Re: [PATCH] Add set/get functions for negative infinity in real.*

[Jeff Law <jeffreyalaw@gmail.com>]

On 8/23/2022 4:33 AM, Aldy Hernandez via Gcc-patches wrote:
> For the frange implementation with endpoints I'm about to contribute,
> we need to set REAL_VALUE_TYPEs with negative infinity.  The support
> is already there in real.cc, but it is awkward to get at.  One could
> call real_inf() and then negate the value, but I've added the ability
> to pass the sign argument like many of the existing real.* functions.
>
> I've declared the functions in such a way to avoid changes to the
> existing code base:
>
> // Unchanged function returning true for either +-INF.
> bool real_isinf (const REAL_VALUE_TYPE *r);
> // New overload to be able to specify the sign.
> bool real_isinf (const REAL_VALUE_TYPE *r, int sign);
> // Replacement function for setting INF, defaults to +INF.
> void real_inf (REAL_VALUE_TYPE *, int sign = 0);
>
> Tested on x86-64 Linux.
>
> OK?
>
> gcc/ChangeLog:
>
> 	* real.cc (real_isinf): New overload.
> 	(real_inf): Add sign argument.
> 	* real.h (real_isinf): New overload.
> 	(real_inf): Add sign argument.

Funny in that I've fairly recently had the desire to do something a bit 
similar.  Let's consider 0.5, which we have a dconsthalf, but we don't 
have dconstmhalf for -0.5.  To get that value I create a dconsthalf 
object and flip its sign.  Similarly for a variety of other special 
constants (particularly powers of two, but a few others as well).

Consider making the "sign" argument a boolean.  It's defined as a single 
bit bitfield in the real_value structure.   We don't want folks to pass 
in values outside [0..1] for the sign if we can easily avoid it:-)



jeff