Re: [PATCH] Enable auto-vectorization at O2 with very-cheap cost model.

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Sep 06, 2021 at 11:18:47AM +0200, Richard Biener wrote:
> On Mon, Sep 6, 2021 at 10:47 AM liuhongt via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > Hi:
> >   As discussed in [1], most of (currently unopposed) targets want
> > auto-vectorization at O2, and IMHO now would be a good time to enable O2
> > vectorization for GCC trunk, so it would leave enough time to expose
> > related issues and fix them.
> >
> >   Bootstrapped and regtested on x86_64-linux-gnu{-m32,}
> >   Ok for trunk?
> 
> It changes the cost model used when the user specifices
> -O2 -ftree-vectorize which used 'cheap' before but now sticks to
> 'very-cheap'.  I guess adjusting the cost model in process_options
> might be possible when any(?) of the vectorizer flags were set
> explicitly?

process_options would mean it affects only the command line and not
__attribute__((optimize ("O2", "ftree-vectorize")))
etc.
So, shouldn't it be instead done in default_options_optimization, somewhere
among the
  if (openacc_mode)
    SET_OPTION_IF_UNSET (opts, opts_set, flag_ipa_pta, true);

  /* Track fields in field-sensitive alias analysis.  */
  if (opt2)
    SET_OPTION_IF_UNSET (opts, opts_set, param_max_fields_for_field_sensitive,
                         100);

  if (opts->x_optimize_size)
    /* We want to crossjump as much as possible.  */
    SET_OPTION_IF_UNSET (opts, opts_set, param_min_crossjump_insns, 1);

  /* Restrict the amount of work combine does at -Og while retaining
     most of its useful transforms.  */
  if (opts->x_optimize_debug)
    SET_OPTION_IF_UNSET (opts, opts_set, param_max_combine_insns, 2);
in there?
Like:
  /* Use -fvect-cost-model=cheap instead of -fvect-cost-mode=very-cheap
     by default with explicit -ftree-{loop,slp}-vectorize.  */
  if (opts->x_optimize == 2
      && (opts_set->x_ftree_loop_vectorize
	  || opts_set->x_ftree_slp_vectorize))
    SET_OPTION_IF_UNSET (opts, opts_set, fvect_cost_model_,
			 VECT_COST_MODEL_CHEAP);
Though, unsure if that will work with -O2 -ftree-vectorize which is an
option without flag with EnabledBy on the other two options.

Also, is:
+    { OPT_LEVELS_2_PLUS, OPT_ftree_loop_vectorize, NULL, 1 },
+    { OPT_LEVELS_2_PLUS, OPT_ftree_slp_vectorize, NULL, 1 },
what we really want, isn't that instead:
+    { OPT_LEVELS_2_PLUS_SPEED_ONLY, OPT_ftree_loop_vectorize, NULL, 1 },
+    { OPT_LEVELS_2_PLUS_SPEED_ONLY, OPT_ftree_slp_vectorize, NULL, 1 },
?  I mean, for -Os vectorization even in very-cheap model I'd think it
usually enlarges code size, and for -Og it is seriously harmful for
debugging experience, especially when DWARF <= 5 doesn't have anything that
would help debugging vectorized loops.

	Jakub