Re: [PATCH] frange: flush denormals to zero for -funsafe-math-optimizations.

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Sep 19, 2022 at 09:37:22AM +0200, Richard Biener wrote:
> On Sat, Sep 17, 2022 at 10:25 AM Aldy Hernandez via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > Jakub has mentioned that for -funsafe-math-optimizations we may flush
> > denormals to zero, in which case we need to be careful to extend the
> > ranges to the appropriate zero.  This patch does exactly that.  For a
> > range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x]
> > we flush to [+0.0, x].
> >
> > It is unclear whether we should do this for Alpha, since I believe
> > flushing to zero is the default, and the port requires -mieee for IEEE
> > sanity.  If so, perhaps we should add a target hook so backends are
> > free to request flushing to zero.
> >
> > Thoughts?
> 
> I'm not sure what the intention of this is - it effectively results in
> more conservative ranges for -funsafe-math-optimizations.  That is,
> if -funsafe-math-optimizations says that denormals don't exist and
> are all zero then doesn't that mean we can instead use the smalles
> non-denormal number less than (or greater than) zero here?

Yes, with -funsafe-math-optimizations we will sometimes have slightly
larger ranges, while when we know or assume we know that denormals
will be honored we can be more precise.
It is similar to the -fno-signed-zeros case, when we are more relaxed
and say we don't care about sign of zeros and optimizations can be
performed to leave the sign of zero in whatever state they like,
we can't have [+0.0, xx] or [xx, -0.0] ranges and need to extend them
to [-0.0, xx] or [xx, +0.0].  The honor denormals case is similar,
if there is the possibility of having denormals flushed to zero,
[+denormal, xx] needs to be extended to [+0.0, xx] and
[xx, -denormal] to [xx, -0.0].  Now, I think we just should add
a separate option whether denormals are honored or not, because
always extending the ranges with denormals on the boundaries might
prevent some useful optimizations.  If we add such an option, it
could default to not honoring them for -funsafe-math-optimizations,
and say on alpha it should default to not honoring them by default
always unless -mieee option is used, but all of this if the user
didn't use the option explicitly, in that case follow what the user
said.  That way if users know they set DTZ flag themselves or use
libraries that do so, they can tell the compiler about it.

> That said, the flushing you do is also "valid" for
> -fno-unsafe-math-optimizations
> in case we don't want to deal with denormals in range boundaries.

It is always valid to extend the range more, but could be harmful to
optimizations.  Say if we have then x > 0.0 test...
Of course, without -ffast-math we won't DCE various floating point
operations because they can raise exceptions, but at least we can DCE
the basic block after it that depends on x > 0.0.

	Jakub