The Atom toolkit includes a programmable instrumentation tool and several prepackaged tools. The prepackaged tools include:

• hiprof
  Produces a flat profile of an application that shows the execution time spent in a given procedure, and a hierarchical profile that shows the execution time spent in a given procedure and all of its descendents.
• pixie
  Produces a profile of an application, by procedure, source line, or instruction. It partitions the application into basic blocks and counts the number of times each basic block is executed.
• third
  Performs memory access checks and detects memory leaks in an application.

To invoke atom tools, use the following general command syntax:

% atom -tool tool-name ...)

For more information, see the DIGITAL UNIX Programmers Guide, atom(1), hiprof(5), pixie(5), and third(5).

Atom does not work on programs built with the -om option.

5.3 Data Alignment Considerations

For optimal performance on Alpha systems, make sure your data is aligned naturally.

A natural boundary is a memory address that is a multiple of the data item's size (data type sizes are described in Table 9-1). 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.

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 DIGITAL Fortran 90 compiler naturally aligns individual data items when it can, certain DIGITAL Fortran 90 statements (such as EQUIVALENCE) can cause data items to become unaligned (see Section 5.3.1).

Although you can use the f90 command -align keyword options to ensure naturally aligned data, you should check and consider reordering data declarations of data items within common blocks and structures. Within each common block, derived type, or record structure, 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 nonnumeric (character) data.

5.3.1 Causes of Unaligned Data and Ensuring Natural Alignment

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

The following declaration statements can force data to be unaligned:

• Common blocks (COMMON statement)
  The order of variables in the COMMON statement determines their storage order.
  Unless you are sure that the data items in the common block will be naturally aligned, specify either the -align commons or -align dcommons option, depending on the largest data size used.
  For examples and more information, see Section 5.3.3.1.
• Derived-type (user-defined) data
  Derived-type data members are declared after a TYPE statement.
  If your data includes derived-type data structures and you specify the -vms option, you should also specify the -align records option, unless you are sure that the data items in derived-type data structures will be naturally aligned.
  If you omit the SEQUENCE statement, the -align record option (default unless -vms option is specified) ensures all data items are naturally aligned.
  If you specify the SEQUENCE statement, the -align record option is prevented from adding necessary padding to avoid unaligned data (data items are packed). When you use SEQUENCE, you should specify data declaration order such that all data items are naturally aligned.
  For an example and more information, see Section 5.3.3.2.
• DIGITAL Fortran record structures (RECORD and STRUCTURE statements)
  DIGITAL Fortran record structures usually contain multiple data items. The order of variables in the STRUCTURE statement determines their storage order. The RECORD statement names the record structure. For examples and more information, see Section 5.3.3.3.
  If your data includes DIGITAL Fortran record structures and you specify the -vms option, you should also specify the -align records option, unless you are sure that the data items in derived-type data and DIGITAL Fortran record structures will be naturally aligned.
  For an example and more information, see Section 5.3.3.3.
• EQUIVALENCE statements
  EQUIVALENCE statements can force unaligned data or cause data to span natural boundaries. For more information, see the DIGITAL Fortran Language Reference Manual.

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 Section 5.3.3).
• 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 DIGITAL Fortran record structure.

When actual arguments from outside the program unit are not naturally aligned, unaligned data access will occur. DIGITAL Fortran 90 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 DIGITAL 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 f90 -align options to supply needed padding to avoid some array elements being unaligned.

If you specify -align norecords or specify -vms without -align 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 f90 command options specified. In this case, some elements will be unaligned.

When -align records 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).

For More Information:

On the -align keyword options, see Section 5.3.4.

5.3.2 Checking for Inefficient Unaligned Data

During compilation, the DIGITAL Fortran 90 compiler naturally aligns as much data as possible. Exceptions that can result in unaligned data are described in Section 5.3.1.

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 Section 5.3.3). 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 Section 5.3.1).

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 option).
• During program execution, warning messages are issued for any data that is detected as unaligned. The message includes the address of the unaligned access. You can use the ladebug debugger to locate unaligned data.
  The following run-time message shows that:
  • The statement accessing the unaligned data (program counter) is located at 3ff80805d60
  • The unaligned data is located at address 140000154

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

  
  To check where the address is located, use the debugger as described in Section 4.10.
  To suppress unaligned access run-time messages, use the uac command (see uac(1)).

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.
• Add small data items of the correct size (or padding) before otherwise unaligned data to ensure natural alignment for the data that follows.
• If your data includes a mixture of character and numeric data, place the numeric data first.

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.

5.3.3.1 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 5-1, if you omit the appropriate f90 command options, the common block will contain unaligned data items beginning at the first array element of IARRY_I.

Figure 5-1 Common Block with Unaligned Data

As shown in Figure 5-2, if you compile the program units that use the common block with the -align commons options, data items will be naturally aligned.

Figure 5-2 Common Block with Naturally Aligned Data

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

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 f90 command options -align commons or -align dcommons :

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 5-3, 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 5-3 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 DIGITAL Fortran 77 use), you can place the data declarations in a module without using a common block.

5.3.3.2 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 f90 options.

DIGITAL Fortran 90 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 f90 options have no effect on unaligned data, so data declarations must be carefully specified to naturally align data.
• If you omit the SEQUENCE statement, DIGITAL Fortran 90 adds the padding bytes needed to naturally align data item components, unless you specify the -align norecords option or the -vms option without -align records .

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

As shown in Figure 5-4, 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 option is in effect.

Figure 5-4 Derived-Type Naturally Aligned Data (in CATALOG_SPRING : ( ,))

DIGITAL Fortran 90 supports record structures provided by DIGITAL Fortran. DIGITAL Fortran record structures use the RECORD statement and optionally the STRUCTURE statement, which are extensions to the FORTRAN-77 and Fortran 90 standards. The order of data items in a STRUCTURE statement determine the order in which the data items are stored.

DIGITAL Fortran 90 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 the -vms option without the -align records option or specify -align norecords , 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:

STRUCTURE /STRA/ CHARACTER*1 CHR INTEGER*4 INT END STRUCTURE . . . RECORD /STRA/ REC

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

Figure 5-5 Memory Diagram of REC for Naturally Aligned Records

The following options control whether the DIGITAL Fortran 90 compiler adds padding (when needed) to naturally align multiple data items in common blocks, derived-type data, and DIGITAL Fortran record structures:

• The -align commons option requests that data in common blocks be aligned on up to 4-byte boundaries, by adding padding bytes as needed. Unless you specify -fast , the default is -align nocommons or arbitrary byte alignment of common block data. In this case, unaligned data can occur unless the order of data items specified in the COMMON statement places the largest numeric data item first, followed by the next largest numeric data (and so on), followed by any character data.
• The -align dcommons option requests that data in common blocks be aligned on up to 8-byte boundaries, by adding padding bytes as needed. Unless you specify -fast , the default is -align nocommons or arbitrary byte alignment of data items in a common data.
  Specify the -align dcommons option for applications that use common blocks, unless your application has no unaligned data or, if the application might have unaligned data, all data items are four bytes or smaller. For applications that use common blocks where all data items are four bytes or smaller, you can specify -align commons instead of -align dcommons .
• The -align norecords option requests that multiple data items in derived-type data and record structures (a DIGITAL Fortran extension) be aligned arbitrarily on byte boundaries instead of being naturally aligned. If you omit the -vms option, the default is -align records . If you specify the -vms option, -align norecords is used (unless you also specify -align records ).
• The -align records option requests that multiple data items in record structures (extension) and derived-type data without the SEQUENCE statement be naturally aligned, by adding padding bytes as needed. You only need to specify -align records if you specify the -vms option.
• The -align recnbyte option requests that fields of records and components of derived types be aligned on either the size byte boundary specified or the boundary that will naturally align them, whichever is smaller. This option does not affect whether common blocks are naturally aligned or packed.
• The -vms option controls certain VMS-related run-time defaults, including alignment (sets -align norecords option). If you specify the -vms option, you may need to also specify the -align records option to ensure that padding bytes are added.

The default behavior is that multiple data items in derived-type data and record structures will be naturally aligned; data items in common blocks will not ( -align records with -align nocommons ). In derived-type data, using the SEQUENCE statement prevents -align records from adding needed padding bytes to naturally align data items.