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Herbert Valerio Riedel authored
The new primop compareByteArrays# :: ByteArray# -> Int# {- offset -} -> ByteArray# -> Int# {- offset -} -> Int# {- length -} -> Int# allows to compare the subrange of the first `ByteArray#` to the (same-length) subrange of the second `ByteArray#` and returns a value less than, equal to, or greater than zero if the range is found, respectively, to be byte-wise lexicographically less than, to match, or be greater than the second range. Under the hood, the new primop is implemented in terms of the standard ISO C `memcmp(3)` function. It is currently an out-of-line primop but work is underway to optimise this into an inline primop for a future follow-up Differential (see D4091). This primop has applications in packages like `text`, `text-short`, `bytestring`, `text-containers`, `primitive`, etc. which currently have to incur the overhead of an ordinary FFI call to directly or indirectly invoke `memcmp(3)` as well has having to deal with some `unsafePerformIO`-variant. While at it, this also improves the documentation for the existing `copyByteArray#` primitive which has a non-trivial type-signature that significantly benefits from a more explicit description of its arguments. Reviewed By: bgamari Differential Revision: https://phabricator.haskell.org/D4090
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