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

Jan Hubicka hubicka@kam.mff.cuni.cz
Fri Nov 19 11:51:01 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,
> 
> Hi,
> 
> This patch turns flag_semantic_interposition to optimization option so
> it can be enabled with per-function granuality.  This is done by adding
> the flag among visibility flags into the symbol table.  This fixes the
> behaviour on the testcase I added to testsuite.
> 
> There are bugs where get_availability misbehaves on partitioned program.
> We can also use the new flag to fix those, but I will do that
> incrementally.
> 
> The -Ofast change should be safe now.

Also forgot to say it explicitly, the patch is OK, so please commit it.

Honza


More information about the Gcc-patches mailing list