-qhot

The -qhot option is by far the most expensive option in terms of adding to the elapsed time of a compile. Adding this option often more than triples the compile time of a code.

The -qhot option performs several loop oriented optimizations:

• restructures loops with 3 or more loop indices to traverse the loop more efficiently.
• pads array dimensions and data objects to minimize cache misses. (Arrays with dimensions that are a power of two are particularly susceptible to cache misses).
• converts certain operations that are done on successive elements of an array (e.g. square root) into a "vector" operation that calculates several results at once at a much faster rate than calculating them sequentially.
• performs these optimization transformations on loops: scalar replacement, loop blocking, distribution, fusion, interchange, reversal, skewing, and unrolling.
• reduces the generation of temporary arrays.

The Fortran option -qreport=hotlist will produce a listing describing all of the transformations done by -qhot. The listing is in a file called program.lst where program.f is the name of the source code file that was compiled.

It is possible for the -qhot option to decrease the performance of a program if the compiler does not have enough information about loop bounds and array dimensions, so that it attempts inappropriate optimizations.