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

[Marek Olšák]

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.

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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