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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:12.0pt;margin-left:10.5pt'>The number of lines of assembly isn't really a good proxy for the performance of some code - mostly due to inlining (one piece of code may be many more lines of assembly because it's not calling large/complicated external functions - or, even taken as a whole (including those external functions) it might still be more efficient to have longer code (because it's more specialized - ie: two calls to one generic function were inlined into two places and each one simplified/optimized a bit for those situations))<o:p></o:p></p><p class=MsoNormal>Yeah, you’re right; that was also pointed out to me by someone on one of the IRC channels I lurk on. A bit more investigation on Godbolt revealed that the difference could be to unrolling. It was certainly a surprise to me, as I expected that libstdc++ and libc++ would have relatively similar implementations that would produce relatively similar outputs. Guess something about libc++’s implementation is a bit easier for Clang to inspect? In any case, let that be a lesson to me to be a bit more careful about drawing conclusions from code size<o:p></o:p></p><p class=MsoNormal style='margin-bottom:12.0pt'><br><br>That said, libc++ does have a bunch of forced inlining that's not for performance reasons, but for linkage reasons (to ensure that certain kinds of changes/updates to libc++ don't break existing compiled code/libraries). It's a tradeoff that not every user of libc++ needs to make & there are steps being taken to make that tradeoff more configurable/optional, so far as I understand it.<o:p></o:p></p><p class=MsoNormal>Huh, that’s interesting. That isn’t what is happening here, though, right? I didn’t see anything that looks like that around the declarations/implementations of emplace() and friends<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><div><div><p class=MsoNormal>On Mon, Jul 23, 2018 at 4:43 PM via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p></div><blockquote style='border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0in;margin-bottom:5.0pt'><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Hello all,<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Just a quick question to make sure I’m not missing something.<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>This program:<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-family:Consolas'>#include <vector></span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-family:Consolas'>void f(std::vector<double>& vec, double val) {</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-family:Consolas'> vals.emplace(std::cbegin(vec), val);</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-family:Consolas'>}</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>When compiled with trunk Clang on Godbolt with <span style='font-family:Consolas'>-O3 -march=haswell -std=c++17 -stdlib=libstdc++</span>, 132 lines of assembly are produced. If <span style='font-family:Consolas'>-stdlib=libc++</span> is used, though, 638 (!) lines of assembly are produced. A few of those lines are due to <span style='font-family:Consolas'>f()</span> itself, but it appears the vast majority are due to the implementation of <span style='font-family:Consolas'>emplace()</span>. As a partial comparison, GCC trunk produced 136 lines of assembly, and seems to have partially inlined <span style='font-family:Consolas'>emplace()</span>, leaving 94 lines of assembly for <span style='font-family:Consolas'>_M_realloc_insert</span>.<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>I can sort of duplicate this on Debian sid, with libc++-dev 6.0.1-1 and <span style='font-family:Consolas'>clang++-7</span> (<span style='font-family:Consolas'>--version</span> doesn’t appear to give a revision number, unfortunately?). Using libstdc++ results in 176 lines of assembly, and libc++ results in 803 lines of assembly (counted by <span style='font-family:Consolas'>wc -l</span>).<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Is this something to be worried about? I’m still rather new to performance-related work, so I’m working from a relatively simplistic view of what could be affecting performance. A 4x difference in what could be a commonly-used function seems rather unusual to me, though.<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Thanks,<o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Alex<o:p></o:p></p></div></div><p class=MsoNormal>_______________________________________________<br>cfe-dev mailing list<br><a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a><br><a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev" target="_blank">http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev</a><o:p></o:p></p></blockquote></div></div></body></html>