Re: Disable all optimization options in GCC

[Lars Nielsen <lars at steinwurf dot com>]

> On 22 Oct 2015, at 14:47, Mohamed Boussaa <mohamedboussaa3@gmail.com> 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
âhelp=optimizers:
	âoptimizersâ
	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