XL Fortran Enterprise Edition for AIX : User's Guide - IBM

-qhot Option
Syntax
-qhot[=suboptions] | -qnohot
HOT[=suboptions] | NOHOT
The -qhot compiler option is a powerful alternative to hand tuning that provides
opportunities to optimize loops and array language. The -qhot compiler option
will always attempt to optimize loops, regardless of the suboptions you specify.
If you do not specify an optimization level of 2 or higher when using -O and
-qhot, the compiler assumes -O2.
For additional information on loop unrolling, see the Directives for Loop
Optimization section in the XL Fortran Enterprise Edition for AIX Language Reference.
Array Padding: In XL Fortran, array dimensions that are powers of two can lead
to a decrease in cache utilization. The arraypad suboption allows the compiler to
increase the dimensions of arrays where doing so could improve the efficiency of
array-processing loops. This can reduce cache misses and page faults that slow
your array processing programs.
Specify -qhot=arraypad when your source includes large arrays with dimensions
that are powers of 2. This can be particularly effective when the first dimension is
a power of 2.
The -C option turns off some array optimizations.
Vectorization: The -qhot compiler option supports the vector suboption that can
optimize loops in source code for operations on array data, by ensuring that
operations run in parallel where applicable. The compiler uses standard registers
with no vector size restrictions. Supporting single and double-precision
floating-point mathematics, users can typically see benefit when applying
–qhot=vector to applications with significant mathematical requirements.
Arguments
arraypad
The compiler pads any arrays where there could be an increase in cache
utilization. Not all arrays will necessarily be padded, and the compiler can
pad different arrays by different amounts.
arraypad=n
The compiler pads each array in the source. The pad amount must be a
positive integer value. Each array will be padded by an integral number of
elements. The integral value n must be multiples of the largest array
element size for effective use of arraypad. This value is typically 4, 8, or 16.
When you specify the arraypad and arraypad=n options, the compiler does not
check for reshaping or equivalences. If padding takes place, your program can
produce unexpected results.
vector | novector
The compiler converts certain operations in a loop that apply to successive
elements of an array into a call to a routine that is in the libxlopt.a library.
This call calculates several results at one time, which is faster than
calculating each result sequentially.
XL Fortran Compiler-Option Reference 171