[petsc-dev] fortran literals

[William Gropp]

Its my mistake - the kind values are not defined, but using the intrinsics to select a kind value based on precision and/or range is well define.  My apologies for the confusion.

Bill

William Gropp
Acting Director and Chief Scientist, NCSA
Director, Parallel Computing Institute
Thomas M. Siebel Chair in Computer Science
University of Illinois Urbana-Champaign





On Sep 2, 2016, at 11:40 AM, Munson, Todd <tmunson at mcs.anl.gov> wrote:

> 
> I am confused by Blaise's statement.  "selected_real_kind(5)" is well defined across
> compilers; it returns an integer value that can be used in the kind statements and
> selected real numbers with at least 5 digits of precision.
> 
> With the exception of -1, -2, and -3 (plus -4 and -5 in Fortran 2008) indicating 
> no support.  The actual integer values returned are compiler specific.
> 
> If we wanted to have a better chance of getting ieee single, double, and quad, 
> we should probably use something like:
> 
>   selected_real_kind(6,37)
>   selected_real_kind(15,307)
>   selected_real_kind(33,4931)
> 
> in PETSc.  You would still need to do some testing during configuration to 
> really ensure that
> 
>   real(kind=selected_real_kind(7,38))
>   real(kind=selected_real_kind(15,307))
>   real(kind=selected_real_kind(34,4931))
> 
> is the same as a C float, double, and __float128 respectively?  
> 
> =====
> 
> Its not until Fortran 2008 that the standard has constants for REAL32, REAL64, 
> and REAL128 defined in the ISO_FORTRAN_ENV module.  However, if we are using
> Fortran 2008 features, go all out and use the ISO_C_BINDING module with 
> the C_FLOAT and C_DOUBLE constants!  Some compilers, such as the gnu
> compilers. extend the constants and include C_FLOAT128.
> 
> I have attached a simple program to output the kinds.  Try it on your machine.
> On my machine with gfortran from gcc version 4.8.5 (MacPorts gcc48 4.8.5_0), I
> get the following:
> 
>  C_FLOAT   :            4
>  C_DOUBLE  :            8
>  C_FLOAT128:           16
>  REAL32    :            4
>  REAL64    :            8
>  REAL128   :           10
>  sel(6)    :            4
>  sel(15)   :            8
>  sel(16)   :           10
>  sel(33)   :           16
> 
> The integers provide the number of bytes on my machine.  Note REAL128 is 
> apparently an 80-bit long double and not a 128-bit __float128!!  The 
> type using selected_real_kind appears correct though.  The same 
> happens for REAL128 with gfortran from gcc 4.9.3, gcc 5.4.0, 
> and gcc 6.1.0 on my machine.
> 
> Todd.
> 
> 
> 
> > On Sep 2, 2016, at 12:51 AM, William Gropp <wgropp at illinois.edu> wrote:
> > 
> > This is true.  The meaning of integer values of kind is up to the implementation, and at least two choices are in use.
> > 
> > Bill
> > 
> > William Gropp
> > Acting Director and Chief Scientist, NCSA
> > Director, Parallel Computing Institute
> > Thomas M. Siebel Chair in Computer Science
> > University of Illinois Urbana-Champaign
> > 
> > 
> > 
> > 
> > 
> > On Sep 1, 2016, at 6:15 PM, Blaise A Bourdin <bourdin at lsu.edu> wrote:
> > 
> >> If I recall correctly, fortran does not mandate that "selected_real_kind(5)" means the same across compilers, so that hardcoding kind values may not be portable.
> > 
> 
> <mytest.f>

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