[llvm-dev] [RFC] Adding a -memeq-lib-function flag to allow the user to specify a memeq function.

[Clement Courbet via llvm-dev]

Hi all,

We'd like to suggest *adding a -memeq-lib-function* flag to allow the user
to specify a `*memeq()*` function to improve string equality check
performance.

Right now, when llvm encounters a *string equality check*, e.g. `if
(memcmp(a, b, s) == 0)`, it tries  to expand to an equality comparison if
`s` is a small compile-time constant, and falls back on calling `memcmp()`
else.

This is sub-optimal because memcmp has to compute much more than equality.

We propose adding a way for the user to specify a `memeq` library function
(e.g. `-memeq-lib-function=user_memeq`) which will be called instead of
`memcmp()` when the result of the memcmp call is only used for equality
comparison.

`memeq` can be made much more efficient than `memcmp` because equality
comparison is trivially parallel while lexicographic ordering has a chain
dependency.

We measured an very large improvement of this approach on our internal
codebase. A significant portion of this improvement comes from the stl,
typically `std::string::operator==()`.

Note that this is a *backend-only change*. Because the c family of
languages do not have a standard `memeq()` (posix used to have `bcmp()` but
it was removed in 2001), c/c++ code cannot communicate the equality
comparison semantics to the compiler.

We did not add an RTLIB entry for memeq because the user environment is not
guaranteed to contain a `memeq()` function as the libc has no such concept.

If there is interest, we could also contribute our optimized `memeq` to
compiler-rt.

A proof of concept patch for this for this RFC can be found here:
https://reviews.llvm.org/D56248

Comments & suggestions welcome !
Thanks,

Clement
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190103/9ba61c16/attachment.html>