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Date: Wed, 14 Nov 2018 13:47:00 -0000
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Jakub Jelinek <jakub@redhat.com>
Cc: Martin Sebor <msebor@gmail.com>, Gcc Patch List <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] diagnose unsupported uses of hardware register variables (PR 88000)
Message-ID: <20181114134709.GA23873@gate.crashing.org>
References: <20181114122250.GZ23873@gate.crashing.org> <20181114122726.GW11625@tucnak>
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On Wed, Nov 14, 2018 at 01:27:26PM +0100, Jakub Jelinek wrote:
> On Wed, Nov 14, 2018 at 06:22:51AM -0600, Segher Boessenkool wrote:
> > Btw, if you just add
> > 
> > void *
> > retsp (void)
> > {
> >   register void *sp __asm ("sp");
> >   asm ("" : "+g" (sp));  // <-- this line
> >   return sp;
> > }
> > 
> > everything works fine.
> 
> Even in what you are proposing, i.e. handle the var as any other var
> in SSA form and only copy into the hard register right before asm and out of
> it after it?

Yes, *only* in that: with current trunk sp lives in the "sp" hard register
at the "return sp", which cannot work reliably (what value is returned?
It is unspecified).

> Because 
> {
>   void *sp;
>   asm ("" : "+g" (sp));
>   return sp;
> }
> would store into the register default definition of the SSA_NAME (the var
> has no initializer).

I'm more concerned about what it looks like in RTL, but sure :-)  What
*should* it do before RTL?  Not much at all I think, just keep track that
this var is a register asm and that's that?


Segher