Re: Disable all optimization options in GCC

[Martin Sebor <msebor at gmail dot com>]

On 10/22/2015 07:11 AM, Lars Nielsen wrote:
> > 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.

Reading beyond the first mention of the options, one sees:

  If the -Q option appears on the command line before the --help=
  option, then the descriptive text displayed by --help= is changed.
  Instead of describing the displayed options, an indication is given
  as to whether the option is enabled, disabled or set to a specific
  value

  The output is sensitive to the effects of previous command-line
  options, so for example it is possible to find out which
  optimizations are enabled at -O2 by using:

          -Q -O2 --help=optimizers

  ...

What's not obvious is that even when many of these fine-grained
optimization options are enabled (either implicitly in the source
code of the compiler or explicitly by the user on the command line
or via a pragma) the corresponding optimizations don't necessarily
run unless other optimizations are also enabled.

For example, the first option that shows up as enabled in the
output, -faggressive-loop-optimizations, is specified as enabled
by default, but only comes into play at -O2 and higher, during
optimizations like dead code elimination, loop unrolling, value
range propagation, etc.

So the output of the -Q --help=optimizers option isn't quite as
meaningful as one might hope.

Martin