> On 22 Oct 2015, at 14:47, Mohamed Boussaa <mohamedboussaa3@xxxxxxxxx> wrote: > > The default optimization level for compiling C programs using GCC is > -O0. which turns off all optimizations according to GCC documentation. > for example: > > gcc -O0 test.c > > However, to check if -O0 is really turning off all optimizations. I > executed this command: > > gcc -Q -O0 --help=optimizers RTFM https://gcc.gnu.org/onlinedocs/gcc/Overall-Options.html —help=optimizers: ‘optimizers’ Display all of the optimization options supported by the compiler It does not show what is activate it shows what is supported. > > And here, I was a bit surprised. I got around 50 options enabled. > Then, I checked the default arguments passed to gcc using this: > > gcc -v > > I got this: > > Using built-in specs. > COLLECT_GCC=gcc > COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper > Target: x86_64-linux-gnu > Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.4- > 2ubuntu1~14.04' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs -- > enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr -- > program-suffix=-4.8 --enable-shared --enable-linker-build-id -- > libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with- > gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls --with- > sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx- > time=yes --enable-gnu-unique-object --disable-libmudflap --enable-plugin -- > with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk- > cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable- > java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --with- > jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64 --with-arch- > directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc- > gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 -- > with-multilib-list=m32,m64,mx32 --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 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04) > > I am seeking to implement from scratch a tool that generates random > sequences of optimization options and compare generated sequences to > default levels O0-3. Just like "acovea". So that, I would like to > compare my generated sequences to a zero-optimization level (which > should be O0) > > Can you explain to me why 50 options are enabled by default in -O0? > > One idea I have in mind is to compile with O0 + turn off default > optimizations in O0 using "-fno-OPTIMIZATION_NAME" 50 times. What do > you think? Kind Regards Lars
