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

[Richard Guenther]

On Sat, Mar 26, 2011 at 3:51 AM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> [sorry for breaking the threading I've deleted the mails I'm answering
> already]
>
> In any case, citing from
>  http://gcc.gnu.org/ml/gcc-patches/2011-03/msg01250.html
>
>> Here's the patch I came up with.  It is on top of the previous one, so
>> if we want to backport to 4.6 later, both are necessary.
>>
>> It also fixes a typo in the in_tree_ld case, which can never have
>> worked.
>>
>> Tested by i386-pc-solaris2.11 bootstraps with either Sun ld or gld 2.21,
>> and by configuring with Sun ld --with-plugin-ld=gld-2.21 and obvserving
>> HAVE_LTO_PLUGIN being set to 1.
> ...
>> @@ -3207,6 +3207,10 @@
>>      elif test "$ld_is_gold" = yes -a "$ld_vers_major" -eq 2 -a "$ld_vers_minor" -eq 20; then
>>        gcc_cv_lto_plugin=1
>>      fi
>> +  elif test x"$ORIGINAL_PLUGIN_LD_FOR_TARGET" != x"$gcc_cv_ld"; then
>> +    # Allow -fuse-linker-plugin if plugin linker differs from
>> +    # default/specified linker.
>> +    gcc_cv_lto_plugin=1
>>    fi
>>  fi
>
> And this '1' is a problem.  Even if I specify
> --with-plugin-ld=some-good-ld (i.e. it can be reasonably assumed that I
> know what I'm doing) the above forces me to still have to use
> -fuse-linker-plugin when I really want to use it.  This is because the
> introduction of three levels of HAVE_LTO_PLUGIN: 0 (-fuse-linker-plugin
> not allowed), 1 (allowed but not default for -flto), 2 (allowed and
> default to on with -flto).
>
> I think if the plugin linker is different from the normal linker we should
> set HAVE_LTO_PLUGIN to 2.

I think we should do the linker version checks which relate to linker-plugin
use on the plugin-linker instead.  So if I specify a separate but known
buggy linker I don't want it to be used by default.

Richard.

>
> Ciao,
> Michael.
>