Implement new `compareByteArrays#` primop

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

1 files changed, 1 insertions, 0 deletions

diff --git a/includes/stg/MiscClosures.h b/includes/stg/MiscClosures.h
index 76cfbd6c8c..66e26545f8 100644
--- a/includes/stg/MiscClosures.h
+++ b/includes/stg/MiscClosures.h
@@ -351,6 +351,7 @@ RTS_FUN_DECL(stg_casArrayzh);
 RTS_FUN_DECL(stg_newByteArrayzh);
 RTS_FUN_DECL(stg_newPinnedByteArrayzh);
 RTS_FUN_DECL(stg_newAlignedPinnedByteArrayzh);
+RTS_FUN_DECL(stg_compareByteArrayszh);
 RTS_FUN_DECL(stg_isByteArrayPinnedzh);
 RTS_FUN_DECL(stg_isMutableByteArrayPinnedzh);
 RTS_FUN_DECL(stg_shrinkMutableByteArrayzh);