[Mesa-dev] [PATCH] nouveau: codegen: Take src swizzle into account on loads

[Ilia Mirkin]

On Fri, Apr 8, 2016 at 5:27 AM, Hans de Goede <hdegoede at redhat.com> wrote:
> Hi,
>
> On 07-04-16 15:58, Ilia Mirkin wrote:
>>
>> That's wrong.
>
>
> It used to work with the old RES[] code and if one cannot specify
> a source swizzle, then how can I do something like
>
> LOAD TEMP[0].y, MEMORY[0], address
>
> And get the data at absolute global memory address "address" into TEMP[0].y
> ?
>
> This is a must-have for llvm to be able to generate working TGSI code,
> I do not see any way around this.
>
> AFAIK this is exactly what src-swizzling is for. Also note that
> this commit does not change anything if no src-swizzling is specified,
> in that case things work exactly as before.
>
>> The spec for the instruction needs to be clarified...
>>
>> The current nouveau impl is correct - only the .x of the address
>> should be loaded, with up to 16 bytes read into the destination.
>
>
> Ah note this is not about swizzling on the address, that indeed
> makes no sense given how the addressing works for BUFFERS / MEMORY,
> no this is about adding a swizlling postfix to the buffer / memory
> resource specification, for example:
>
> LOAD TEMP[0].y, MEMORY[0].xxxx, TEMP[0]
>
> See the swizzling is done on the resource, not on the address, so
> the swizzling specifies swizzling of the up to 16 bytes read from
> address, it does not influence the address handling at all.
>
> I now see I made an error in my commit msg, it gives the following
> example:
>
> LOAD TEMP[0].y, MEMORY[0].xxxx, TEMP[0].x
>
> This clearly is wrong, the last TEMP[0].x is not even valid TGSI,
> the correct example would be:
>
> LOAD TEMP[0].y, MEMORY[0].xxxx, TEMP[0]

I stand by my comment of "working as intended". But that doesn't mean
the intent can't be changed :)

For memory/buffers, LOAD takes the address at TEMP[0].x and loads 16
bytes (4 words), and sticks them into the destination's .xyzw. If you
happen to have a writemask, then only some of those are written out.

It seems that you're trying to add additional meaning to the swizzle
on the "memory" argument. However I don't believe that such a thing is
defined. (And definitely not used anywhere, at least not on purpose.)

Why does this cause you issues with LLVM-generated TGSI?

  -ilia