Using high-order loop analysis and transformations
High-order transformations are optimizations that specifically improve the performance of loops through techniques such as interchange, fusion, and unrolling.
The goals of these loop optimizations include:
- Reducing the costs of memory access through the effective use of caches and address translation look-aside buffers
- Overlapping computation and memory access through effective utilization of the data prefetching capabilities provided by the hardware
- Improving the utilization of microprocessor resources through reordering and balancing the usage of instructions with complementary resource requirements
- Generating SIMD vector instructions
- Generating calls to vector math library functions
To enable high-order loop analysis and transformations, use the -qhot option, which implies an optimization level of -O2. The following table lists the suboptions available for -qhot.
| Suboption | Behavior |
|---|---|
| level=0 | Instructs the compiler to perform a subset of high-order transformations that enhance performance by improving data locality. This suboption implies -qhot=novector and -qhot=noarraypad. This level is automatically enabled if you compile with -O3. |
| level=1 | This is the default suboption if you specify -qhot with no suboptions. This level is also automatically enabled if you compile with -O4 or -O5. This is equivalent to specifying -qhot=vector. |
| vector | When specified with -qnostrict and -qignerrno, or -O3 or a higher optimization level, instructs the compiler to transform some loops to use the optimized versions of various math functions contained in the MASS libraries, rather than use the system versions. The optimized versions make different trade-offs with respect to accuracy and exception-handling versus performance. This suboption is enabled by default if you specify -qhot with no suboptions. Also, specifying -qhot=vector with -O3 implies -qhot=level=1. |
| arraypad | Instructs the compiler to pad any arrays where it infers there might be a benefit and to pad by whatever amount it chooses. |
