On Thu, Apr 20, 2023 at 09:14:10PM -0400, Siddhesh Poyarekar wrote:
> On 2023-04-20 13:57, Siddhesh Poyarekar wrote:
> > For bounds that aren't representable, one could get error bounds from
> > libm-test-ulps data in glibc, although I reckon those won't be
> > exhaustive. From a quick peek at the sin/cos data, the arc target seems
> > to be among the worst performers at about 7ulps, although if you include
> > the complex routines we get close to 13 ulps. The very worst
> > imprecision among all math routines (that's gamma) is at 16 ulps for
> > power in glibc tests, so maybe allowing about 25-30 ulps error in bounds
> > might work across the board.
>
> I was thinking about this a bit more and it seems like limiting ranges to
> targets that can generate sane results (i.e. error bounds within, say, 5-6
> ulps) and for the rest, avoid emitting the ranges altogether. Emitting a bad
> range for all architectures seems like a net worse solution again.
Well, at least for basic arithmetics when libm functions aren't involved,
there is no point in disabling ranges altogether.
And, for libm functions, my plan was to introduce a target hook, which
would have combined_fn argument to tell which function is queried,
machine_mode to say which floating point format and perhaps a bool whether
it is ulps for these basic math boundaries or results somewhere in between,
and would return in unsigned int ulps, 0 for 0.5ulps precision.
So, we could say for CASE_CFN_SIN: CASE_CFN_COS: in the glibc handler
say that ulps is say 3 inside of the ranges and 0 on the boundaries if
!flag_rounding_math and 6 and 2 with flag_rounding_math or whatever.
And in the generic code don't assume anything if ulps is say 100 or more.
The hooks would need to be a union of precision of supported versions of
the library through the history, including say libmvec because function
calls could be vectorized.
And default could be that infinite precision.
Back in November I've posted a proglet that can generate some ulps from
random number testing, plus on glibc we could pick maximums from ulps files.
And if needed, say powerpc*-linux could override the generic glibc
version for some subset of functions and call default otherwise (say at
least for __ibm128).
Jakub
