On Mon, Aug 29, 2022 at 03:13:21PM +0200, Aldy Hernandez wrote:
> It seems to me we can do this optimization regardless, but then treat
> positive and negative zero the same throughout the frange class.
> Particularly, in frange::singleton_p(). We should never return TRUE
> for any version of 0.0. This will keep VRP from propagating an
> incorrect 0.0, since all VRP does is propagate when a range is
> provably a singleton. Also, frange::zero_p() shall return true for
> any version of 0.0.
Well, I think for HONOR_SIGNED_ZEROS it would be nice if frange was able to
differentiate between 0.0 and -0.0.
One reason is e.g. to be able to optimize copysign/signbit - if we can
prove that the sign bit on some value will be always cleared or always set,
we can fold those.
On the other side, with -fno-signed-zeros it is invalid to use
copysign/signbit on values that could be zero (well, nothing guarantees
whether the sign bit is set or clear), so for MODE_HAS_SIGNED_ZEROS &&
!HONOR_SIGNED_ZEROS it is best to treat contains_p as {-0.0,0.0} being
one thing (just not singleton_p) and not bother with details like whether
a range ends or starts with -0.0 or 0.0, either of them would work the same.
And for !MODE_HAS_SIGNED_ZEROS, obviously 0.0 can be singleton_p.
Jakub
