Finding the optimization that is making the change

[Will Hawkins]

On Fri, Aug 11, 2017 at 1:14 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> On 11 August 2017 at 18:09, Will Hawkins <whh8b@virginia.edu> wrote:
>> Hello everyone!
>>
>> First, thank you all for your participation in the gcc community -- I
>> firmly believe that one of the great strengths of free software is the
>> community of people that participate in its development, maintenance
>> and support. So, thank you!
>>
>> I have a simple C program and I am attempting to determine which of
>> the optimizations at O1 cause a particular transformation. In order to
>> isolate the optimizations enabled at O1 vs O0, I followed an idea set
>> out in the gcc man page and ran the following command:
>>
>> $ diff <(gcc -Q -O1 --help=optimizers) <(gcc -Q --help=optimizers) |
>> grep enabled | awk '{print $2;}' > optimizations
>>
>> Then I compiled with the following command:
>>
>> gcc -o scfi.poptim `cat optimizations | tr '\n' ' '` scfi.c
>>
>> I compared simple.poptim with simple.optim that came from running this command:
>>
>> gcc -o simple.optim -O1 simple.c
>>
>> I expected that simple.optim and simple.poptim would be (largely)
>> identical. That is not the case, however. It does not look like the
>> scfi.poptim program has been optimized at all.
>
> Because you didn't specify any -O optimization option, which means
> there is no optimization done at all. See
> https://gcc.gnu.org/wiki/FAQ#optimization-options
>
> Options to enable/disable individual optimizations have no effect if
> the optimizers aren't run at all.
>
>> I was wondering if anyone could shed some light on why this is not the
>> case. I ask only because the gcc man page seems to imply that this is
>> the "right" way to isolate the different optimizations performed at
>> different levels.
>
> No, you need to use -O1 -fno-xxx -fno-yyy -fno-zzz
>
> i.e. turn on optimization, then disable individual passes. You can't
> start from nothing and enable individual ones, that gives you nothing.

Wow! That makes perfect sense. Thank you so much! I will give it a try
and let you know what I find. Thanks for the quick response!

Will