[Mesa-dev] TGSI 16-bit support

[Marek Olšák]

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.

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
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