The Shape of Code

Commercial compiler implementors have to produce compilers that are capable of being used on a typical developer computer. A whole bunch of optimization techniques were known for years but could not be used because few computers had the available memory capacity (in the days when 2M was a lot of memory your author once attended a talk that presented some impressive results and was frustrated to learn that the typical memory footprint was 160M, who would ever imagine developers having so much memory to work within?) These days the available of gigabytes of storage has means that likely computer storage capacity is rarely a reason not to use some optimization technique, although the whole program optimization people are still out in the cold.

What is new these days is the general availability of multiple processors. The obvious use of multiple processors is to have make distribute the compilation load. The more interesting use is having the compiler apply different sets of optimizations techniques on different processors, picking the one that produces the highest quality code.

Optimizing code generation algorithms don’t appear to leave anything to chance and individually they generally don’t. However, selecting an order in which to apply individual optimization algorithms is something of a black art. In some cases code transformations made by one algorithm can interfere with the performance of another algorithm. In some cases the possibility of the interference is known and applies in one direction, choosing the appropriate relative ordering of the two algorithms solves the problem. In other cases the way in which two algorithms interfere with each other depends on the code being translated, now the ordering of the two algorithms becomes problematic. The obvious solution is to try both orderings and pick the one that produces the best result.

Several research groups have investigated the use of machine learning in compiler optimization. cTuning.org is a new project that aims to bring together groups interested in self-tuning adaptive computing systems based on statistical and machine learning techniques.
Commercial pressure is always forcing compiler implementors to produce faster code and use of machine learning techniques can produce some impressive results. Now that multi-processor systems are common it will not be long before compilers writers start to make use of the extra resources now available to them.

The safety critical people have problems trying to show the correctness of compiler output that has been generated by ‘fixed’ algorithms. It is not hard to envisage that in 10 years time all large production quality compilers will be using machine learning.