User's Guide

Glossary

This glossary defines terms that are commonly used in this book. It includes definitions that were developed by the American National Standards Institute (ANSI) and entries from the IBM Dictionary of Computing.

A

active processors

See online processors.

alias

An alternative label for a data object or point in a computer program.

American National Standards Institute (ANSI)

An organization that is sponsored by the Computer and Business Equipment Manufacturers Association through which accredited organizations create and maintain voluntary industry standards.

asynchronously

The way in which signals that are caused by the execution of some code of a thread are generated.

automatic parallelization

The process by which the compiler attempts to parallelize both explicitly coded DO loops and DO loops generated by the compiler for array language.

B

bss storage

Uninitialized static storage.

busy-wait

The state in which a thread keeps executing in a tight loop looking for more work once it has completed all of its work and there is no new work to do.

C

chunk

A subset of consecutive loop iterations.

compile

To translate a program that is written in a high-level programming language into a machine language program. The program that performs this task is called a compiler.

D

data object

A variable, constant, or subobject of a constant.

data striping

Spreading data across multiple storage devices so that I/O operations can be performed in parallel for better performance. Also known as disk striping.

data type

The properties and internal representation that characterize data and functions.

denormalized number

A nonzero floating-point number that has the following properties:

• Its exponent has a reserved value, usually the format's minimum.
• Its explicit or implicit leading significand bit is zero.

dynamic dimensioning

The process of re-evaluating the bounds of a pointee array each time the pointee is referenced.

E

environment variable

A variable that describes the operating environment of the process.

executable program

A program that can be run. It consists of a main program, and, optionally, one or more subprograms or non-Fortran-defined external procedures, or both.

external name

The name of a common block, subroutine, or other global procedure, which the linker uses to resolve references from one compilation unit to another.

F

floating-point number

A real number that is represented by a pair of distinct numerals. The real number is the product of the fractional part, one of the numerals, and a value obtained by raising the implicit floating-point base to a power indicated by the second numeral.

Fortran (Formula Translation)

A high-level programming language that is used primarily for scientific, engineering, and mathematical applications.

function

A procedure that returns the value of a single variable and that usually has a single exit.

H

hard limit

A system resource limit that can only be raised or lowered by using root authority.

high order transformations

A type of optimization that restructures loops.

Hollerith constant

A string of any characters capable of representation by XL Fortran and preceded with nH, where n is the number of characters in the string.

I

IPA

Interprocedural analysis, a type of optimization that allows optimizations to be performed across procedure boundaries and across calls to procedures in separate source files.

i-node

The internal structure that describes the individual files in the operating system. There is one i-node for each file. An i-node contains the node, type, owner, and location of a file. A table of i-nodes is stored near the beginning of a file system. Synonym for file index.

intrinsic procedures

Fortran defines a number of procedures, called intrinsic procedures, that are available to any program.

L

link-edit

To create a loadable computer program by means of a linker.

linker

A program that resolves cross-references between separately compiled or assembled object modules and then assigns final addresses to create a single relocatable load module. If a single object module is linked, the linker simply makes it relocatable.

load balancing

An optimization strategy that aims at evenly distributing the work load among processors.

License Use Management (LUM)

For those customers who require a method of auditing the use of the compiler, the XL Fortran compiler can be license management (LM) controlled using LUM (License Use Management), previously known as the NetLS / iFOR/LS product.

M

_main

The default name given to a main program by the compiler if the main program was not named by the programmer.

main program

The first program unit to receive control when a program is run.

N

NaN

See not-a-number.

not-a-number (NaN)

A symbolic entity encoded in floating-point format. There are two types of NaNs. Signalling NaNs signal the invalid operation exception whenever they appear as operands. Quiet NaNs propagate through almost every arithmetic operation without signaling exceptions. Both types of NaNs represent anything that is not a number. The intent of the signaling NaN is to catch program errors, such as using an uninitialized variable. The intent of a quiet NaN is to propagate a NaN result through subsequent computations.

O

online processors

Also known as active processors. In a multiprocessor machine, this refers to the processors which the system administrator has decided to activate (bring online). This number is less than or equal to the number of physical processors actually installed in the machine.

one-trip DO-loop

A DO loop that is executed at least once, if reached, even if the iteration count is equal to 0. (This type of loop is from FORTRAN 66.)

P

paging space

Disk storage for information that is resident in virtual memory but is not currently being accessed.

PDF

Profile-directed feedback, a type of optimization that uses information collected during application execution to improve performance of conditional branches and in frequently executed sections of code.

pointee array

Explicit-shape or assumed-size arrays that are declared in INTEGER pointer statements or other specification statements.

procedure

A computation that may be invoked during program execution. It may be a function or a subroutine. It may be an intrinsic procedure, an external procedure, a module procedure, an internal procedure, a dummy procedure, or a statement function. A subprogram may define more than one procedure if it contains ENTRY statements.

S

semantics

The relationships of characters or groups of characters to their meanings, independent of the manner of their interpretation and use. Contrast with syntax.

sleep

The state in which a thread completely suspends execution until another thread signals it that there is work to do.

soft limit

A system resource limit that is currently in effect for a process. The value of a soft limit can be raised or lowered by a process, without requiring root authority. The soft limit for a resource cannot be raised above the setting of the hard limit.

SMP

Symmetric Multi-Processor. A machine where the system looks exactly the same to each processor on the system.

spill space

The stack space reserved in each subprogram in case there are too many variables to hold in registers and the program needs temporary storage for register contents.

stanza

A group of lines in a file that together have a common function or define a part of the system. Stanzas are usually separated by blank lines or colons, and each stanza has a name.

subroutine

A procedure that is invoked by a CALL statement or by a defined assignment statement.

Symmetric Multi-Processor

See SMP.

synchronously

The way in which signals caused by interrupts are generated.

syntax

The rules for the construction of a statement. Constrast with semantics.

T

thread

A collection of processes whose order determines the process eligible for execution. A thread is the element that is scheduled and to which resources such as time slices, locks and queues may be assigned.

time slice

An interval of time on the processing unit allocated for use in performing a task. After the interval has expired, processing unit time is allocated to another task, so a task cannot monopolize processing unit time beyond a fixed limit.

trigger constant

Sequences of characters that identify comment lines as compiler comment directives.

U

Unicode

The informal name for the Universal Coded Character set (UCS), which is the name of the ISO 10646 standard that defines a single code for the representation, interchange, processing, storage, entry, and presentation of the written form of the world's major languages.

unsafe option

Any option that could result in grossly incorrect results if used in the incorrect context. Other options may result in very small variations from the default result, which is usually acceptable. Typically, using an unsafe option is an assertion that your code is not subject to the conditions that make the option unsafe.