[Mesa-dev] [PATCH 04/15] i965/fs: Get rid of reladdr

[Jason Ekstrand]

On Dec 10, 2015 12:40 PM, "Matt Turner" <mattst88 at gmail.com> wrote:
>
> On Thu, Dec 10, 2015 at 11:11 AM, Jason Ekstrand <jason at jlekstrand.net>
wrote:
> >
> > On Dec 10, 2015 6:58 AM, "Francisco Jerez" <currojerez at riseup.net>
wrote:
> >>
> >> Jason Ekstrand <jason at jlekstrand.net> writes:
> >>
> >> > We aren't using it anymore.
> >>
> >> It seems useful to me to be able to represent indirect access as part
of
> >> any instruction source or destination register.
> >>
> >> The following:
> >>
> >> | mov_indirect g0, g1, a0
> >> | foo g2, g0
> >>
> >> and the converse case with indirect destination offset (which you don't
> >> seem to represent currently) can be implemented by the hardware more
> >> efficiently using a single instruction in certain cases.  The current
IR
> >> is able to represent what the hardware can do, but supporting the
> >> MOV_INDIRECT instruction only would force us to keep the indirection
> >> separate from the instruction that uses it, so it seems like a less
> >> expressive representation to me than the current approach, unless
you're
> >> willing to add _INDIRECT variants of most hardware opcodes.
> >
> > Yes and, mostly, no.  Yes, you can put an indirect on almost anything
but it
> > has substantial restrictions:
> >
> > 1) Destination indirects must be uniform (I'm only 95% sure this is the
> > case)
>
> Almost true. The Register Region Restrictions page gives an example:
>
> > // Example:
> > mov (16) r[a0.0]:f r10:f
> > // The above instruction behaves the same as the following two
instructions:
> > mov (8) r[a0.0]:f r10:f
> > mov (8) r[a0.1]:f r11:f
>
> But that's all you get -- you still have to use 1x1 mode.
>
> And that behavior changes on BDW:
>
> > // Example:
> > mov (16) r[a0.0]:f r10:f
> > // The above instruction behaves the same as the following two
instructions:
> > mov (8) r[a0.0]:f r10:f
> > mov (8) r[a0.0, 8*4]:f r11:f
>
>
> The BRW_VERTICAL_STRIDE_ONE_DIMENSIONAL (0xF) vertical stride is
> necessary for Vx1 or VxH, and the destination does not have vertical
> stride. It's also not usable in align16 mode or on src1.

Thanks for digging up the details. That all makes sense and is still insane.
--Jason
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