[Mesa-dev] [PATCH] radeonsi: compute the absolute value before RSQ

[Roland Scheidegger]

Am 05.01.2017 um 20:43 schrieb Samuel Pitoiset:
> 
> 
> On 01/05/2017 06:49 PM, Roland Scheidegger wrote:
>> Meh, I'm not really a big fan of such hacks. GPUs have support for NaNs
>> since ages, and while glsl is lenient the point stands that returning a
>> NaN is a more correct result, so doing extra work to get a wrong result
>> doesn't look all that great to me.
>> FWIW dx10 requires NaNs as results (for both sqrt and rsq). Maybe app
>> specific quirks (if said apps can't be fixed) would be better...
>> But well, it's your driver, so whatever floats your boat.
> 
> I think the main idea here is to reduce the number of bug reports when
> applications use D3D-style for RSQ/SQRT instead of what GLSL spec says.

Well if someone ports from d3d10, he might well expect NaNs since d3d10
guarantees you get a NaN there...
(That said, with d3d10 NaNs will get converted to zeros on pixel shader
export if the RT isn't a float type, I don't know what mesa drivers to
there.)
Not saying it wouldn't make sense doing such hackery at glsl level but
imho it still should be a quirk.

Roland


>>
>> Roland
>>
>>
>> Am 05.01.2017 um 18:37 schrieb Marek Olšák:
>>> Shouldn't we also use abs for SQRT? For example, this adds abs for
>>> both RSQ and SQRT:
>>>
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__cgit.freedesktop.org_-7Emareko_mesa_commit_-3Fid-3D5e0fb661a8e6ac5f7b2245dd31595155128e0664&d=DgIGaQ&c=uilaK90D4TOVoH58JNXRgQ&r=_QIjpv-UJ77xEQY8fIYoQtr5qv8wKrPJc7v7_-CYAb0&m=wUYi2DhIZXHwD19wGIoEgceIF0cE0chTe7swjZ4MLVs&s=ZqrwtvXGdvXw-YJQtVnWo7yBeH5lhM3jtnYaa6vFZJY&e=
>>>
>>>
>>> Marek
>>>
>>> On Thu, Jan 5, 2017 at 5:47 PM, Samuel Pitoiset
>>> <samuel.pitoiset at gmail.com> wrote:
>>>> As explained by Nicolai, it seems like D3D always compute the
>>>> absolute value while GLSL says that the result of inversesqrt()
>>>> is undefined if x <= 0. Using the absolute value looks like safer
>>>> especially when the game has been ported from D3D to GL.
>>>>
>>>> This gets rid of the NaN values in the "Spec Ops: The Line" game
>>>> as well as the black squares.
>>>>
>>>> Bugzilla:
>>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__bugs.freedesktop.org_show-5Fbug.cgi-3Fid-3D97338&d=DgIGaQ&c=uilaK90D4TOVoH58JNXRgQ&r=_QIjpv-UJ77xEQY8fIYoQtr5qv8wKrPJc7v7_-CYAb0&m=wUYi2DhIZXHwD19wGIoEgceIF0cE0chTe7swjZ4MLVs&s=gjxDxmhEtQDYO34rucXJ6nIDojabOhpTAx1rNUP6X-g&e=
>>>>
>>>>
>>>> Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
>>>> ---
>>>>
>>>> Nouveau also computes the absolute value before emitting RSQ.
>>>>
>>>>  src/gallium/drivers/radeonsi/si_shader_tgsi_alu.c | 9 ++++++---
>>>>  1 file changed, 6 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/src/gallium/drivers/radeonsi/si_shader_tgsi_alu.c
>>>> b/src/gallium/drivers/radeonsi/si_shader_tgsi_alu.c
>>>> index 1966752cc0..ec6d6b0534 100644
>>>> --- a/src/gallium/drivers/radeonsi/si_shader_tgsi_alu.c
>>>> +++ b/src/gallium/drivers/radeonsi/si_shader_tgsi_alu.c
>>>> @@ -754,9 +754,12 @@ static void emit_rsq(const struct
>>>> lp_build_tgsi_action *action,
>>>>                      struct lp_build_tgsi_context *bld_base,
>>>>                      struct lp_build_emit_data *emit_data)
>>>>  {
>>>> -       LLVMValueRef sqrt =
>>>> -               lp_build_emit_llvm_unary(bld_base, TGSI_OPCODE_SQRT,
>>>> -                                        emit_data->args[0]);
>>>> +       LLVMValueRef abs, sqrt;
>>>> +
>>>> +       abs = lp_build_emit_llvm_unary(bld_base, TGSI_OPCODE_ABS,
>>>> +                                      emit_data->args[0]);
>>>> +
>>>> +       sqrt = lp_build_emit_llvm_unary(bld_base, TGSI_OPCODE_SQRT,
>>>> abs);
>>>>
>>>>         emit_data->output[emit_data->chan] =
>>>>                 lp_build_emit_llvm_binary(bld_base, TGSI_OPCODE_DIV,
>>>> -- 
>>>> 2.11.0
>>>>
>>>> _______________________________________________
>>>> mesa-dev mailing list
>>>> mesa-dev at lists.freedesktop.org
>>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.freedesktop.org_mailman_listinfo_mesa-2Ddev&d=DgIGaQ&c=uilaK90D4TOVoH58JNXRgQ&r=_QIjpv-UJ77xEQY8fIYoQtr5qv8wKrPJc7v7_-CYAb0&m=wUYi2DhIZXHwD19wGIoEgceIF0cE0chTe7swjZ4MLVs&s=boy8sEj2W3FP0LnjHsSnUxWrR98aM22Qq-uxvFIqMlQ&e=
>>>>
>>> _______________________________________________
>>> mesa-dev mailing list
>>> mesa-dev at lists.freedesktop.org
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.freedesktop.org_mailman_listinfo_mesa-2Ddev&d=DgIGaQ&c=uilaK90D4TOVoH58JNXRgQ&r=_QIjpv-UJ77xEQY8fIYoQtr5qv8wKrPJc7v7_-CYAb0&m=wUYi2DhIZXHwD19wGIoEgceIF0cE0chTe7swjZ4MLVs&s=boy8sEj2W3FP0LnjHsSnUxWrR98aM22Qq-uxvFIqMlQ&e=
>>>
>>>
>>