Re: [PATCH] [range-ops] Implement sqrt.

[Jeff Law <jeffreyalaw@gmail.com>]

On 11/14/22 00:45, Aldy Hernandez via Gcc-patches wrote:
> On Sun, Nov 13, 2022 at 9:39 PM Jakub Jelinek <jakub@redhat.com> wrote:
>> On Sun, Nov 13, 2022 at 09:05:53PM +0100, Aldy Hernandez wrote:
>>> It seems SQRT is relatively straightforward, and it's something Jakub
>>> wanted for this release.
>>>
>>> Jakub, what do you think?
>>>
>>> p.s. Too tired to think about op1_range.
>> That would be multiplication of the same value twice, i.e.
>> fop_mult with trio that has op1_op2 () == VREL_EQ?
>> But see below, as sqrt won't be always precise, we need to account for
>> some errors.
>>
>>> gcc/ChangeLog:
>>>
>>>        * gimple-range-op.cc (class cfn_sqrt): New.
>>>        (gimple_range_op_handler::maybe_builtin_call): Add cases for sqrt.
>> Yes, I'd like to see SQRT support in.
>> The only thing I'm worried is that unlike {+,-,*,/}, negation etc. typically
>> implemented in hardware or precise soft-float, sqrt is often implemented
>> in library using multiple floating point arithmetic functions.  And different
>> implementations have different accuracy.
>>
>> So, I wonder if we don't need to add a target hook where targets will be
>> able to provide upper bound on error for floating point functions for
>> different floating point modes and some way to signal unknown accuracy/can't
>> be trusted, in which case we would give up or return just the range for
>> VARYING.
>> Then, we could write some tests that say in a loop constructs random
>> floating point values (perhaps sanitized to be non-NAN), calls libm function
>> and the same mpfr one and return maximum error in ulps.
>> And then record those, initially for glibc and most common targets and
>> gradually maintainers could supply more.
>>
>> If we add an infrastructure for that within a few days, then we could start
>> filling the details.  One would hope that sqrt has < 10ulps accuracy if not
>> already the 0.5ulp one, but for various other functions I think it can be
> I don't know what would possess me to think that sqrt would be easy
> ;-).  Sure, I can sink a few days to flesh this out if you're willing
> to review it.

To Jakub's concern.  I thought sqrt was treated like +-/* WRT accuracy 
requirements by IEEE.   ie, for any input there is a well defined answer 
for a confirming IEEE implementation.   In fact, getting to that .5ulp 
bound is a significant amount of the  cost for a NR or Goldschmidt (or 
hybrid) implementation if you've got a reasonable (say 12 or 14 bit) 
estimator and high performance fmacs.


Jeff