[Mesa-dev] [PATCH] nv50/ir: saturate FRC result to avoid completely bogus values

[Ilia Mirkin]

On Tue, Nov 18, 2014 at 8:54 AM, Roland Scheidegger <sroland at vmware.com> wrote:
> Am 18.11.2014 um 05:03 schrieb Ilia Mirkin:
>> For values above integer accuracy in floats, val - floor(val) might
>> actually produce a value greater than 1. For such large floats, it's
>> reasonable to be imprecise, but it's unreasonable for FRC to return a
>> value that is not between 0 and 1.
>>
>> Signed-off-by: Ilia Mirkin <imirkin at alum.mit.edu>
>> ---
>>  src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp b/src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp
>> index 41b91e8..e5b767f 100644
>> --- a/src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp
>> +++ b/src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp
>> @@ -2512,7 +2512,8 @@ Converter::handleInstruction(const struct tgsi_full_instruction *insn)
>>           src0 = fetchSrc(0, c);
>>           val0 = getScratch();
>>           mkOp1(OP_FLOOR, TYPE_F32, val0, src0);
>> -         mkOp2(OP_SUB, TYPE_F32, dst0[c], src0, val0);
>> +         mkOp2(OP_SUB, TYPE_F32, val0, src0, val0);
>> +         mkOp1(OP_SAT, TYPE_F32, dst0[c], val0);
>>        }
>>        break;
>>     case TGSI_OPCODE_ROUND:
>>
>
> I don't understand the math behind this. For any such large number, as
> far as I can tell floor(val) == val and hence the end result ought to be
> zero. Or doesn't your floor work like that?

I could be thinking about this backwards, but let's say that floats
lose integer precision at 10.0. And I do floor(12.5)... normally this
would be 12.0, but since that's not exactly representable, it might
actually be 11.0. (Or would it be 11.9987? I didn't consider that
possibility...) And then 12.5 - 11 = 1.5. Or am I thinking about this
backwards? I guess ideally I'd do something along the lines of y = x -
floor(x); return y - floor(y). That seems like it might be more
accurate... not sure.