Re: [build, lto] Only accept -fuse-linker-plugin if linker supports -plugin (PR lto/46944)

[Rainer Orth <ro at CeBiTec dot Uni-Bielefeld dot DE>]

Richard Guenther <rguenther@suse.de> writes:

> If I read this patch correctly then
>
>  1) it doesn't change the condition under which lto-plugin/ is built
>     (good)

Right.

>  2) it makes -fuse-linker-plugin the default for and only for
>     known good linkers (GNU binutils >= 2.21) (good)

Indeed.

>  3) it makes it impossible to use -fuse-linker-plugin explicitly
>     for other linkers or linkers that were not installed during
>     configuring gcc (bad - esp. the latter)
>
> can you please try avoiding 3) at this stage?  Or is the whole
> point of this patch 3) to be able to fix PR46944?

That was my goal: the Solaris linker accepts -p <auditlib>, which causes
confusion when it is called with -plugin.

> Ideally we'd reject broken linkers at runtime, but that would
> require some major collect2 massaging (eventually falling back
> to collect2 or simply reporting an error).

What about making LTO_PLUGIN 3-valued?

2	linker used has full -plugin support, i.e. gld/gold >= 2.21
1       linker has some -plugin support, i.e. gold >= 2.20 < 2.21
0       linker has no known plugin support, i.e. everything else, in
        particular vendor linkers

We'd default to -fuse-linker-plugin for 2, accept it if given explicitly
for 1, and reject it for 0.

This would be similar to the first version of my patch, with the
difference that we don't try to determine the level of -plugin support
from trying to run the configured linker with -plugin and check if it
works, but instead hardcode that knowledge.

> That said, I'm not 100% happy with 3) at this point (though
> 2) is very desirable).
>
> Can we to fix 46944 change the dejagnu require-linker-plugin
> to check if a linker plugin is used by default and not add
> -fuse-linker-plugin?

That might be involved since it requires parsing gcc -Wl,-debug output.
I suppose the solution outlined above is simpler and thus more robust.

	Rainer

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Rainer Orth, Center for Biotechnology, Bielefeld University