http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60621
Bug ID: 60621 Summary: std::vector::emplace_back generates massively more code than push_back Product: gcc Version: 4.7.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: mutz at kde dot org Created attachment 32429 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32429&action=edit Source of the programme used to generate the mentioned numbers Compiling the attached program on Linux AMD64 with the following command lines: $ g++ -O2 -std=c++11 -o emplace-vs-push_back{.pb,.cpp} $ g++ -O2 -std=c++11 -o emplace-vs-push_back{.eb,.cpp} -DEMPLACE_BACK and stripping the resulting executables: $ strip emplace-vs-push_back.* I get the following sizes: $ size emplace-vs-push_back.* text data bss dec hex filename 5570 696 40 6306 18a2 emplace-vs-push_back.eb 4338 672 40 5050 13ba emplace-vs-push_back.pb IOW: the emplace_back version generates roughly 1K more text (code). This is surprising, since functionally, emplace_back is the same as push_back(S&&), except that it saves one move ctor and one dtor call due to in-place construction. This should result in _less_ code generated, not more. $ g++ -v Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.7/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Debian 4.7.2-5' --with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs --enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.7 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.7 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --enable-objc-gc --with-arch-32=i586 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.7.2 (Debian 4.7.2-5)