[Mesa-dev] TGSI 16-bit support

[Nicolai Hähnle]

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.

Cheers,
Nicolai


> Marek
> 
>>
>> On Tue, Aug 22, 2017 at 1:10 PM, Marek Olšák <maraeo at gmail.com> wrote:
>>> Hi,
>>>
>>> I'd like to discuss 16-bit float and integer support in TGSI. I'm
>>> proposing this:
>>>
>>>   struct tgsi_instruction
>>>   {
>>>      unsigned Type       : 4;  /* TGSI_TOKEN_TYPE_INSTRUCTION */
>>>      unsigned NrTokens   : 8;  /* UINT */
>>>      unsigned Opcode     : 8;  /* TGSI_OPCODE_ */
>>>      unsigned Saturate   : 1;  /* BOOL */
>>>      unsigned NumDstRegs : 2;  /* UINT */
>>>      unsigned NumSrcRegs : 4;  /* UINT */
>>>      unsigned Label      : 1;
>>>      unsigned Texture    : 1;
>>>      unsigned Memory     : 1;
>>>      unsigned Precise    : 1;
>>> -   unsigned Padding    : 1;
>>> +   unsigned HalfPrecision : 1;
>>>   };
>>>
>>> There won't be any 16-bit TEMPs in TGSI, but each instruction will
>>> have the HalfPrecision flag, which is a hint for drivers that they can
>>> use a 16-bit opcode. Even texture, load, and store instructions can
>>> set HalfPrecision, which means they can accept and return 16-bit
>>> values.
>>>
>>> The catch is that drivers will have to insert 16-bit <-> 32-bit
>>> conversions manually, because they won't be present in TGSI. The
>>> advantage is that we don't have to add 200 new opcodes for the 3 new
>>> 16-bit types.
>>>
>>> What do you think?
>>>
>>> Marek
>>> _______________________________________________
>>> mesa-dev mailing list
>>> mesa-dev at lists.freedesktop.org
>>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
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