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Re: rs6000 toc reference rtl again
- From: Alan Modra <amodra at gmail dot com>
- To: gcc-patches at gcc dot gnu dot org, David Edelsohn <dje dot gcc at gmail dot com>, rdsandiford at googlemail dot com
- Date: Wed, 4 Apr 2012 10:33:58 +0930
- Subject: Re: rs6000 toc reference rtl again
- References: <20120327075431.GA12569@bubble.grove.modra.org> <firstname.lastname@example.org>
On Tue, Apr 03, 2012 at 07:49:04PM +0100, Richard Sandiford wrote:
> Alan Modra <email@example.com> writes:
> > Now that we are back in stage1, I'd like to apply
> > http://gcc.gnu.org/ml/gcc-patches/2011-09/msg00304.html, a change to
> > toc reference rtl in order to properly specify r2 dependencies. More
> > commentary in that url. I'm reposting the patch here since the old
> > one no longer applies cleanly, and I've added some ENABLE_CHECKING
> > code in rs6000_delegitimize_address.
> Sorry to be a pain, but I don't think HIGH is supposed contain
> regs either. Both HIGH and CONST are supposed to be true constants.
Eh, so the existing use of CONST is wrong then. ;-)
(unspec [(symbol_ref sym) (reg r2)] UNSPEC_TOCREL)
for the small model, and
(high (unspec [(symbol_ref sym) (reg r2)] UNSPEC_TOCREL)))
(lo_sum (reg hi) (unspec [(symbol_ref sym) (reg r2)] UNSPEC_TOCREL))
for medium/large model.
You can see why I'd like to keep it this way; The medium/large rtl is
a natural split of the small rtl. (I'm going to experiment with
splitting the small rtl after reload for medium/large to see whether
that helps our usage of call-saved regs in loops.)
I'm not wedded to the representation, *but* we do want gcc to treat
the high part as a constant. That's important because we don't ever
want reload saving the high part to a stack slot! Which is what does
happen if you don't somehow tell gcc it is a constant.
Besides, the high part *is* a constant within any given function. So
is the low part for that matter. The only reason I want r2 mentioned
in this rtl is for register liveness, eg. so that a load of a function
pointer (which loads r2) for an indirect call doesn't get scheduled
before any uses of the old r2.
The alternative of removing r2 from the unspec and attaching a
(use (reg r2)) to all instructions that have this addressing form
might be clean but will require major duplication of patterns in
rs6000.md, won't it?
Australia Development Lab, IBM