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registers `al`, `cl`, `dl`, and `bl` could have two different
representations - with `rex.w` and without. these two forms of `RegSpec`
would not compare equal, nor has the same, so for code relying on
`RegSpec` to faithfully represent a 1-1 mapping to x86 registers, these
synonyms would introduce bugs in register analysis.
for example, in `yaxpeax-core`, this would result in instructions
writing to `rex.w al` not being visible as definitions for a future
read of `!rex.w al`.
fix this in `x86_64` code, add new test cases about the confusion,
adjust register names to make this situation more clearly a bug, and
introduce two new fuzz targets that would have helped spot this error.
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