Disable all optimization options in GCC

[Martin Sebor]

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
> —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.

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