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<div class="moz-cite-prefix">On 1/3/19 3:29 AM, Clement Courbet via llvm-dev wrote:<br>
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<div>Hi all,</div>
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<div>We'd like to suggest <b>adding a -memeq-lib-function</b> flag to allow the user to specify a `<b>memeq()</b>` function to improve string equality check performance.
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<p>Hi, Clement,</p>
<p>We really shouldn't be adding backend flags for anything at this point (except for debugging and the like). A function attribute should be fine, or global metadata if necessary. A function attribute should play better with LTO, and so that's generally the
recommended design point.</p>
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<div>Right now, when llvm encounters a <b>string equality check</b>, 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.</div>
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<div>This is sub-optimal because memcmp has to compute much more than equality.</div>
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<div>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.</div>
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<div>`memeq` can be made much more efficient than `memcmp` because equality comparison is trivially parallel while lexicographic ordering has a chain dependency.</div>
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<div>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==()`.</div>
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<div>Note that this is a <b>backend-only change</b>. 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.</div>
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<div>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.</div>
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<div>If there is interest, we could also contribute our optimized `memeq` to compiler-rt.</div>
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<p>That would be useful.<br>
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<p>Thanks again,</p>
<p>Hal</p>
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<div>A proof of concept patch for this for this RFC can be found here: <a href="https://reviews.llvm.org/D56248" moz-do-not-send="true">https://reviews.llvm.org/D56248</a></div>
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<div>Comments & suggestions welcome !</div>
<div>Thanks,</div>
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<div>Clement</div>
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<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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