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

[Roland Scheidegger]

Am 18.11.2014 um 15:05 schrieb 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.
> 
If your float is large enough that the next closest float is more than
1.0 away, then that float would have been an exact integer, thus floor()
doing nothing.

Roland