Re: RFA: Darwin x86 alignment

[Tobias Schlüter <tobias dot schlueter at physik dot uni-muenchen dot de>]

Mark Kettenis wrote:
>    From: Dale Johannesen <dalej@apple.com>
>    Date: Thu, 21 Jul 2005 16:56:01 -0700
> 
>    On x86 currently the alignments of double and long long are linked:
>    they are either 4 or 8 depending on whether -malign-double is set.
>    This follows the documentation of -malign-double.  But it's wrong for
>    what we want the Darwin ABI to be:  the default should be that double
>    is 4 bytes and long long is 8 bytes.
> 
> I have a strong suspicion there is a reason why the two are linked,
> and that that reason is FORTRAN.  A lot of FORTRAN code assumes
> EQUIVALENCE of floating-point and integer types of equal size.  Such
> code will in all likelyhood break if those types have different
> alignment.  For x86 this means that int/float and long long/double
> will have to have the same alignment.

This might indeed be a problem, as the alignments not only have to be the same
if they appear in an equivalence, but also in arrays or when using the
TRANSFER intrinsic.  Out of the types dicussed, the standard only specifies
this for default INTEGERs (=int in C) and default REALs (=float in C), but
users do expect this to consistently extend to bigger types, otherwise they
consider the compiler buggy instead of their code.

More precisely, the standard says this: a scalar variable of a certain type
occupies a certain number of "storage units".  Default INTEGERs, and REALs
take one storage unit, default COMPLEX and DOUBlE PRECISION (= REAL*8 = double
in C) take two storage units.  Finally, arrays of these types take a sequence
of contiguous storage units.  Details can be found in section 14.6.3 of the
Fortran 95 standard.  These sequences of storage units can be equivalenced
(i.e. a certain overlap between the sequences is requested by the user.  This
is only allowed for variables of the same type, but the extension to different
types is common and widely used).  Furthermore the TRANSFER intrinsic can be
used to copy the binary representation of variable/array A into variable/array
B; it is therefore also sensitive to memory layout.

- Tobi