[Fortran, Patch] PR 34192 - Fix compile-time evaluation of NEAREST

[Jerry DeLisle]

Tobias Burnus wrote:
> :ADDPATCH fortran:
> 
> We had several problems with the compile-time evaluation for NEAREST for
> 0.0 and subnormal numbers.
> 
>>From the standard:
> 
> -------------------- cut ---------------
> 
> "NEAREST (X, S)
> Description. Returns the nearest different machine-representable number in a
> given direction.
>   X shall be of type real.
>   S shall be of type real and not equal to zero.
> Result Characteristics. Same as X.
> Result Value. The result has a value equal to the machine-representable number
> distinct from X and nearest to it in the direction of the infinity with the
> same sign as S.
> 
> NOTE 13.16 Unlike other floating-point manipulation functions, NEAREST operates
> on machine-representable numbers rather than model numbers. On many systems
> there are machine-representable numbers that lie between adjacent model
> numbers."
> 
> -------------------- cut ---------------
> 
> 
> Problems:
> 
> a) gfortran gave an range error for the returned subnormal number
> b) gfortran returned 0.0 as nearest number of 0.0, which is plainly wrong.
> 
> To (a): With the current patch, only for NaN an error is printed; one
> might extend it to +/-INF if it is regarded as useful.
> 
> To (b): The fix was done in three steps:
> 
> 1. Reverting to Steve's old patch
> 2. Fixing the returned result using Steve's suggestion
> 3. Fixing rounding of subnormal numbers
> 
> Thanks to Steve for the help! And to Paul Zimmermann, who send a similar
> suggestion as Steve's for (2).
> 
> Build and regression tested on x86-64. OK for the trunk?
> 
> Tobias
> 

Tobias, this looks good.  OK

> PS: Any idea how to test for INF and NAN? If I do "0.0/0.0" I get a
> compile-time error and -fno-range-check does not help. I cannot assign
> to a variable first as otherwise NEAREST is not evaluated at compile time.
> 

You can write a value to a string and test for "NaN" or "Infinity".  Its crude 
but effective.

Jerry