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

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Nov 17, 2022 at 8:38 PM Jakub Jelinek via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Thu, Nov 17, 2022 at 06:59:45PM +0000, Joseph Myers wrote:
> > On Thu, 17 Nov 2022, Aldy Hernandez via Gcc-patches wrote:
> >
> > > So... is the optimization wrong?  Are we not allowed to substitute
> > > that NAN if we know it's gonna happen?  Should we also allow F F F F F
> > > in the test?  Or something else?
> >
> > This seems like the usual ambiguity about what transformations
> > -ftrapping-math (on by default) is meant to prevent.
> >
> > Generally it's understood to prevent transformations that add *or remove*
> > exceptions, so folding a case that raises "invalid" to a NaN (with
> > "invalid" no longer raised) is invalid with -ftrapping-math.  But that
> > doesn't tend to be applied if the operation raising the exceptions has a
> > result that is otherwise unused - in such a case the operation may still
> > be removed completely (the exception isn't properly treated as a side
> > effect to avoid dead code elimination; cf. Marc Glisse's -ffenv-access
> > patches from August 2020).  And it may often also not be applied to
> > "inexact".
>
> The problem is that the above model I'm afraid is largely incompatible with
> the optimizations ranger provides.
> A strict model where no operations that could raise exceptions are discarded
> is easy, we let frange optimize as much as it wants and just tell DCE not to
> eliminate operations that can raise exceptions.
> But in the model where some exceptions can be discarded if results are unused
> but not others where they are used, there is no way to distinguish between
> the result of the operation really isn't needed and ranger figured out a
> result (or usable range of something) and therefore the result of the
> operation isn't needed.
> Making frange more limited with -ftrapping-math, making it punt for
> operations that could raise an exception would be quite drastic
> pessimization.  Perhaps for -ftrapping-math we could say no frange value is
> singleton and so at least for most of operations we actually wouldn't
> optimize out the whole computation when we know the result?  Still, we could
> also just have
> r = long_computation (x, y, z);
> if (r > 42.0)
> and if frange figures out that r must be [256.0, 1024.0] and never NAN, we'd
> still happily optimize away the comparison.

Yes, I don't think singling out the singleton case will help.
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?

Richard.

>
>         Jakub
>