On Sun, Apr 14, 2013 at 2:36 PM, Marek Olšák <mar...@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 <srol...@vmware.com> > wrote: >> >> Am 14.04.2013 18:39, schrieb Marek Olšák: >> > On Sun, Apr 14, 2013 at 5:24 PM, Roland Scheidegger <srol...@vmware.com >> > <mailto:srol...@vmware.com>> wrote: >> > >> > Am 14.04.2013 10:12, schrieb jfons...@vmware.com >> > <mailto:jfons...@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 > > > > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > http://lists.freedesktop.org/mailman/listinfo/mesa-dev > _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
