[Mesa-dev] [PATCH 2/2] [RFC] radv: add scratch support for spilling.

[Marek Olšák]

I'd like the scratch buffer to be the first descriptor in RW_BUFFERS,
that means s_load_dwordx2 ptr, s[0:1], 0x0.

Please don't add any other user data SGPRs to radeonsi.

Marek

On Mon, Oct 10, 2016 at 10:06 AM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> On 10.10.2016 05:45, Dave Airlie wrote:
>>
>> On 10 October 2016 at 13:25, Dave Airlie <airlied at gmail.com> wrote:
>>>
>>> From: Dave Airlie <airlied at redhat.com>
>>>
>>> This is a bit of a hack due to how llvm currently handles
>>> spilling in it's shader ABI. Currently llvm amdgpu backend
>>> uses relocations to patch the shader with the address of
>>> the tmpring. The driver loads the shader and patches the
>>> relocations.
>>>
>>> However for vulkan this doesn't work so well for a few reasons
>>> a) when we build/load the shaders we aren't constructing the
>>> command stream yet, and the same shader could be using in multiple
>>> command streams.
>>>
>>> b) multiple command execution engines for compute shaders.
>>>
>>> So ideally we'd fix LLVM to understand the ABI convention, possibly
>>> we'd fix it so user sgpr 0,1 are used (this hack uses 10/11).
>>>
>>> This patch when it gets the shader back from llvm it patches
>>> the relocation dword to a nop, and patches to previous mov command
>>> to move from SGPR 10 and 11. This works usually as it seems the
>>> SGPR loading of the spill stuff is at the start of shaders always
>>> so the 10/11 user sgprs haven't been trashed yet. I'm not 100%
>>> sure this will work all the time, but for now this should allow
>>> us to pass a bunch more CTS tests and make the Sascha computeshader
>>> demo to work.
>>
>>
>> So I found a shader that this doesn't work so well with unfortunately, so
>> while I'd like this as a temporary solution I probably need to start
>> digging into llvm.
>>
>> My current plan is to add a flag to llvm to denote ability to spill to
>> the address
>> in userdata sgpr 0/1, have llvm preload those and use them instead.
>>
>> Now the other question I have is, should I be killing two user data sgprs
>> for
>> this purpose, or should we define a better ABI, so that the first
>> descriptor
>> in the buffer pointed to by these is the scratch buffer, and other things
>> could be queued after it, (like push constants and dynamic descriptor).
>>
>> Bas, nha? not sure if Matt is on this list.
>
>
> Getting rid of the relocations would be nice for OpenGL as well. It always
> feels like a bit of a hack.
>
> For OpenGL, it's important that non-monolithic shaders work. In practice,
> this means that it must be possible to pass the scratch buffer pointer
> through the prolog shader into the main part.
>
> It seems to me the simplest way to ensure this is to add an explicit
> function argument. Something like:
>
> define amdgpu_XX TYPE @prolog(i64 inreg %spillptr, ....)
> "amdgpu-spill-ptr"="0" {
>    ...
>    %retval = insertvalue TYPE %prev, i64 %spillptr, 0
>    ret TYPE %retval
> }
>
> amdgpu-spill-ptr would be a function attribute indicating which function
> argument (by position) holds the base pointer for spilling.
>
> I also think that loading the spill pointer indirectly makes a lot of sense.
> After all, user data is scarce and most shaders don't spill. This could
> easily fit in the function attribute scheme. Something like
> "amdgpu-spill-ptr=1:16" to indicate that the second function argument is a
> 64-bit pointer, and the spill pointer should be loaded from there with a 16
> byte offset.
>
> FWIW, even with the indirect load, I think we should only load a 64-bit
> pointer instead of a full descriptor. This is because we should really set
> the size limit of the spill descriptor correctly to implement robustness
> semantics (this is relevant because the spill descriptor is also used for
> temporary arrays; not sure if Vulkan cares, but OpenGL definitely does).
>
> Cheers,
> Nicolai
>
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