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

[Michael Matz]

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.


Ciao,
Michael.