Setting Data Type and Alignment

Alignment of data affects these kinds of variables:

• Those that are dynamically allocated. (Dynamically allocated data allocated with ALLOCATE is 8-byte aligned.)

• Those that are members of a data structure

• Those that are global or local variables

• Those that are parameters passed on the stack

For best performance, align data as follows:

• Align 8-bit data at any address.

• Align 16-bit data to be contained within an aligned four byte word.

• Align 32-bit data so that its base address is a multiple of four.

• Align 64-bit data so that its base address is a multiple of eight.

• Align 128-bit data so that its base address is a multiple of sixteen (8-byte boundaries).

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® compiler naturally aligns individual data items when it can, certain Fortran statements can cause data items to become unaligned.

You can use the align command-option 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:

• Carefully specify the order and sizes of data declarations to ensure naturally aligned data.

• Start with the largest size numeric items first, followed by smaller size numeric items, and then non-numeric (character) data.

The !DEC$ ATTRIBUTES ALIGN directive specifies the byte alignment for a variable. It is not supported for ALLOCATABLE/POINTER variables.

Common blocks (COMMON statement), derived-type data, and Fortran 77 record structures (RECORD statement) usually contain multiple items within the context of the larger structure.

The following statements can cause unaligned data:

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

• For new programs or for programs where the source code declarations can be modified easily, plan the order of data declarations with care. For example, you should order variables in a COMMON statement such that numeric data is arranged from largest to smallest, followed by any character data (see the data declaration rules in Ordering Data Declarations to Avoid Unaligned Data below.

• For existing programs where source code changes are not easily done or for array elements containing derived-type or record structures, you can use command line options to request that the compiler align numeric data by adding padding spaces where needed.

Other possible causes of unaligned data include unaligned actual arguments and arrays that contain a derived-type structure or Intel Fortran record structure as detailed below.

• When actual arguments from outside the program unit are not naturally aligned, unaligned data access occurs. Intel Fortran assumes all passed arguments are naturally aligned and has no information at compile time about data that will be introduced by actual arguments during program execution.

• For arrays where each array element contains a derived-type structure or Intel Fortran record structure, the size of the array elements may cause some elements (but not the first) to start on an unaligned boundary.

• Even if the data items are naturally aligned within a derived-type structure without the SEQUENCE statement or a record structure, the size of an array element might require use of the -align records (Linux) or /align:records (Windows) option to supply needed padding to avoid some array elements being unaligned.

• If you specify -align norecords (Linux) or /align:norecords (Windows) or specify -vms (Linux) or /Qvms (Windows) without RECORDS no padding bytes are added between array elements. If array elements each contain a derived-type structure with the SEQUENCE statement, array elements are packed without padding bytes regardless of the Fortran command options specified. In this case, some elements will be unaligned.

• When the -align records (Linux) or /align:records (Windows)option is in effect, the number of padding bytes added by the compiler for each array element is dependent on the size of the largest data item within the structure. The compiler determines the size of the array elements as an exact multiple of the largest data item in the derived-type structure without the SEQUENCE statement or a record structure. The compiler then adds the appropriate number of padding bytes. For instance, if a structure contains an 8-byte floating-point number followed by a 3-byte character variable, each element contains five bytes of padding (16 is an exact multiple of 8). However, if the structure contains one 4-byte floating-point number, one 4-byte integer, followed by a 3-byte character variable, each element would contain one byte of padding (12 is an exact multiple of 4).

Checking for Inefficient Unaligned Data

During compilation, the Intel® 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:

• Double-check data declarations within common block, derived-type data, or record structures to ensure all data items are naturally aligned (see the data declaration rules in the subsection below). Using modules to contain data declarations can ensure consistent alignment and use of such data.

• Avoid the EQUIVALENCE statement or use it in a manner that cannot cause unaligned data or data spanning natural boundaries.

• Ensure that passed arguments from outside the program unit are naturally aligned.

• Check that the size of array elements containing at least one derived-type data or record structure (extension) cause array elements to start on aligned boundaries (see the previous subsection).

• There are two ways unaligned data might be reported:

• • During compilation, warning messages are issued for any data items that are known to be unaligned (unless you specify the -warn noalignments (or -W0) (Linux) or /warn:noalignments (/W0) (Windows) option that suppresses all warnings).

  • During program execution, warning messages are issued for any data that is detected as unaligned. The message includes the address of the unaligned access.

Consider the following run-time message:

This message shows that:

• The statement accessing the unaligned data (program counter) is located at 3ff80805d60

• The unaligned data is located at address 140000154

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 data, record structure, or data items made equivalent by an EQUIVALENCE statement will be naturally aligned.

Use the following rules to prevent unaligned data:

• Always define the largest size numeric data items first.

• If your data includes a mixture of character and numeric data, place the numeric data first.

• Add small data items of the correct size (or padding) before otherwise unaligned data to ensure natural alignment for the data that follows.

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 following compiler options 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 (Linux) or /align:keyword (Windows) options to add padding bytes to ensure naturally aligned data. In cases where the -align keyword (Linux) or /align:keyword (Windows) 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 determines the order in which the data items are stored. Consider the following declaration of a common block named X:

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 (Linux) or /align:commons (Windows) 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 (Linux) or /align:commons (Windows) 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 (Linux) or /align:commons (Windows) 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 Fortran command options -align commons (Linux) or -align dcommons (Windows) or -align commons (Linux) or /align:commons (Windows):

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 data may contain multiple data items (members).

Data item components within derived-type data will be naturally aligned on up to 64-bit boundaries, with certain exceptions related to the use of the SEQUENCE statement and Fortran options. See Alignment Options for links to resources about these exceptions.

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

• If you specify the SEQUENCE statement, the first data item is in the first storage location and the last data item is in the last storage location. The data items appear in the order in which they are declared. The Fortran options have no effect on unaligned data, so data declarations must be carefully specified to naturally align data. The -align sequence (Linux) or /align:sequence (Windows) option specifically aligns data items in a SEQUENCE derived-type on natural boundaries.

• If you omit the SEQUENCE statement, the Intel Fortran adds the padding bytes needed to naturally align data item components, unless you specify the -align norecords (Linux) or /align:norecords (Windows) option.

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

As shown in Figure 1-4, the largest numeric data items are defined first and the character data type is defined last. There are no padding characters between data items and all items are naturally aligned. The trailing padding byte is needed because CATALOG_SPRING is an array; it is inserted by the compiler when the -align records (Linux) or /align:records (Windows) option is in effect.

Figure 1-4 Derived-Type Naturally Aligned Data (in CATALOG_SPRING : ( ,))

Arranging Data Items in Intel Fortran Record Structures

Intel Fortran supports record structures provided by Intel Fortran. Intel Fortran record structures use the RECORD statement and optionally the STRUCTURE statement, which are extensions to the Fortran 77 and Fortran standards. The order of data items in a STRUCTURE statement determines the order in which the data items are stored.

Intel Fortran stores a record in memory as a linear sequence of values, with the record's first element in the first storage location and its last element in the last storage location. Unless you specify -align norecords (Linux) or /align:norecords (Windows) padding bytes are added if needed to ensure data fields are naturally aligned.

The following example contains a structure declaration, a RECORD statement, and diagrams of the resulting records as they are stored in memory:

 Figure 1-5 shows the memory diagram of record REC for naturally aligned records.

Figure 1-5 Memory Diagram of RECfor Naturally Aligned Records