[Mesa-dev] About tolerance calculation on specific (builtin) functions

[Andres Gomez]

On Wed, 2016-05-04 at 13:48 -0400, Ilia Mirkin wrote:
> On Wed, May 4, 2016 at 1:41 PM, Connor Abbott <cwabbott0 at gmail.com>
> wrote:
> > 
> > On Wed, May 4, 2016 at 1:05 PM, Andres Gomez <agomez at igalia.com>
> > wrote:
> > > 
> > > 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.
> > > 
> > > 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?
> > Hi,
> > 
> > If you look at the GLSL 4.40 spec, in section 4.7.1 ("Range and
> > Precision") you'll find a table listing the precision of various
> > operations. Your intuition about floor(), ceil(), etc. needing the
> > exact result is correct, as you can see. For doubles, it says "The
> > precision of double-precision operations is at least that of single
> > precision." Now, it's up for interpretation whether that means that
> > they must have the same *absolute* precision or the same ULP's (if
> > the
> > operation is not exact). For example inversesqrt() is listed at 2
> > ULP
> > for single precision, which means that there must be 24 - 2 = 22
> > bits
> > of precision. For doubles, are there still 22 bits of precision
> > required, or is the requirement really that there still be 2 ULP's
> > of
> > precision in which case there are 53 - 2 = 51 bits of precision. I
> > wrote the original lowering pass for inversesqrt() and friends
> > assuming the latter was correct, since it seems like the most sane
> > to
> > me (or else doubles would have no advantage over floats for
> > anything
> > except addition and multiplication).
> All of this is added by ARB_shader_precision (part of GLSL 4.10). It
> also punts on doubles, good to know that the latest specs have
> maintained the status quo. One interpretation of it is "you can
> implement doubles with float ops". Another is that the ULP's apply to
> doubles as well.
> 
> Prior to that ext, the precision of everything was to 10^-5.

Thanks Ilia and Connor, I really missed this one :/

I will check, then, this table and see if we can do something to
enhance the current tests.

-- 
-- 
Br,

Andres

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