Re: Type Alignment

[Jeff Squyres <jsquyres at open-mpi dot org>]

To clarify, we are perfectly happy to use sequenced derived  
datatypes.  Our goal is only to find out what the alignment of a  
given type is.  Over time, we independently grew 2 different tests  
for type alignment (the F77 and F90 tests that Craig sent), and only  
recently [accidentally] noticed that they were returning different  
answers.

Here's what we need:

1. What is a reliable / correct test to determine the alignment of a  
Fortran type?  (I don't care a whit about the previous tests -- if  
they're wrong, we'll throw them out)

2. Will this alignment be the same between compatible Fortran 77 and  
Fortran 90 compilers?  (e.g., compilers of the same vendor/version,  
or, in a trivial case, using the same "gfortran" to compile f77 and  
f90 compilation units)

3. Am I reading these replies correctly that the *alignment* of a  
type may change depending on whether a derived type is sequenced or  
not?  I can understand how members might be re-ordered in memory by  
the compiler (and that's not a problem for us), but can it really  
change the datatype's alignment?  If so, this could be a Big Problem  
for us.

Thanks for your time.



On Feb 21, 2006, at 5:19 PM, Richard E Maine wrote:

> On Feb 21, 2006, at 1:12 PM, Janne Blomqvist wrote:
>
> > There is no rule stating that aligment must be the same for common  
> > and
> > derived types, so perhaps both your tests are correct. Common is
> > limited in the sense that storage association is required to work,  
> > but
> > for a derived type the compiler is free to align in whatever way it
> > thinks provides the best performance. Unless you make the derived  
> > type
> > a sequence type, in which case the derived type should also obey the
> > rules of storage association.
>
> For nonsequence derived types, it is worse than that. Not only can  
> the compiler align things in any way, it can even re-order them. It  
> is *VERY* plausible that a compiler might do something like re- 
> order to put the things with the largest alignment requirements  
> first, thus minimizing padding requirements. I think I might have  
> seen compilers that did such reordering, though I wouldn't swear to  
> it.
>
> --
> Richard Maine                |  Good judgment comes from experience;
> Richard.Maine@nasa.gov       |  experience comes from bad judgment.
>                             |        -- Mark Twain


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