Developer Guide and Reference

Contents

GAP Message (Diagnostic ID 30519)

Message

Insert a "%s parallel" statement right before the loop at line %d to parallelize the loop.

Advice

Add
"#pragma parallel"
before the specified loop. This
pragma
enables the parallelization of the loop at the specified line by ignoring assumed cross-iteration data dependencies.
Example
Consider the following:
void foo(float *a, int m, int n) { int i; for (i=0; i<n; i++) { a[i] = a[i+m]+1; } return; }
In this case, the compiler is unable to parallelize the loop without further information about
m
. For example, if
m
is negative, then each iteration will be dependent on the previous iteration. However, if
m
is known to be greater than
n
, you can parallelize the loop.
If you determine it is safe to do so, you can add the
pragma
as follows:
void foo(float *a, int m, int n) { int i; #pragma parallel for (i=0; i<n; i++) { a[i] = a[i+m]+1; } return; }

Verify

Confirm that any arrays in the loop do not have cross-iteration dependencies. A cross-iteration dependency exists if a memory location is modified in an iteration of the loop and accessed by a
read or a write statement
in another iteration of the loop.

Product and Performance Information

1

Intel's compilers may or may not optimize to the same degree for non-Intel microprocessors for optimizations that are not unique to Intel microprocessors. These optimizations include SSE2, SSE3, and SSSE3 instruction sets and other optimizations. Intel does not guarantee the availability, functionality, or effectiveness of any optimization on microprocessors not manufactured by Intel. Microprocessor-dependent optimizations in this product are intended for use with Intel microprocessors. Certain optimizations not specific to Intel microarchitecture are reserved for Intel microprocessors. Please refer to the applicable product User and Reference Guides for more information regarding the specific instruction sets covered by this notice.

Notice revision #20110804