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

[Richard Biener via Gcc-patches]

> Am 18.11.2022 um 11:44 schrieb Jakub Jelinek <ja...@redhat.com>:
> 
> 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)?

I think it’s just like -frounding-math and Fenv access - the intent is there 
but the implementation is known buggy (and disabling optimizations doesn’t 
fully fix it).

Richard 

>    Jakub
>