Hi Jason, The DX10 version of min/max is indeed commutative. However, GLSL specs define min/max using a conditional assignment:
mix(x) = y < x ? y : x; max(x) = x < y ? y : x; which means that if either argument is NaN, x is returned., which means that you can combine open-coded min/max functions into min/max instructions if your hardware implements this behavior. It also means that if your hardware doesn't implement this behavior, you should lower min/max to conditional assignments if you want it to be super correct. Marek On Sat, Dec 20, 2014 at 3:09 AM, Jason Ekstrand <ja...@jlekstrand.net> wrote: > > > On Fri, Dec 19, 2014 at 6:02 PM, Matt Turner <matts...@gmail.com> wrote: >> >> On Fri, Dec 19, 2014 at 5:04 PM, Jason Ekstrand <ja...@jlekstrand.net> >> wrote: >> > v3: fmin and fmax technically aren't commutative or associative. Things >> > get funny when one of the arguments is a NaN. >> >> I have a difficult time believing that the GLSL definitions of min() >> and max() intentionally have that effect. >> >> To recap, the definition of min() in GLSL is >> >> > Returns y if y < x, otherwise it returns x. >> >> and since comparisons against NaN are always false, min(x, y) where x >> or y is NaN will always return x, regardless of which argument is NaN. >> That doesn't seem like a useful behavior. >> >> Indeed the GLSL spec says >> >> > Operations and built-in functions that operate on a NaN are not required >> > to return a NaN as the result. >> >> I think based on that text alone, min() and max() should be considered >> commutative and associative. > > > As a datapoint, apparently Merek (cc'd) bumped into this at one point on > some unreal game. Perhaps he could shed some light. > >> >> Ian, perhaps we should clarify this with Khronos? >> >> An IEEE 754 spec draft [1] defines minNum(x, y) as "y if x<y, x if >> y<x, the canonicalized floating-point >> number if one operand is a floating-point number and the other a NaN. >> Otherwise it is either x >> or y." > > > That seems to imply that min(x, NaN) = min(NaN, x) = x which would make it > commutative and associative. > >> >> i965's SEL instruction implements that behavior. >> >> [1] http://754r.ucbtest.org/drafts/archive/2006-10-04.pdf _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
