Re: PIC question

[Johnny Billquist <bqt%softjar.se@localhost>]

Matt Thomas wrote:
> On May 27, 2010, at 4:57 PM, Johnny Billquist wrote:
>
> > Martin Husemann wrote:
> > > I'm new to vax asm and have not been fully assimilated, it seems - I have
> > > some asm code that is not PIC and so when compiled into a .so dies due
> > > to .text relocs. (Linked static it all works)
> > > It does kindof Duff's device in asm - there is an unrolled loop starting
> > > at .Loop and a jump with offset into the loop is calculated in %r5, then the
> > > code does:
> > >         jmp     .Loop(%r5)
> > > .Loop:  ...
> > > This causes a 32bit relocation for the .Loop address inside the
> > > indirect jump with offset instructions.
> > > What is the pic way to do this? Some trick? There doesn't seem to be a br +
> > > register offset instruction.
> > > Or along the lines of:
> > >         addl2   %pc,%r5
> > >         add     .Loop - ., %r5
> > >         jmp     (%r5)
> > > .Loop:  ...
> > > (but the .Loop-. is not accepted by gas)
> > > ?
> > Maybe a stupid solution, and I'm sure there is a more kosher one, but...
> >
> >         addl2   %r5,%pc
>
> Bzzt.  Can't do that to the PC register.

Ah! Darn. I had some nagging feeling that there might have been some 
problem with that.
Its perfectly legal on the PDP-11 though. :-)

>         jmp     .Loop[%r5]

Hmm, so deferring won't work, but indexing will? That sounds weird...
Is that some limitation on gas then?

> is the right answer.
>
> or      movab   .Loop[%r5], %r5
>         jmp     (%r5)
>
> But the first one is simpler.

Indeed.

        Johnny

--
Johnny Billquist                  || "I'm on a bus
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