[Bug fortran/57871] gfortran -freal-4-real-16 gives wrong result for selected_real_kind(1)

["harper at msor dot vuw.ac.nz" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57871

--- Comment #5 from harper at msor dot vuw.ac.nz ---
I have now found two more oddities of type promotion but I don't claim 
that these are gfortran bugs, only that the mmanual might need amending.

Oddity 1. Although -freal-4-real-8 does what the manual implies with
the program below (making the real kinds 8,10,16 on an x86_64 system),
-fdefault-real-8 keeps the available kinds at 4,8,10,16 but merely
changes the defaults. I suggest that the following sentence 
be added to the manual sewction on -fdefault-real-8 at the end:

If "REAL(KIND=4)" is a real type available with no -f options, then it
remains available with -fdefault-real-8 though not with -freal-4-real-8.

Oddity 2. If one uses -fdefault-double-8 in a system where default real
was 4 bytes wide then one must also use -fdefault-real-8. The manual
entries for -fdefault-double-8 and -fdefault-real-8 are both silent 
on what happens if -fdefault-double-8 is given but -fdefault-real-8 is 
not. I won't suggest an amendment here because I don't know what happens 
in a system whose default real with no -f options was 8 bytes wide.

Evidence:

cayley[~/Jfh] % cat gfkinds.f90
implicit none
real     xdefault
real(4 ) x4
real(8 ) x8
real(10) x10
real(16) x16
print *,kind(xdefault),kind(x4),kind(x8),kind(x10),kind(x16)
end
cayley[~/Jfh] % gf -freal-4-real-8 gfkinds.f90
cayley[~/Jfh] % ./a.out
            8           8           8          10          16
cayley[~/Jfh] % gf -fdefault-real-8 gfkinds.f90
cayley[~/Jfh] % ./a.out
            8           4           8          10          16
cayley[~/Jfh] % gf -fdefault-double-8 gfkinds.f90
<built-in>:0:0: fatal error: Use of -fdefault-double-8 requires 
-fdefault-real-8
compilation terminated.
cayley[~/Jfh] % alias gf
/local/scratch/gf/bin/gfortran
cayley[~/Jfh] % gf -v
Using built-in specs.
COLLECT_GCC=/local/scratch/gf/bin/gfortran
COLLECT_LTO_WRAPPER=/local/scratch/gf/libexec/gcc/x86_64-unknown-linux-gnu/4.8.1/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /local/scratch/gcc-4.8-20130530/configure 
--prefix=/local/scratch/gf --enable-languages=c,fortran --disable-libada 
--with-local-prefix=/local/scratch --with-gmp=/local/scratch
Thread model: posix
gcc version 4.8.1 20130530 (prerelease) (GCC)
cayley[~/Jfh] %


On Wed, 10 Jul 2013, sgk at troutmask dot apl.washington.edu wrote:

> Date: Wed, 10 Jul 2013 20:02:43 +0000
> From: sgk at troutmask dot apl.washington.edu <gcc-bugzilla@gcc.gnu.org>
> To: john.harper@vuw.ac.nz
> Subject: [Bug fortran/57871] gfortran -freal-4-real-16 gives wrong result for
>     selected_real_kind(1)
> Resent-Date: Wed, 10 Jul 2013 20:02:59 +0000
> Resent-From: <john.harper@vuw.ac.nz>
> 
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57871
>
> --- Comment #3 from Steve Kargl <sgk at troutmask dot apl.washington.edu> ---
> On Wed, Jul 10, 2013 at 05:42:03PM +0000, kargl at gcc dot gnu.org wrote:
>>   implicit none
>>   integer,parameter:: p1 = selected_real_kind(1)
>>
>> This initialization expression is requesting the type with
>> smallest precision that exceeds 1.  With -freal-4-real-16,
>> that type is double precision (ie kind=8).  The problem
>> lies in gfc_simplify_selected_real_kind.  It has no knowledge
>> of the -freal-*-real-* options.  The loop (lines 5447-5457)
>
> (remove loop code)
>
>> searchs the ordered set [24, 53, 53, 113] (on FreeBSD-i386) or
>> [24, 53, 64, 113] (on FreeBSD-amd64).  In either case, the
>> initialization expression returns REAL(4).  However, with
>> -freal-4-real-16,  one might expect the ordered set to be
>> searched is [113, 53, 64, 113] as REAL(4) has been promoted
>> to REAL(16).
>
> The results of running John's code are odder than what one
> might expect.
>
> The normal results:
>
> % gfc4x -o foo foo.f90 && ./foo
> kind(1.0_p1)           4 precision(1.0_p1)           6
> kind(1.0_dp)           8 precision(1.0_dp)          15
>
> One might expect the first line to be 8 and 15, respectively.
>
> % gfc4x -o foo foo.f90 -freal-4-real-16 && ./foo
> kind(1.0_p1)          16 precision(1.0_p1)          33
> kind(1.0_dp)           8 precision(1.0_dp)          15
>
> Here the default real kind is promoted to REAL(8), but literal
> constants declared with a kind type suffix are not promoted.
> The first line is the correct output, although one might
> anticipate 8 and 15.  But, what may be surprising is that the
> double precision kind has been bumped to quad precision.
>
> troutmask:sgk[207] gfc4x -o foo foo.f90 -fdefault-real-8 && ./foo
> kind(1.0_p1)           4 precision(1.0_p1)           6
> kind(1.0_dp)          16 precision(1.0_dp)          33
>
> I've always hated these type options as a lzy programmer's
> crutch who refuses to properly port an algorithm from
> one precision to another.
>
> -- 
> You are receiving this mail because:
> You reported the bug.
>
>
>


-- John Harper, School of Mathematics Statistics and Operations Research
Victoria University, PO Box 600, Wellington 6140, New Zealand
e-mail john.harper@vuw.ac.nz phone (+64)(4)463 5276 fax (+64)(4)463 5045