Re: Add -fsection-anchors (4.2 project)

[Andrew Pinski <pinskia at physics dot uc dot edu>]

> This is the main part for the 4.2 section anchor project:
> 
>     http://gcc.gnu.org/wiki/Section%20Anchor%20Optimisations
> 
> The idea is to introduce anchor symbols that can be used to access
> several nearby objects.  For example, if we have:
> 
>     static int a, b, c;
>     int foo (void) { return a + b + c; }
> 
> gcc will normally perform separate symbolic address calculations for
> "a", "b" and "c".  The idea of this patch is to introduce a new anchor
> symbol and access "a", "b" and "c" relative to that anchor.
> 
> The main motivation is to reduce GOT size, but the patch also has
> the potential to make some code faster.  SPEC results are attached
> and described below.

First I want to say thanks for doing this optimization.
Second, this should fix PR 17106 which is actually asking for
this optimization.

> The only extra failures were in objc:
> 
>     FAIL: objc/execute/class-13.m compilation
>     FAIL: objc/execute/class-6.m compilation
>     FAIL: objc/execute/object_is_class.m compilation
>     FAIL: objc/execute/object_is_meta_class.m compilation
> 
> (options snipped).  These failures are caused by objc creating two decls
> for _OBJC_METACLASS_* objects.  I think it would be better to modify the
> old decl once the initialiser is known, but I don't know enough about
> objc to do that.

Thrid, give me a few days to debug this.  If your patch is approved make sure that these
tests get xfailed and a bug(s) filed for this regression.

Thanks,
Andrew Pinski