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

[Jose Fonseca <jfonseca-pghWNbHTmq7QT0dZR+AlfA@public.gmane.org>]

On 18/11/14 14:34, Roland Scheidegger wrote:
> Am 18.11.2014 um 15:05 schrieb Ilia Mirkin:
>> On Tue, Nov 18, 2014 at 8:54 AM, Roland Scheidegger <sroland@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@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

Roland's right -- it takes less mantissa bits to represent an integer x, 
than a fractional number between x and x + 1

The only case where `frac(x) = x - floor(x)` fails is when x is a 
negative denormal. It might give 1.0f instead of 0.0f, if the hardware 
is not setup to flush denormals to zero properly.

Jose

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