Default flags enabled by GCC at -O3 by default appear to be out of sync with the GCC documentation

[InBetweenNames]

Hi everyone,

I was working on refining make.conf today and I noticed a bit of an oddity. On GCC's docs, they give a list of all optimizations enabled by default at O3:

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

Curiously, they list -ftree-loop-distribution as being enabled by default by O3, but in my own GCC it remains disabled:

>gcc --version
gcc (Gentoo 7.2.0 p1.1) 7.2.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

>gcc -c -Q -O3 --help=optimizers | grep distribut                                                                                                                                                                                                                       Tue 12 Sep 2017 09:37:44 AM EDT
  -ftree-loop-distribute-patterns       [enabled]
  -ftree-loop-distribution              [disabled]

I was wondering if anyone could check if this is the case on their systems too? Please include your GCC version in your response. If I can get at least one person to reproduce this bug I'll make a bug report in GCC.

[darkbasic]

ArchLinux:

$ gcc --version
gcc (GCC) 7.2.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc -c -Q -O3 --help=optimizers | grep distribut
  -ftree-loop-distribute-patterns       [enabled]
  -ftree-loop-distribution              [disabled]

P.S.
You removed your GCC version from the output.

[InBetweenNames]

Whoops--copied in some extraneous spaces from my terminal that hid it. Thanks!

[InBetweenNames]

That's interesting. I'll check GCC HEAD to see if it's enabled in there.

[mgomersbach]

Same for 6.4.0:

$ gcc --version 
gcc (Gentoo 6.4.0 p1.0) 6.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.



$ gcc -c -Q -O3 --help=optimizers | grep distribut
  -ftree-loop-distribute-patterns 	[enabled]
  -ftree-loop-distribution    		[disabled]

The documentation changed between 4.9.4 and 5.1.0

[preney]

If the documentation changed between 4.9.4 and 5.1.0, then that doesn't explain why my GCC output for v4.9.4 is the same as v5.4.0, v6.4.0, and v7.2.0, e.g.,

    $ gcc-4.9.4 --version
    gcc-4.9.4 (Gentoo 4.9.4 p1.0, pie-0.6.4) 4.9.4
    Copyright (C) 2015 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
    
    $ gcc-4.9.4 -c -Q -O3 --help=optimizers | grep distribut
         -ftree-loop-distribute-patterns       [enabled]
         -ftree-loop-distribution              [disabled]

i.e., the -c -Q -O3 --help=optimizers outputs are all identical --so the GCC documentation differs from the settings (unless this is due to a Gentoo patch on GCC --I didn't check).

[preney]

It is entirely possible that the graphite or some other Gentoo USE flag changes this setting. Checking with GCC pristine sources is the only definitive way to know for sure if the docs are out-of-sync.

[InBetweenNames]

Per the GCC 7.2.0 sources, file gcc/opts.c, starting from line 528:

    { OPT_LEVELS_3_PLUS, OPT_ftree_loop_distribute_patterns, NULL, 1 },
    { OPT_LEVELS_3_PLUS, OPT_fpredictive_commoning, NULL, 1 },
    { OPT_LEVELS_3_PLUS, OPT_fsplit_paths, NULL, 1 },
    /* Inlining of functions reducing size is a good idea with -Os
       regardless of them being declared inline.  */
    { OPT_LEVELS_3_PLUS_AND_SIZE, OPT_finline_functions, NULL, 1 },
    { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_finline_functions_called_once, NULL, 1 },
    { OPT_LEVELS_3_PLUS, OPT_fsplit_loops, NULL, 1 },
    { OPT_LEVELS_3_PLUS, OPT_funswitch_loops, NULL, 1 },
    { OPT_LEVELS_3_PLUS, OPT_fgcse_after_reload, NULL, 1 },
    { OPT_LEVELS_3_PLUS, OPT_ftree_loop_vectorize, NULL, 1 },
    { OPT_LEVELS_3_PLUS, OPT_ftree_slp_vectorize, NULL, 1 },
    { OPT_LEVELS_3_PLUS, OPT_fvect_cost_model_, NULL, VECT_COST_MODEL_DYNAMIC },
    { OPT_LEVELS_3_PLUS, OPT_fipa_cp_clone, NULL, 1 },
    { OPT_LEVELS_3_PLUS, OPT_ftree_partial_pre, NULL, 1 },
    { OPT_LEVELS_3_PLUS, OPT_fpeel_loops, NULL, 1 },

Note in that list, ftree_loop_distribution is not present. I think either the docs should be amended or the O3 options should be.

[InBetweenNames]

Just built GCC 7.2.0 from scratch just to be sure there weren't any build time modifications that would change the state of the flag.

host-x86_64-pc-linux-gnu/gcc/cc1 -c -Q -O3 --help=optimizers | grep distribu                                                                                                                                                                                                   Tue 12 Sep 2017 05:05:16 PM EDT
cc1: error: command line option ‘-c’ is valid for the driver but not for C
cc1: error: command line option ‘-Q’ is valid for the driver but not for C
  -ftree-loop-distribute-patterns       [enabled]
  -ftree-loop-distribution              [disabled]

Unless someone has an objection, I'm going to go ahead and file a bug report upstream

[InBetweenNames]

Bug report is here, for interested parties: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82197

[InBetweenNames]

Turns out this documentation is actually for GCC trunk. TIL! The behaviour is correct for 6.4.0 and 7.2.0. I'll make a note to revise the flags for the next GCC release.