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

[Roland Mainz]

On Mon, Jan 12, 2015 at 5:27 PM, Tom Stellard <tom at stellard.net> wrote:
> On Thu, Jan 08, 2015 at 01:46:32PM +0100, Marek Olšák wrote:
>> 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.
>>
>
> Is it documented anywhere how min/max and other tgsi comparison instructions
> are supposed to handle NaN and +-inf?  If not, we really need to get this
> documented somewhere.

Well, for |fmin()| and |fmax()| in ISO C99/C11 it works like this
(using ksh93 since it exposes these APIs directly has arithmetric
functions):
-- snip --
$ ksh93 -c 'float x=5 ; print -- $(( fmin(x,nan) ))'
5
$ ksh93 -c 'float x=5 ; print -- $(( fmin(x,-nan) ))'
5
$ ksh93 -c 'float x=5 ; print -- $(( fmax(x,-nan) ))'
5
$ ksh93 -c 'float x=5 ; print -- $(( fmax(x,nan) ))'
5
$ ksh93 -c 'float x=+inf ; print -- $(( fmax(x,nan) ))'
inf
$ ksh93 -c 'float x=-inf ; print -- $(( fmax(x,nan) ))'
-inf
-- snip --
(so not-a-number always "looses" in favor of a real number or +/-inf)

Note that Nan's have a sign (+/-) which must be honored and that
future revisions of the ISO C standard may include functions to access
the ISO754 Nan payloads (but that's off-topic))

----

Bye,
Roland

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