[PATCH][LTO/PGO] Warn when both -flto and -fprofile-generate are enabled

[Richard Biener]

On Wed, Apr 2, 2014 at 1:50 PM, Markus Trippelsdorf
<markus@trippelsdorf.de> wrote:
> It is a common mistake to enable both -flto and -fprofile-generate when
> building projects. This is not a good idea, because memory use will
> skyrocket due to instrumentation. So just warn the user.
>
> OK for next stage1?

I'd rather see if we can fix the underlying issue.  For example as we
are now instrumenting as IPA pass we can allocate a single
counter array (if the number of global vars is the issue).  Basically
split analysis and instrumentation into two phases for that.

Or even better, do profile instrumentation as "real" IPA pass.

Richard.

> 2014-04-02  Markus Trippelsdorf  <markus@trippelsdorf.de>
>
>         * common.opt (fprofile-generate): Add flag.
>         * opts.c (finish_options): Add new warning.
>         (common_handle_option): Set flag.
>
> diff --git a/gcc/common.opt b/gcc/common.opt
> index 62c72f0d2fbf..61e9adfa0df5 100644
> --- a/gcc/common.opt
> +++ b/gcc/common.opt
> @@ -1689,7 +1689,7 @@ Common Report Var(flag_profile_correction)
>  Enable correction of flow inconsistent profile data input
>
>  fprofile-generate
> -Common
> +Common Var(flag_profile_generate)
>  Enable common options for generating profile info for profile feedback directed optimizations
>
>  fprofile-generate=
> diff --git a/gcc/opts.c b/gcc/opts.c
> index fdc903f9271a..b62a0d626d94 100644
> --- a/gcc/opts.c
> +++ b/gcc/opts.c
> @@ -833,6 +833,9 @@ finish_options (struct gcc_options *opts, struct gcc_options *opts_set,
>         error_at (loc, "only one -flto-partition value can be specified");
>      }
>
> +  if (opts->x_flag_generate_lto && opts->x_flag_profile_generate)
> +    warning_at (loc, 0, "Enabling both -fprofile-generate and -flto is a bad idea.");
> +
>    /* We initialize opts->x_flag_split_stack to -1 so that targets can set a
>       default value if they choose based on other options.  */
>    if (opts->x_flag_split_stack == -1)
> @@ -1728,6 +1731,7 @@ common_handle_option (struct gcc_options *opts,
>
>      case OPT_fprofile_generate_:
>        opts->x_profile_data_prefix = xstrdup (arg);
> +      opts->x_flag_profile_generate = true;
>        value = true;
>        /* No break here - do -fprofile-generate processing. */
>      case OPT_fprofile_generate:
> --
> Markus