Setting Data Type and Alignment

Data alignment considerations apply to the following kinds of variables:

For best performance, align data as follows:

Causes of Unaligned Data and Ensuring Natural Alignment

For optimal performance, make sure your data is aligned naturally. A natural boundary is a memory address that is a multiple of the data item's size. For example, a REAL (KIND=8) data item aligned on natural boundaries has an address that is a multiple of 8. An array is aligned on natural boundaries if all of its elements are so aligned.

All data items whose starting address is on a natural boundary are naturally aligned. Data not aligned on a natural boundary is called unaligned data.

Although the Intel® Fortran Compiler naturally aligns individual data items when it can, certain Fortran statements can cause data items to become unaligned.

You can use the command-line option -align to ensure naturally aligned data, but you should check and consider reordering data declarations of data items within common blocks, derived-type structures, and record structures as follows:

The following statements can cause unaligned data:

To avoid unaligned data in a common block, derived-type structure, or record structure, use one or both of the following:

Other possible causes of unaligned data include unaligned actual arguments and arrays that contain a derived-type structure or record structure:

Checking for Inefficient Unaligned Data

During compilation, the Intel Fortran compiler naturally aligns as much data as possible. Exceptions that can result in unaligned data are described above.

Because unaligned data can slow run-time performance, it is worthwhile to:

There are two ways unaligned data might be reported:

Consider the following run-time message:

Unaligned access pid=24821 <a.out> va=140000154, pc=3ff80805d60, ra=1200017bc

This message shows that:

Ordering Data Declarations to Avoid Unaligned Data

For new programs or when the source declarations of an existing program can be easily modified, plan the order of your data declarations carefully to ensure the data items in a common block, derived-type structure, record structure, or data items made equivalent by an EQUIVALENCE statement will be naturally aligned.

Use the following rules to prevent unaligned data:

When declaring data, consider using explicit length declarations, such as specifying a KIND parameter. For example, specify INTEGER(KIND=4) (or INTEGER(4)) rather than INTEGER. If you do use a default size (such as INTEGER, LOGICAL, COMPLEX, and REAL), be aware that the compiler options
or -real_size{32|64|128} can change the size of an individual field's data declaration size and thus can alter the data alignment of a carefully planned order of data declarations.

Using the suggested data declaration guidelines minimizes the need to use the -align keyword options to add padding bytes to ensure naturally aligned data. In cases where the -align keyword options are still needed, using the suggested data declaration guidelines can minimize the number of padding bytes added by the compiler.

Arranging Data Items in Common Blocks

The order of data items in a common statement determine the order in which the data items are stored. Consider the following declaration of a common block named x:

logical (kind=2) flag
integer          iarry_i(3)
character(len=5) name_ch
common /x/ flag, iarry_i(3), name_ch

As shown in Figure 1-1, if you omit the appropriate Fortran command options, the common block will contain unaligned data items beginning at the first array element of iarry_i.

Figure 1-1 Common Block with Unaligned Data

As shown in Figure 1-2, if you compile the program units that use the common block with the
-align commons
option, data items will be naturally aligned.

Figure 1-2 Common Block with Naturally Aligned Data

Because the common block x contains data items whose size is 32 bits or smaller, specify the
-align commons
option. If the common block contains data items whose size might be larger than 32 bits (such as REAL (KIND=8) data), use the -align commons option.

If you can easily modify the source files that use the common block data, define the numeric variables in the COMMON statement in descending order of size and place the character variable last. This provides more portability, ensures natural alignment without padding, and does not require the command-line options -align commons or -align dcommons option:

logical (kind=2) flag
integer          iarry_i(3)
character(len=5) name_ch
common /x/ iarry_i(3), flag, name_ch

As shown in Figure 1-3, if you arrange the order of variables from largest to smallest size and place character data last, the data items will be naturally aligned.

Figure 1-3 Common Block with Naturally Aligned Reordered Data

When modifying or creating all source files that use common block data, consider placing the common block data declarations in a module so the declarations are consistent. If the common block is not needed for compatibility (such as file storage or Fortran 77 use), you can place the data declarations in a module without using a common block.

Arranging Data Items in Derived-Type Data

Like common blocks, derived-type structures can contain multiple data items (members).

Data item components within derived-type structures are naturally aligned on up to 64-bit boundaries, with certain exceptions related to the use of the SEQUENCE statement and Fortran options. See Options Controlling Alignment for information about these exceptions.

Intel Fortran stores a derived data type as a linear sequence of values, as follows:

Consider the following declaration of array CATALOG_SPRING of derived-type PART_DT:

module data_defs
type part_dt
integer           identifier
real              weight
character(len=15) description
end type part_dt
type(part_dt) catalog_spring(30)
end module data_defs