Re: [PATCH] Decimal floats can never be an frange::singleton_p.

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Sep 05, 2022 at 03:47:01PM +0200, Aldy Hernandez wrote:
> As Jakub mentioned in the PR, because many numbers have multiple
> possible representations, we can't reliably return true here.
> 
> I'll commit this if tests pass.
> 
> I wonder if its worth even handling decimal floats in frange, since
> there's a lot of things we can't represent.  I suppose even though we
> could never propagate an actual value with VRP, we could fold
> conditionals (symbolic and stuff outside ranges, etc) ??.  
> 
> Thoughts?
> 
> 	PR middle-end/106831
> 
> gcc/ChangeLog:
> 
> 	* value-range.cc (frange::singleton_p): Return false for
> 	DECIMAL_FLOAT_TYPE_P.
> ---
>  gcc/value-range.cc | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/gcc/value-range.cc b/gcc/value-range.cc
> index c3f668a811a..12a3750d078 100644
> --- a/gcc/value-range.cc
> +++ b/gcc/value-range.cc
> @@ -647,6 +647,14 @@ frange::singleton_p (tree *result) const
>        if (HONOR_NANS (m_type) && !get_nan ().no_p ())
>  	return false;
>  
> +      // Because the significand is not normalized (there is no
> +      // implicit leading "1").  Most values with less than 7
> +      // significant digits have multiple possible representations.
> +      // Zero has 192 possible representations (or twice that for
> +      // signed zeros).

The exact details (7, 192) are dependent on the format, for
decimal64 it is (16, 768) and for decimal128 (34, 12288).

	Jakub