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

[Aldy Hernandez <aldyh@redhat.com>]

On 11/18/22 11:44, Jakub Jelinek wrote:
> On Fri, Nov 18, 2022 at 11:37:42AM +0100, Aldy Hernandez wrote:
>>> Practically strictly
>>> preserving IEEE exceptions is only important for a very small audience, and
>>> for that even INEXACT will matter (but we still have -ftrapping-math
>>> by default).
>>> For that audience likely all constant / range propagation is futile and thus the
>>> easiest thing might be to simply cut that off completely?
>>>
>>> I'd say what ranger does is reasonable with -ftrapping-math given the current
>>> practice of handling this option.  There's no point in trying to preserve the
>>> (by accident) "better" handling without ranger.  Instead as Joseph says somebody
>>> would need to sit down, split -ftrapping-math, adjust the default and thorougly
>>> document things (also with -fnon-call-exceptions which magically makes
>>> IEEE flag raising operations possibly throw exceptions).  As there's currently
>>> no code motion barriers for FP code with respect to exception flag inspection
>>> any dead code we preserve is likely going to be unhelpful.
>>>
>>> So for now simply amend the documentation as to what -ftrapping-math
>>> currently means with respect to range/constant propagation?
>>
>> So something like "Even in the presence of -ftrapping-math, VRP may fold
>> operations that may cause exceptions  For example, an addition that is
>> guaranteed to produce a NAN, may be replaced with a NAN, thus eliding the
>> addition.  This may cause any exception that may have been generated by the
>> addition to not appear in the final program."
>>
>> ??
> 
> If we just adjust user expectations for -ftrapping-math, shouldn't we
> introduce another option that will make sure we never optimize away floating
> point operations which can trap (and probably just disable frange for that
> mode)?

That seems like a big hammer, but sure.  We could change 
frange::supports_p() to return false for flag_severely_limiting_option :).

Aldy