HP Fortran Programmer's Guide (September 2007)

Performance and optimization
Vectorization
Chapter 6160
If your PA2.0 application uses very large arrays, compiling with both +Ovectorize and
+Odataprefetch may also increase performance. The math library contains special
prefetching versions of the vector routines that are called if you specify both options.
If you compile with the +Ovectorize and +Oinfo options, the optimizer will identify which
loops it vectorized. If you find that the extent of vectorization is not significant, you may want
to consider some other optimization, such as parallelization.
Controlling vectorization locally
When you compile with the +Ovectorize option, the optimizer considers all loops in the
source file as candidates for vectorization. The *$* [NO]VECTORIZE directive enables you to
limit vectorization. You use the *$* NOVECTORIZE form of the directive to disable
vectorization and the *$* VECTORIZE form to enable it. The directive applies to the beginning
of the next loop and remains in effect for the rest of the program unit or until superseded by a
later directive. The directive is ignored if you do not compile with the +Ovectorize option
and specify an optimization of 3 or higher.
For example, if a file containing the following code segment were compiled with +Ovectorize,
only one loop would be considered as a candidate for vectorization:
! This is line 1 of the source file.
!*$* NOVECTORIZE
.
.
.
!*$* VECTORIZE
DO i = 1, 100
.
.
.
sdot Compute the dot product of two single-precision
vectors.
vec_damax Find the maximum absolute value in a
double-precision vector.
vec_dmult_add Multiply a scalar by a vector and add the result to
the result vector, using double-precision operands.
vec_dsum Sum the elements of a double-precision vector.
Table 6-5 Vector routines called by +Ovectorize (Continued)
Vector routine Description