Re: [Patch] Testsuite/Fortran: gfortran.dg/pr96711.f90 - fix expected value for PowerPC [PR96983]

[Segher Boessenkool <segher@kernel.crashing.org>]

On Wed, May 19, 2021 at 02:32:02PM +0200, Tobias Burnus wrote:
> Regarding gfortran.dg/pr96711.f90:
> 
> On my x86-64-gnu-linux, it PASSes.
> On our powerpc64le-linux-gnu it FAILS with
> 'STOP 3' (→ also scan-dump count) and 'STOP 4'.
> 
> Contrary to PR96983's bug summary, I don't get an ICE.
> 
> 
> On powerpc64le-linux-gnu, the following condition evaluates true (→ 'STOP 
> 3'):
> 
>    real(16)               :: y   ! 128bit REAL
>    integer(16), parameter :: k2 = nint (2 / epsilon (y), kind(k2))
>    integer(16), parameter :: m2 = 10384593717069655257060992658440192_16 
>    !2**113
>    if (k2 /= m2) stop 3
> 
> On x86_64-linux-gnu, k2 == m2 — but on powerpc64le-linux-gnu,
> k2 == 2**106 instead of 2**113.
> 
> My solution is to permit also 2**106 besides 2**113.
> 
> @PowerPC maintainers: Does this make sense? – It seems to work on our 
> PowerPC
> but with all the new 'long double' changes, does it also work for you?

I do not understand Fortran well enough, could you explain what the code
is supposed to do?

> 	PR fortran/96983
> 	* gfortran.dg/pr96711.f90:

You're missing the actual entry here, fwiw.

> -  integer(16), parameter :: m2 = 10384593717069655257060992658440192_16 !2**113
> +  integer(16), parameter :: m2 = 10384593717069655257060992658440192_16 !2**113  ! Some systems like x86-64
> +  integer(16), parameter :: m2a = 81129638414606681695789005144064_16   !2**106  ! Some systems like PowerPC

If you use double-double ("ibm long double") a number is represented as
the sum of two double precision numbers, while if you use IEEE quad
precision floating point you get a 112-bit fraction (and a leading one).
The most significant of the two DP numbers is the whole rounded to DP.
The actual precision varies, it depends on various factors :-/


Segher


>    integer(16), volatile  :: m
>    x = 2 / epsilon (x)
>    y = 2 / epsilon (y)
>    m = nint (x, kind(m))
>  ! print *, m
>    if (k1 /= m1) stop 1
>    if (m  /= m1) stop 2
>    m = nint (y, kind(m))
>  ! print *, m
> -  if (k2 /= m2) stop 3
> -  if (m  /= m2) stop 4
> +  if (k2 /= m2 .and. k2 /= m2a) stop 3
> +  if (m  /= m2 .and. m /= m2a) stop 4
>  end program