Re: [PATCH] [ranger] x == -0.0 does not mean we can replace x with -0.0

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Aug 26, 2022 at 05:46:06PM +0200, Aldy Hernandez wrote:
> On the true side of x == -0.0, we can't just blindly value propagate
> the -0.0 into every use of x because x could be +0.0 (or vice versa).
> 
> With this change, we only allow the transformation if
> !HONOR_SIGNED_ZEROS or if the range is known not to contain 0.
> 
> Will commit after tests complete.
> 
> gcc/ChangeLog:
> 
> 	* range-op-float.cc (foperator_equal::op1_range): Do not blindly
> 	copy op2 range when honoring signed zeros.
> ---
>  gcc/range-op-float.cc | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/gcc/range-op-float.cc b/gcc/range-op-float.cc
> index ad2fae578d2..ff9fe312acf 100644
> --- a/gcc/range-op-float.cc
> +++ b/gcc/range-op-float.cc
> @@ -252,8 +252,21 @@ foperator_equal::op1_range (frange &r, tree type,
>    switch (get_bool_state (r, lhs, type))
>      {
>      case BRS_TRUE:
> -      // If it's true, the result is the same as OP2.
> -      r = op2;
> +      if (HONOR_SIGNED_ZEROS (type)
> +	  && op2.contains_p (build_zero_cst (type)))

What exactly does op2.contains_p for zero?
Does it use real_compare/real_equal under the hood, so it is
equivalent to op2 == 0.0 or op2 == -0.0, where both will be
true whether op2 is -0.0 or 0.0?  Or is it more strict and
checks whether it is actually a positive zero?
In any case, for HONOR_SIGNED_ZEROS, VARYING is unnecessary, all you
can do is extend the r range to contain both -0.0 and +0.0 if it contains
at least one of them.

> +	{
> +	  // With signed zeros, x == -0.0 does not mean we can replace
> +	  // x with -0.0, because x may be either +0.0 or -0.0.
> +	  r.set_varying (type);
> +	}
> +      else
> +	{
> +	  // If it's true, the result is the same as OP2.
> +	  //
> +	  // If the range does not actually contain zeros, this should
> +	  // always be OK.
> +	  r = op2;
> +	}

!HONOR_SIGNED_ZEROS doesn't imply that negative zeros won't appear,
but says that user doesn't care if he gets a positive or negative zero
(unless !MODE_HAS_SIGNED_ZEROS - in that case -0.0 doesn't exist
and one doesn't need to bother with it).

Now, if all the code setting franges makes sure that for
MODE_HAS_SIGNED_ZEROS && !HONOR_SIGNED_ZEROS if +0.0 or -0.0 are inside
of a range, then both -0.0 and +0.0 are in the range, then yes,
you can use r = op2;

>        // The TRUE side of op1 == op2 implies op1 is !NAN.
>        r.set_nan (fp_prop::NO);
>        break;

	Jakub