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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal>Hello all,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Just a quick question to make sure I’m not missing something.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>This program:<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><span style='font-family:Consolas'>#include <vector><o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'>void f(std::vector<double>& vec, double val) {<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'> vals.emplace(std::cbegin(vec), val);<o:p></o:p></span></p><p class=MsoNormal><span style='font-family:Consolas'>}<o:p></o:p></span></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>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><o:p> </o:p></p><p class=MsoNormal>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><o:p> </o:p></p><p class=MsoNormal>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><o:p> </o:p></p><p class=MsoNormal>Thanks,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Alex<o:p></o:p></p></div></body></html>