[PATCH] options: Make -Ofast switch off -fsemantic-interposition

Martin Jambor mjambor@suse.cz
Fri Nov 12 22:14:06 GMT 2021


Hi,

On Fri, Nov 12 2021, Martin Jambor wrote:
> Hi,
>
> using -fno-semantic-interposition has been reported by various people
> to bring about considerable speed up at the cost of strict compliance
> to the ELF symbol interposition rules  See for example
> https://fedoraproject.org/wiki/Changes/PythonNoSemanticInterpositionSpeedup
>
> As such I believe it should be implied by our -Ofast optimization
> level, not only so that benchmarks that can benefit run faster, but
> also so that people looking at -Ofast documentation for options that
> could speed their programs find it.
>
> I have verified that with the following patch IPA-CP sees
> flag_semantic_interposition set to zero at Ofast and that info and pdf
> manual builds fine with the documentation change.  I am bootstrapping
> and testing it now in order to comply with submission criteria but I
> don't think an Ofast change gets much tested.
>
> Assuming it passes, is the patch OK?  (If it is, I will also add a note
> about it in the "Caveats" section in gcc-12/changes.html of wwwdocs
> after I commit the patch.)
>

Unfortunately, I was wrong, there are testcases which use the optimize
attribute to switch a function to Ofast and those ICE because
-fsemantic-interposition is not an optimization flag and only
optimization flags can change in an optimize attribute (AFAIK, I only
had a quick glance at the results).

I am not sure what is the right way to tackle this, whether to set the
flag at Ofast in some nonstandard way or make the flag an optimization
flag - probably affecting function definitions, having it affect
call-sites seems too fine-grained.  I will try to discuss this on IRC on
Monday (and hope such change is still doable early stage3).

Sorry for posting this a bit prematurely,

Martin

>
>
> gcc/ChangeLog:
>
> 2021-11-12  Martin Jambor  <mjambor@suse.cz>
>
> 	* opts.c (default_options_table): Switch off
> 	flag_semantic_interposition at Ofast.
> 	* doc/invoke.texi (Optimize Options): Document that Ofast switches off
> 	-fsemantic-interposition.
> ---
>  gcc/doc/invoke.texi | 1 +
>  gcc/opts.c          | 1 +
>  2 files changed, 2 insertions(+)
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 2ea23d07c4c..fd16c91aec8 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -10551,6 +10551,7 @@ valid for all standard-compliant programs.
>  It turns on @option{-ffast-math}, @option{-fallow-store-data-races}
>  and the Fortran-specific @option{-fstack-arrays}, unless
>  @option{-fmax-stack-var-size} is specified, and @option{-fno-protect-parens}.
> +It turns off @option {-fsemantic-interposition}.
>  
>  @item -Og
>  @opindex Og
> diff --git a/gcc/opts.c b/gcc/opts.c
> index caed6255500..3da53d8f890 100644
> --- a/gcc/opts.c
> +++ b/gcc/opts.c
> @@ -682,6 +682,7 @@ static const struct default_options default_options_table[] =
>      /* -Ofast adds optimizations to -O3.  */
>      { OPT_LEVELS_FAST, OPT_ffast_math, NULL, 1 },
>      { OPT_LEVELS_FAST, OPT_fallow_store_data_races, NULL, 1 },
> +    { OPT_LEVELS_FAST, OPT_fsemantic_interposition, NULL, 0 },
>  
>      { OPT_LEVELS_NONE, 0, NULL, 0 }
>    };
> -- 
> 2.33.0


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