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* [Bug libstdc++/60621] New: std::vector::emplace_back generates massively more code than push_back
@ 2014-03-22 23:32 mutz at kde dot org
2014-03-23 8:24 ` [Bug libstdc++/60621] " glisse at gcc dot gnu.org
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: mutz at kde dot org @ 2014-03-22 23:32 UTC (permalink / raw)
To: gcc-bugs
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)
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug libstdc++/60621] std::vector::emplace_back generates massively more code than push_back
2014-03-22 23:32 [Bug libstdc++/60621] New: std::vector::emplace_back generates massively more code than push_back mutz at kde dot org
@ 2014-03-23 8:24 ` glisse at gcc dot gnu.org
2014-03-23 11:26 ` marc at kdab dot com
2015-02-11 11:13 ` marc at kdab dot com
2 siblings, 0 replies; 4+ messages in thread
From: glisse at gcc dot gnu.org @ 2014-03-23 8:24 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60621
--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
Some things that help:
-fabi-version=0
-fwhole-program (so it knows emplace_back won't be used anywhere else, and it
can inline it and remove the unneeded paths)
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug libstdc++/60621] std::vector::emplace_back generates massively more code than push_back
2014-03-22 23:32 [Bug libstdc++/60621] New: std::vector::emplace_back generates massively more code than push_back mutz at kde dot org
2014-03-23 8:24 ` [Bug libstdc++/60621] " glisse at gcc dot gnu.org
@ 2014-03-23 11:26 ` marc at kdab dot com
2015-02-11 11:13 ` marc at kdab dot com
2 siblings, 0 replies; 4+ messages in thread
From: marc at kdab dot com @ 2014-03-23 11:26 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60621
--- Comment #2 from marc at kdab dot com ---
Yes, that helps a bit, but emplace_back still generates larger code than the
corresponding rvalue-push_back. Considering that the latter also needs to
generate the implicitly defined move ctor for S, this is still somewhat
surprising and runs counter to the motivation to have emplace_back in the first
place.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug libstdc++/60621] std::vector::emplace_back generates massively more code than push_back
2014-03-22 23:32 [Bug libstdc++/60621] New: std::vector::emplace_back generates massively more code than push_back mutz at kde dot org
2014-03-23 8:24 ` [Bug libstdc++/60621] " glisse at gcc dot gnu.org
2014-03-23 11:26 ` marc at kdab dot com
@ 2015-02-11 11:13 ` marc at kdab dot com
2 siblings, 0 replies; 4+ messages in thread
From: marc at kdab dot com @ 2015-02-11 11:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60621
marc at kdab dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
Attachment #32429|0 |1
is obsolete| |
--- Comment #4 from marc at kdab dot com ---
Created attachment 34723
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34723&action=edit
New version of the test programme.
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2015-02-11 11:13 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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2014-03-22 23:32 [Bug libstdc++/60621] New: std::vector::emplace_back generates massively more code than push_back mutz at kde dot org
2014-03-23 8:24 ` [Bug libstdc++/60621] " glisse at gcc dot gnu.org
2014-03-23 11:26 ` marc at kdab dot com
2015-02-11 11:13 ` marc at kdab dot com
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