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
