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(g77-295.info)Aligned Data

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Aligned Data
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   On some systems, such as those with Pentium Pro CPUs, programs that
make heavy use of `REAL(KIND=2)' (`DOUBLE PRECISION') might run much
slower than possible due to the compiler not aligning these 64-bit
values to 64-bit boundaries in memory.  (The effect also is present,
though to a lesser extent, on the 586 (Pentium) architecture.)

   The Intel x86 architecture generally ensures that these programs will
work on all its implementations, but particular implementations (such
as Pentium Pro) perform better with more strict alignment.  (Such
behavior isn't unique to the Intel x86 architecture.)  Other
architectures might *demand* 64-bit alignment of 64-bit data.

   There are a variety of approaches to use to address this problem:

   * Order your `COMMON' and `EQUIVALENCE' areas such that the
     variables and arrays with the widest alignment guidelines come
     first.

     For example, on most systems, this would mean placing
     `COMPLEX(KIND=2)', `REAL(KIND=2)', and `INTEGER(KIND=2)' entities
     first, followed by `REAL(KIND=1)', `INTEGER(KIND=1)', and
     `LOGICAL(KIND=1)' entities, then `INTEGER(KIND=6)' entities, and
     finally `CHARACTER' and `INTEGER(KIND=3)' entities.

     The reason to use such placement is it makes it more likely that
     your data will be aligned properly, without requiring you to do
     detailed analysis of each aggregate (`COMMON' and `EQUIVALENCE')
     area.

     Specifically, on systems where the above guidelines are
     appropriate, placing `CHARACTER' entities before `REAL(KIND=2)'
     entities can work just as well, but only if the number of bytes
     occupied by the `CHARACTER' entities is divisible by the
     recommended alignment for `REAL(KIND=2)'.

     By ordering the placement of entities in aggregate areas according
     to the simple guidelines above, you avoid having to carefully
     count the number of bytes occupied by each entity to determine
     whether the actual alignment of each subsequent entity meets the
     alignment guidelines for the type of that entity.

     If you don't ensure correct alignment of `COMMON' elements, the
     compiler may be forced by some systems to violate the Fortran
     semantics by adding padding to get `DOUBLE PRECISION' data
     properly aligned.  If the unfortunate practice is employed of
     overlaying different types of data in the `COMMON' block, the
     different variants of this block may become misaligned with
     respect to each other.  Even if your platform doesn't require
     strict alignment, `COMMON' should be laid out as above for
     portability.  (Unfortunately the FORTRAN 77 standard didn't
     anticipate this possible requirement, which is
     compiler-independent on a given platform.)

   * Use the (x86-specific) `-malign-double' option when compiling
     programs for the Pentium and Pentium Pro architectures (called 586
     and 686 in the `gcc' configuration subsystem).  The warning about
     this in the `gcc' manual isn't generally relevant to Fortran, but
     using it will force `COMMON' to be padded if necessary to align
     `DOUBLE PRECISION' data.

     When `DOUBLE PRECISION' data is forcibly aligned in `COMMON' by
     `g77' due to specifying `-malign-double', `g77' issues a warning
     about the need to insert padding.

     In this case, each and every program unit that uses the same
     `COMMON' area must specify the same layout of variables and their
     types for that area and be compiled with `-malign-double' as well.
     `g77' will issue warnings in each case, but as long as every
     program unit using that area is compiled with the same warnings,
     the resulting object files should work when linked together unless
     the program makes additional assumptions about `COMMON' area
     layouts that are outside the scope of the FORTRAN 77 standard, or
     uses `EQUIVALENCE' or different layouts in ways that assume no
     padding is ever inserted by the compiler.

   * Ensure that `crt0.o' or `crt1.o' on your system guarantees a 64-bit
     aligned stack for `main()'.  The recent one from GNU (`glibc2')
     will do this on x86 systems, but we don't know of any other x86
     setups where it will be right.  Read your system's documentation
     to determine if it is appropriate to upgrade to a more recent
     version to obtain the optimal alignment.

   Progress is being made on making this work "out of the box" on
future versions of `g77', `gcc', and some of the relevant operating
systems (such as GNU/Linux).

   A package that tests the degree to which a Fortran compiler (such as
`g77') aligns 64-bit floating-point variables and arrays is available
at `ftp://alpha.gnu.org/gnu/g77/align/'.