and gcc, --fast-math makes us ignore corner cases of floating-point math like NaN changes and (not done yet) lack of associativity and so forth. In the future we may want to have separate fast math flags for each specific thing, like gcc and clang do. This undoes some changes (#2958 and #3096) where we assumed it was ok to not change NaN bits, but @binji corrected us. We can only do such things in fast math mode. This puts those optimizations behind that flag, adds tests for it, and restores the interpreter to the simpler code from before with no special cases.
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Similar to #2958, but for multiplication. I thought this was limited
only to division (it doesn't happen for addition, for example), but the
fuzzer found that it does indeed happen for multiplication as well.
Overall these are kind of workarounds for the interpreter doing
normal f32/f64 multiplications using the host CPU, so we pick up
any oddness of its NaN behavior. Using soft float might be safer
(but much slower).