To enable the auto-parallelizer, use the -parallel (Linux* and Mac OS* X) or /Qparallel (Windows*) option. This option detects parallel loops capable of being executed safely in parallel and automatically generates multi-threaded code for these loops.
Note
You might need to set the KMP_STACKSIZE environment variable to an appropriately large size to enable parallelization with this option.
An example of the command using auto-parallelization is as follows:
Operating System |
Description |
|---|---|
Linux and Mac OS X |
ifort -c -parallel myprog.f |
Windows |
ifort -c /Qparallel myprog.f |
Auto-parallelization uses two specific directives, !DEC$ PARALLEL and !DEC$ NO PARALLEL.
Auto-parallelization Directives Format and Syntax
The format of an auto-parallelization compiler directive is:
Syntax |
|---|
<prefix> <directive> |
where the brackets above mean:
<xxx>: the prefix and directive are required
For fixed form source input, the prefix is !DEC$ or CDEC$
For free form source input, the prefix is !DEC$ only
The prefix is followed by the directive name; for example:
Syntax |
|---|
!DEC$ PARALLEL |
Since auto-parallelization directives begin with an exclamation point, the directives take the form of comments if you omit the -parallel (Linux) or /Qparallel (Windows) option.
The !DEC$ PARALLEL directive instructs the compiler to ignore dependencies that it assumes may exist and which would prevent correct parallelization in the immediately following loop. However, if dependencies are proven, they are not ignored.
The !DEC$ NOPARALLEL directive disables auto-parallelization for the following loop:
Example |
|---|
|
program main parameter (n=100 integer x(n),a(n) !DEC$ NOPARALLEL do i=1,n x(i) = i enddo !DEC$ PARALLEL do i=1,n a( x(i) ) = i enddo end |
Auto-parallelization Environment Variables
Auto-parallelization uses the following OpenMP* environment variables.
OMP_NUM_THREADS
OMP_SCHEDULE
KMP_STACKSIZE
See OpenMP* Environment Variables for more information about the default settings and how to use these variables.