[Mesa-dev] TGSI 16-bit support

[Ilia Mirkin]

On Wed, Aug 23, 2017 at 10:30 AM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> On 23.08.2017 15:15, Nicolai Hähnle wrote:
>>
>> On 22.08.2017 19:32, Marek Olšák wrote:
>>>
>>> On Tue, Aug 22, 2017 at 7:28 PM, Ilia Mirkin <imirkin at alum.mit.edu>
>>> wrote:
>>>>
>>>> How do you propose defining the semantics for e.g. loading a 16-bit
>>>> value from a constbuf/ssbo? Would those get separate instructions?
>>>
>>>
>>> st/mesa should use UP2H, PK2H and similar opcodes for I16 and U16, and
>>> drivers can replace them with MOV if HalfPrecision == 1.
>>
>>
>> You mean, if HalfPrecision == 1 for subsequent operations?
>>
>> How *do* we implement this for LLVM, anyway? Downcast (fptrunc) from float
>> to half whenever we're loading operands of a HalfPrecision == 1 instruction,
>> and then casting (fpext) back up before storing the result?
>>
>> LLVM instcombine seems quite capable of seeing through that in simple
>> code, but I worry about control flow.
>
>
> Thinking about this some more, having the precision a property of the
> temporaries like Ilia suggested would probably help with emitting LLVM IR
> that behaves well across control flow, but complicate st_glsl_to_tgsi. Hard
> to say what the tradeoff is there.

Why would it complicate glsl_to_tgsi? At the GLSL level, it's not the
operations that have precision, but variables. And those variables map
to temp's... we'd have to create a separate pool of high- vs
low-precision temps, but that's about it.

  -ilia