<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Mon, May 1, 2017 at 10:15 AM, Craig Topper via cfe-dev <span dir="ltr"><<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Adding cfe-dev. Removing llvm-dev. Frontend and libary discussions are better represented on that list.</div><div class="gmail_extra"><br clear="all"><div><div class="m_-4321941588692057028gmail_signature" data-smartmail="gmail_signature">~Craig</div></div>
<br><div class="gmail_quote"><div><div class="h5">On Sat, Apr 29, 2017 at 7:13 AM, Daniel Cooke via llvm-dev <span dir="ltr"><<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>></span> wrote:<br></div></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div class="h5"><div style="word-wrap:break-word">Hi,<div><br></div><div>I recently posted a question on StackOverflow regarding the performance of std::find and std::count when provided with char* input: <a href="http://stackoverflow.com/questions/43483378/why-arent-stdcount-and-stdfind-optimised-to-use-memchr" target="_blank">http://stackoverflow.co<wbr>m/questions/43483378/why-arent<wbr>-stdcount-and-stdfind-optimise<wbr>d-to-use-memchr</a>.</div><div><br></div><div>I would propose adding overloads of these functions for char* and const char* inputs that delegate to memchr, something like:</div><div><br></div><div><div>inline const char* find(const char* first, const char* const last, const char value)</div><div>{</div><div><span class="m_-4321941588692057028m_-4336508853031706628Apple-tab-span" style="white-space:pre-wrap"> </span>const auto result = std::memchr(first, value, last - first);</div><div><span class="m_-4321941588692057028m_-4336508853031706628Apple-tab-span" style="white-space:pre-wrap"> </span>return result != nullptr ? static_cast<const char*>(result) : last;</div><div>}</div></div><div><br></div><div><div>inline typename std::iterator_traits<const char*>::difference_type</div><div>count(const char* first, const char* const last, const char value)</div><div>{</div><div><span class="m_-4321941588692057028m_-4336508853031706628Apple-tab-span" style="white-space:pre-wrap"> </span>typename std::iterator_traits<const char*>::difference_type result {0};</div><div><span class="m_-4321941588692057028m_-4336508853031706628Apple-tab-span" style="white-space:pre-wrap"> </span>while (first && first != last) {</div><div><span class="m_-4321941588692057028m_-4336508853031706628Apple-tab-span" style="white-space:pre-wrap"> </span>if ((first = static_cast<const char*>(std::memchr(first, value, last - first)))) {</div><div><span class="m_-4321941588692057028m_-4336508853031706628Apple-tab-span" style="white-space:pre-wrap"> </span>++result;</div><div><span class="m_-4321941588692057028m_-4336508853031706628Apple-tab-span" style="white-space:pre-wrap"> </span>++first;</div><div><span class="m_-4321941588692057028m_-4336508853031706628Apple-tab-span" style="white-space:pre-wrap"> </span>}</div><div><span class="m_-4321941588692057028m_-4336508853031706628Apple-tab-span" style="white-space:pre-wrap"> </span>}</div><div><span class="m_-4321941588692057028m_-4336508853031706628Apple-tab-span" style="white-space:pre-wrap"> </span>return result;</div><div>}</div></div><div><br></div><div>I’ve never contributed to LLVM, so I’m not sure how to proceed, if this is a change that is likely to be accepted?</div></div></div></div></blockquote></div></div></blockquote><div><br></div><div>If the change *measurably* improves performance while maintaining correctness then yes, it is very likely to be accepted.</div><div> </div><div>The first step would be to write a benchmark that demonstrates the performance difference by adding a test in `libcxx/benchmarks/algorithms.bench.cpp`.</div><div>Personally I would write two different benchmarks, one that uses `std::find` and another that uses `std::memchr` on the same set of input.</div><div><br></div><div>/Eric</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div class="h5"><div style="word-wrap:break-word"><div><br></div><div>Best,</div><div>Dan</div><div><br></div></div><br></div></div>______________________________<wbr>_________________<br>
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