Andres Gomez <ago...@igalia.com> writes: > Hi, > > as part of the work done to "Add FP64 support to the i965 shader > backends" at: > https://bugs.freedesktop.org/show_bug.cgi?id=92760 > > I've been working to add piglit tests that would check the new features > added by this addition. > > Due to this, I've been checking and making modifications into the > builtin_functions*py module used by some generators. These modules use > automatic calculation of the tolerance when distance checking the > result of a function. > > As already stated in the good documentation of the module, the > tolerance is computed following what it is stated in OpenGL's specs: > > From the OpenGL 1.4 spec (2.1.1 "Floating-Point Computation"): > > "We require simply that numbers' floating-point parts contain > enough bits ... so that individual results of floating-point > operations are accurate to about 1 part in 10^5." > > Although the text is open to interpretation, and for specific > operations we take a little bit more flexible approach, basically, the > tolerance is calculated as: > > tolerance = <expected_value> / 10⁵ > > This makes sense since the precision of a floating point value gets > reduced while the number gets bigger[1]. > > Following this approach, for a number in the order of 40*10⁵, the > tolerance used is ~40. While this should be OK for most of the > functions, it seems to me that such a high tolerance should not be used > with specific functions, if any tolerance should be used at all. > > For example, when testing the "sign()" function, seems pretty obvious > that using a value of 40 in the tolerance of a function that should > return either 1.0, 0.0 or -1.0 doesn't make much sense.
That's an interesting interpretation. I would assume that a tolerance of 1 part in 10^5 for any of these three values would be somewhat less than 40 :) Although that does bring up an interesting point: To get things correct, probably the tolerance has to apply to all values in the chain, both the inputs, outputs and any intermediates. Consider for example the case of a = 40 * 10^5 + 1 b = 40 * 10^5 - 1 c = a - b What is the correct tolerance for c? eirik > A similar case is the "trunc" function and probably others, like > "floor", "ceil", "abs", etc. > > My conclusion is that it should be safe to assume no tolerance in this > functions and I could modify the algorithm used for them in the python > module but I wanted to have some feedback in case I'm not taking into > account something that advices against doing these modifications. > > Opinions? > > [1] https://en.wikipedia.org/wiki/IEEE_floating_point#Basic_and_interch > ange_formats > -- > Br, > > Andres > > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/mesa-dev _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
