Avoid double mangling at WHOPR

[Richard Guenther]

On Mon, 10 Oct 2011, Jan Hubicka wrote:

> > > Actually it seems to me that mangling at WPA makes more sense - you don't get
> > > symbol name sensitive on partitioning decisions so things go a bit more
> > > consistently. Partitioning depends on global properties of program so any code
> > > that depends on particular partitioning decisions will have tendency to
> > > randomly break and unbreak.
> > > 
> > > We can not really make any promises about our ability to not mangle particular
> > > symbol (for use in asm code or whatever):  we need to mangle on the occasion of
> > > conflict with another static symbol but also when we decide to promote it
> > > hidden. We have no information about another hidden symbol in the non-LTO
> > > world.  Either linker plugin API needs to be extended by providing us with list
> > > of forbidden names or ability to have "hidden in LTO world" visibility or we
> > > probably need to start using random seeds on all promoted symbols.
> > > (in fact I already do sort of mangling to avoid conflict on comdats that has been
> > > brought local and then again promoted global, I just did not noticed it is a general
> > > problem back then when I first saw the linker complaining).
> > 
> > Ok, I see why it makes sense on WPA time.  But then why not mangle
> > during partitioning?  I think it still makes sense to avoid mangling local
> > decls, if not for debugging experience.
> 
> Yeah, we could do that. Debugging experience is quite good reason (though it will
> also make bogus asm statements magically work and break on random basis. In a way
> just breaking them seems more sensible behaviour to me ;) ).

;)

> > We do mangle late when we bring symbols local anyway, no?  I also
> > seem to remember we mangle at LTO time, too ...
> 
> Hmm, I think we mangle at stream in since we do make hashtables based on symbol names
> that are supposed to be unique, but perhaps I am wrong.

I think we do not hash local symbols, so that shouldn't be an issue.

> LTO time you mean at compilation time when streaming out? I am not aware of that.

Maybe that changed then or I misremember.

Can you try the "obvious" and simply mangle all local statics at
partitioning time?  (leaving the non-conflict case for a further
improvement)

Richard.