[Mesa-dev] [PATCH 2/2] gallium: Desambiguate TGSI_OPCODE_IF.

[Alex Deucher]

On Sun, Apr 14, 2013 at 2:36 PM, Marek Olšák <maraeo at gmail.com> wrote:
> The R600 ISA documentation only says that the DX10 variants of MIN and MAX
> use DX10 handling of NaNs. It does not say anything about the non-DX10
> variants.

The difference is the NaN behavior.  The dx10 versions of MIN/MAX are
NaN safe.  There are also DX10 and non-DX10 versions of the SET*
opcodes.  The difference there is in the result:

SETE             A == B ? 1.0 : 0.0
SETE_DX10   A == B ?   -1 : 0
etc.

Alex

>
> Marek
>
>
> On Sun, Apr 14, 2013 at 8:16 PM, Roland Scheidegger <sroland at vmware.com>
> wrote:
>>
>> Am 14.04.2013 18:39, schrieb Marek Olšák:
>> > On Sun, Apr 14, 2013 at 5:24 PM, Roland Scheidegger <sroland at vmware.com
>> > <mailto:sroland at vmware.com>> wrote:
>> >
>> >     Am 14.04.2013 10:12, schrieb jfonseca at vmware.com
>> >     <mailto:jfonseca at vmware.com>:> -  TBD
>> >     > +  Start an IF ... ELSE .. ENDIF block.  Condition evaluates to
>> >     true if
>> >     > +
>> >     > +    src0.x != 0.0
>> >     > +
>> >     > +  where src0.x is interpreted as a floating point register.
>> >     Maybe should say something wrt evaluation of NaNs? I know we haven't
>> >     really established rules for comparisons etc. wrt NaNs but those
>> >     bools-as-float make me cry. I guess it is no different though than
>> > other
>> >     float opcodes, if we now really have a definition saying IF takes
>> > _any_
>> >     float not just a bool-as-float which was loosely implied before.
>> >
>> >
>> > I don't know where the term "bool-as-float" came from, but I'd rather
>> > not use it unless it's properly defined somewhere, and TGSI doesn't have
>> > bools anyway, so why bother? The GLSL compiler or glsl-to-tgsi is
>> > responsible for converting bools to either floats or ints and TGSI
>> > shouldn't need to care. Both r300g and r600g use (src0.x != 0.0) for IF
>> > and (src0.x != 0) for UIF (r600-only), so there is always the
>> > "not-equal-to" operator, which is also well defined for NaNs.
>> That depends on your definition of "well defined". llvm for instance has
>> both "ordered not equal" and "unordered not equal" operators for
>> precisely this reason. But yes I guess ieee-754 has some defined
>> behavior there.
>> That "bool-as-float" essentially comes from state trackers, because the
>> language they are translating from require bools as "if" inputs - hence
>> the input value always should have been the result of some comparison
>> (or similar) operation (which in turn return these fake bools).
>> But I agree this was never really documented, so just clearly stating
>> you can pass in any float is just fine (it means that state trackers now
>> are explicitly allowed to omit the comparison for simple cases like this
>> one, "if(a != 0)...", well if they can detect it, it was not really
>> obvious without documentation before if that would be ok). So in that
>> sense nothing more needs to be said about NaNs, since they just adhere
>> to the same rules as in other places (meaning pretty much undefined for
>> most things, currently).
>>
>> >
>> > Also if you care about NaNs, we should start by defining how
>> > instructions should handle them, e.g. how relational operators handle
>> > NaNs, whether the multiplication operator follows the rule 0*anything =
>> > 0 (MUL, MAD, DP4, ...), etc.
>> >
>> > R600 have separate opcodes depending on what behavior you want, for
>> > example:
>> > - The MUL opcode follows the rule 0*anything = 0. (DX9)
>> > - The MUL_IEEE opcode follows the IEEE behavior.
>> >
>> > The other opcodes with both the DX9 and IEEE behavior are: MAD, DP4,
>> > EX2, LG2, RCP, RSQ. There are also separate MIN and MAX opcodes for DX9
>> > and DX10. We should choose our opcodes carefully depending on whether we
>> > are implementing a DX9, DX10, OpenGL, or OpenCL state tracker.
>>
>> Yes indeed. d3d10 has quite strict rules which are mostly ieee754 (or
>> ieee754r) but with some deviations. Other specs tend to be more lenient,
>> and requiring strict rules could add quite some overhead, so we might
>> want to introduce additional opcodes. How does MIN/MAX work for dx9 btw?
>> DX10 will require you to give back the non-NaN value if only one
>> argument is NaN (which seems to be ieee754r behavior), which for
>> instance unfortunately doesn't translate well to sse2 code (as sse2 will
>> just give you the second source if there's a NaN in either src which
>> means you had to use cmp/select instead and be careful about what
>> comparison you use there since the cpu doesn't support the full set of
>> "ordered" and "unordered" comparisons unless you've got avx though
>> presumably llvm would take care of that if you use the right comparison
>> ops there).
>>
>> Roland
>
>
>
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