PGI Fortran Reference manual - The Portland Group

OpenMP Directives for FortranIn the presence of an IF clause, the parallel region is executed in parallel only if thecorresponding scalar_logical_expression evaluates to .TRUE.. Otherwise, the codewithin the region is executed by a single processor, regardless of the value of the environmentvariable OMP_NUM_THREADS.8.6.7. LASTPRIVATE(list)The LASTPRIVATE(list) clause specifies that the enclosing context's version of the variableis set equal to the private version of whichever thread executes the final iteration (for-loopconstruct).8.6.8. NOWAITThe NOWAIT clause overrides the barrier implicit in a directive. When you specify NOWAIT, itremoves the implicit barrier synchronization at the end of a for or sections construct.8.6.9. NUM_THREADSThe NUM_THREADS clause sets the number of threads in a thread team. The num_threadsclause allows a user to request a specific number of threads for a parallel construct. If thenum_threads clause is present, then8.6.10. ORDEREDThe ORDERED clause specifies that a loop is executed in the order of the loop iterations. Thisclause is required on a parallel FOR statement when an ordered directive is used in the loop.You use this clause in conjunction with a DO or SECTIONS construct to impose a serial order onthe execution of a section of code. If ORDERED constructs are contained in the dynamic extentof the DO construct, the ordered clause must be present on the DO directive.8.6.11. PRIVATEThe PRIVATE clause specifies that each thread should have its own instance of a variable.Therefore, variables specified in a PRIVATE list are private to each thread in a team. In effect,the compiler creates a separate copy of each of these variables for each thread in the team. Whenan assignment to a private variable occurs, each thread assigns to its local copy of the variable.When operations involving a private variable occur, each thread performs the operations using itslocal copy of the variable.Tips about private variables:‣ Variables declared private in a parallel region are undefined upon entry to the parallelregion. If the first use of a private variable within the parallel region is in a right-hand-sideexpression, the results of the expression will be undefined, indicating the probability of acoding error.PGI Fortran Reference Guide 161