Re: Avoiding paradoxical subregs

["Paulo J. Matos" <paulo at matos-sorge dot com>]

On 18/01/14 20:11, pinskia@gmail.com wrote:
> > On Jan 18, 2014, at 12:04 PM, "Paulo J. Matos" <paulo@matos-sorge.com> wrote:
> >
> > On 17/01/14 17:36, Eric Botcazou wrote:
> > > > I am not implying that this is a GCC bug, unless you think
> > > > WORD_REISTER_OPERATIONS should have avoided the creation of such
> > > > paradoxical subreg.
> > >
> > > No, that's precisely the contrary, WORD_REGISTER_OPERATIONS tends to create
> > > paradoxical subregs.
> >
> > I might then, be misunderstanding the reason to enable WORD_REGISTER_OPERATIONS. From the documentation:
> >
> > "Macro: WORD_REGISTER_OPERATIONS
> >
> >     Define this macro if operations between registers with integral mode smaller than a word are always performed on the entire register. Most RISC machines have this property and most CISC machines do not. "
> >
> > The machine I am targeting is a 64bit vector machine. When you perform a 16 bit operation, the whole register is changed because the operation actually changes every 16bit group of the register. So it seems I should enable WORD_REGISTER_OPERATIONS.
> >
> > As an aside, is there any machine in gcc mainline like this?
>
> Yes spu.  It is a 128bit vector machine, operations happen in that size and compares are done in the same size.

Thanks for the reference.

> I think you misunderstood WORD_REGISTER_OPERATIONS, it means the operation happens on the whole register in that a 16bit add is the same as a 32bit one.

Oh... my... you're right. If I now read the doc for 
WORD_REGISTER_OPERATIONS what you're saying makes sense. I shouldn't 
have it enabled.

Thanks for your help.

Paulo Matos

> Thanks,
> Andrew Pinski


--
PMatos