[Mesa-dev] [PATCH 0/7] RadeonSI 32-bit pointers v2 & Gallium changes

[Marek Olšák]

On Sat, Feb 3, 2018 at 2:55 AM, Roland Scheidegger <sroland at vmware.com> wrote:
> Am 03.02.2018 um 00:31 schrieb Marek Olšák:
>> On Sat, Feb 3, 2018 at 12:01 AM, Roland Scheidegger <sroland at vmware.com> wrote:
>>> Am 02.02.2018 um 23:39 schrieb Marek Olšák:
>>>> On Fri, Feb 2, 2018 at 10:26 PM, Roland Scheidegger <sroland at vmware.com> wrote:
>>>>> Am 02.02.2018 um 21:48 schrieb Marek Olšák:
>>>>>> Hi,
>>>>>>
>>>>>> This is the second and hopefully final version of 32-bit pointer
>>>>>> support for radeonsi.
>>>>>>
>>>>>> Constant buffer 0 now has restrictions on which buffers can be set
>>>>>> in that slot.
>>>>>>
>>>>>> I plan to push this when my LLVM patch lands in 6.0 (hopefully it
>>>>>> will be accepted there).
>>>>>>
>>>>>> There will also be a dependency on new libdrm (not included in this
>>>>>> series).
>>>>>>
>>>>>> Please review.
>>>>>>
>>>>>
>>>>> From a api cleanliness point of view, I don't like this much.
>>>>> First, you're making the hack case the default and even require it. IMHO
>>>>> a driver should be able to bind ordinary UBOs to all buffer slots. This
>>>>> is really not a nice burden to put on state trackers to do something
>>>>> special for just slot 0. The gallium API should stay reasonable imho,
>>>>> that's a bit too much custom tailoring for GL for my liking.
>>>>>
>>>>> Maybe I'm missing something but I can't quite see why you can't handle
>>>>> this transparently inside the driver. Can't you just create a different
>>>>> shader depending on what kind of buffer is bound or what's the problem?
>>>>> (You wouldn't expect it to change therefore you should not have to
>>>>> recompile.)
>>>>
>>>> We don't recompile shaders in the vast majority of cases. When shader
>>>> compilation stalls rendering, the gaming experience is destroyed.
>>>>
>>>> There is no alternative. Our shader ABI will be set up such that it
>>>> only has 32-bit pointers in shader registers. There are
>>>> performance-related reasons for that.
>>>
>>> That seems to be quite limited, why can't you have a shader ABI which
>>> can do either 32 or 64 bit pointers?
>>
>> Good questions. GCN shaders have only 16 dwords of constant memory
>> (GFX9 has 32). There are no shader resource slots and the pixel shader
>> is the only one to have real inputs. All other stages don't have any
>> shader inputs except for system values.
>>
>> The 16 dwords contain pointers and states to load inputs and load
>> descriptions of resource slots from memory. One of the pointers
>> sometimes points to constant buffer 0. If it's a VS, there are only 13
>> dwords, because 3 are reserved for baseinstance, basevertex, and
>> drawID. We can also put some other data into that constant memory to
>> skip load instructions. There is a huge incentive to free those
>> precious dwords and use them for something else, like skipping some
>> load instructions. I've been also considering 16-bit pointers (e.g.
>> 32-bit pointers aligned to 64KB).
>>
>
> Ok, so for other buffers you can't really do anything special? You just
> go through a pointer to array-of-pointer lookup?

By default, the shader gets a pointer that points to a merged list of
constant buffer and shader buffer descriptions in memory. If a shader
only uses constant buffer 0 and no shader buffers, that pointer points
to constant buffer 0 directly.

> I thought "proper" apps would just use UBOs for everything these days
> (hence nothing really much need for tuning slot 0). But maybe that's not
> actually true... I can see that you'd want to optimize usage of this
> precious space. I suppose GL doesn't give you much help there with its
> iffy buffer handling.

Yes, games use UBOs.

Marek