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 <imir...@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.
Cheers,
Nicolai
Cheers,
Nicolai
Marek
On Tue, Aug 22, 2017 at 1:10 PM, Marek Olšák <mar...@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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