Re: [EXTERNAL] Re: Advice with finding speed between O2 and O3

[Steve Kargl <sgk@troutmask.apl.washington.edu>]

On Thu, May 25, 2023 at 04:05:11PM +0000, Thompson, Matt (GSFC-610.1)[SCIENCE SYSTEMS AND APPLICATIONS INC] via Fortran wrote:
> Thomas,
> 
> Well, the code did not change. Period. Neither did the compiler. It was 12.3. (We can't use GCC 13 because it seems not to like something in our advanced Fortran code (lots of OO, submodules, string fun...)).
> 
> And I did a run with essentially all the GNU checks on (our Debug build mode) and it happily runs!
> 
> That said, I did some further tests and I am *really* confused. This fails:
> 
> -O3 -march=haswell -mtune=generic -funroll-loops -g
> 
> And this works:
> 
> -O2 -march=haswell -mtune=generic -funroll-loops -g
> 
> Now I just tried:
> 
> -O2     -fgcse-after-reload    -fipa-cp-clone    -floop-interchange    -floop-unroll-and-jam    -fpeel-loops    -fpredictive-commoning    -fsplit-loops    -fsplit-paths    -ftree-loop-distribution    -ftree-partial-pre    -funroll-completely-grow-size    -funswitch-loops    -fversion-loops-for-strides  -march=haswell -mtune=generic -funroll-loops -g
> 
> which as far as I can see from the gcc man page:
> 
>        -O3 Optimize yet more.  -O3 turns on all optimizations specified by -O2 and also turns on the following optimization flags:
> 
>            -fgcse-after-reload -fipa-cp-clone -floop-interchange -floop-unroll-and-jam -fpeel-loops -fpredictive-commoning -fsplit-loops -fsplit-paths -ftree-loop-distribution -ftree-partial-pre -funswitch-loops
>            -fvect-cost-model=dynamic -fversion-loops-for-strides
> 
> means I am running essentially -O3.
> 
> But it works.
> 
> I'm...baffled. Is there something that *gfortran* enables with -O3 that isn't visible from the *gcc* man page?
> 

gcc/gcc/opts.cc also shows some fiddling with parameters.

  /* -O3 parameters.  */
  { OPT_LEVELS_3_PLUS, OPT__param_max_inline_insns_auto_, NULL, 30 },
  { OPT_LEVELS_3_PLUS, OPT__param_early_inlining_insns_, NULL, 14 },
  { OPT_LEVELS_3_PLUS, OPT__param_inline_heuristics_hint_percent_, NULL, 600 },
  { OPT_LEVELS_3_PLUS, OPT__param_inline_min_speedup_, NULL, 15 },
  { OPT_LEVELS_3_PLUS, OPT__param_max_inline_insns_single_, NULL, 200 },

AFAICT, gfortran does not add or change anything with -O3.
Out of curosity, does it compile and run with -O3 if you 
remove one or both of '-march=haswell -mtune=generic'?

One other possibility is an issue with signed integer overflow,
but I don't remember if the change that causes the issue has
reached 12.x.  Does the code run if you add -fwrapv to your
options list?
-- 
Steve