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POLambda slower with g++ 4.7 - faster with g++ 4.6
 

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  1. POLambda slower with g++ 4.7 - faster with g++ 4.6
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    <p>I was using g++ 4.7 because it's one of the latest release of g++ and it's the first one that adds a real support for <code>c++11</code>.</p> <p>For testing purpose I'm considering code taken <a href="https://stackoverflow.com/questions/8611756/why-c-lambda-is-slower-than-ordinary-function-when-called-multiple-times">from here</a>.</p> <p><a href="http://pastebin.com/33pR9nSf" rel="nofollow noreferrer">You can find the complete source code here.</a></p> <p>I name this source lambda.cpp and i compile it with:</p> <pre><code>g++-4.6 -std=c++0x lambda.cpp -o lambda46 g++-4.7 -std=c++11 lambda.cpp -o lambda47 </code></pre> <p>the lambda47 executable is about an half second slower than lambda46 when it comes to lambda performances, the surprise it's that the iterator part is usually faster than lambda46.</p> <p>I have also tried to use</p> <pre><code>g++-4.7 -std=c++0x lambda.cpp -o lambda47-0x </code></pre> <p>but basically g++-4.6 always generates faster code than g++-4.7.</p> <p>This is a common behaviour or it's a bug ?</p> <p>There is a compiler that will perform better with C++11 ?</p> <hr> <p>g++-4.7 it has been compiled with</p> <blockquote> <p>Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.7.2-4precise1' --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 --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu</p> </blockquote> <p>g++-4.6 with</p> <blockquote> <p>Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.3-1ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.6 --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++/4.6 --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 --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu</p> </blockquote>
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