[Mesa-dev] [PATCH 24/53] st/nine: Handle RSQ special cases

[Marek Olšák]

On Wed, Jan 7, 2015 at 7:09 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> On Wed, Jan 7, 2015 at 11:36 AM, Axel Davy <axel.davy at ens.fr> wrote:
>> We should use the absolute value of the input as input to ureg_RSQ.
>>
>> Moreover, an input of 0.0 should return FLT_MAX.
>>
>> Reviewed-by: David Heidelberg <david at ixit.cz>
>> Signed-off-by: Axel Davy <axel.davy at ens.fr>
>>
>> Cc: "10.4" <mesa-stable at lists.freedesktop.org>
>> ---
>>  src/gallium/state_trackers/nine/nine_shader.c | 13 ++++++++++++-
>>  1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/gallium/state_trackers/nine/nine_shader.c b/src/gallium/state_trackers/nine/nine_shader.c
>> index da77da5..4dee5f5 100644
>> --- a/src/gallium/state_trackers/nine/nine_shader.c
>> +++ b/src/gallium/state_trackers/nine/nine_shader.c
>> @@ -1957,6 +1957,17 @@ DECL_SPECIAL(POW)
>>      return D3D_OK;
>>  }
>>
>> +DECL_SPECIAL(RSQ)
>> +{
>> +    struct ureg_program *ureg = tx->ureg;
>> +    struct ureg_dst dst = tx_dst_param(tx, &tx->insn.dst[0]);
>> +    struct ureg_src src = tx_src_param(tx, &tx->insn.src[0]);
>> +    struct ureg_dst tmp = tx_scratch(tx);
>> +    ureg_RSQ(ureg, tmp, ureg_abs(src));
>> +    ureg_MIN(ureg, dst, ureg_imm1f(ureg, FLT_MAX), ureg_src(tmp));
>
> When would this MIN not return the value in tmp? In the description
> you say that RSQ(0.0) should return FLT_MAX... is the theory that
> MIN(NaN, FLT_MAX) == FLT_MAX? Is RSQ(0) in tgsi defined to return NaN?

No, because legacy versions of MIN and MAX aren't commutative. To
clarify this more, the legacy versions of MIN/MAX return x if either
operand is NaN (non-commutative), while DX10 versions return the one
that isn't NaN (commutative). For compatibility with the legacy
versions, the FLT_MAX constant must be in x.

Marek