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<p>Thanks, I didn't that __builtin_assume existed in Clang.</p>
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<p>So, IIRC to proceed with a patch, I should define a _LIBCPP_ASSUME macro wrapping it. Also, if I'm not mistaken, libc++ is sometimes used with g++ and MSVC, so the macro would either use __builtin_assume, __builtin_unreachable or __assume depending on the
target. I guess that <algorithm> isn't the right place to put such a macro, but I don't know which is the right place.</p>
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<p>Would you provide some guidance?</p>
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<p>Thanks in advance :)</p>
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<p>Morwenn<br>
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<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" color="#000000" face="Calibri, sans-serif"><b>De :</b> Hal Finkel <hfinkel@anl.gov><br>
<b>Envoyé :</b> lundi 31 octobre 2016 18:56<br>
<b>À :</b> Morwenn Ed<br>
<b>Cc :</b> cfe-dev@lists.llvm.org<br>
<b>Objet :</b> Re: [cfe-dev] Potential optimization for std::inplace_merge</font>
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<b>From: </b>"Morwenn Ed via cfe-dev" <cfe-dev@lists.llvm.org><br>
<b>To: </b>cfe-dev@lists.llvm.org<br>
<b>Sent: </b>Thursday, October 27, 2016 2:12:39 PM<br>
<b>Subject: </b>[cfe-dev] Potential optimization for std::inplace_merge<br>
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[...]
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<p>However, at this point, the new "optimization" made the code dramatically slower than it was before when sorting a vector of one million integers with a mergesort built on the top of
<span style="font-family:"Courier New",monospace">std::inplace_merge</span>. It kind of baffled me until I finally noticed that the second loop had some information that the first didn't have: the second loop knows that
<span style="font-family:"Courier New",monospace">first1 != last1</span> and <span style="font-family:"Courier New",monospace">
first2 != last2</span> while it runs. However, it is apparently possible to give that information to the first loop too. I defined the following macro:</p>
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<p><span id="DWT3344" style="font-family:"Courier New",monospace"> #define ASSUME(cond) do { if (!(cond)) __builtin_unreachable(); } while(0)</span></p>
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FYI, for Clang, you'll need to use __builtin_assume(cond) (http://clang.llvm.org/docs/LanguageExtensions.html#builtin-functions).<br>
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-Hal<br>
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-- <br>
<div><span name="x"></span>Hal Finkel<br>
Lead, Compiler Technology and Programming Languages<br>
Leadership Computing Facility<br>
Argonne National Laboratory<span name="x"></span><br>
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