Posted on February 2, 2018 by Troels Henriksen

A new version of Futhark has been released - source tarballs and full changelog here. By our usual standards, this release contains quite few breaking language changes. In fact, there are only two of them worth mentioning:

Range literals are now written without brackets. The syntax (-x) can no longer be used as an operator section partially applying subtraction. This means map (-x) a is no longer valid. The reason for this change is that we recently made a grammar simplification that made it ambiguous whether an expression (-x) should be interpreted as a parenthesised negation, or as an operator section. We picked the former (as does Haskell, from which we cribbed the syntax in the first place). The grammar simplification that brought up this issue is necessary to eventually support higher-order functions, so it is a small sacrifice compared to what we will gain.

However, there has been a good number of internal changes and improvements, which is exactly how I prefer it. Futhark really need not be a volatile or complex language; parallelism is supposed to be simple! The smarts should be in the compiler, not the language.

The most significant improvement is probably the new memory management strategy, which got its own blog post. There has also been a number of improvements to the included basis library (“futlib”), of which the most significant is probably the inclusion of a Fast Fourier Transform, which was developed as part of a bachelor’s project at DIKU.

Most of my development effort was actually spent on developing our upcoming technique of incremental flattening. This is a technique that attempts to address the issue that nested parallelism cannot be compiled in a way that is optimal for all data sets. Consider this simple case of nested parallelism:

map (\xs -> reduce (+) 0 xs) xss

Depending on the shape of the two-dimensional array xss, the optimal way of executing this expression may be to parallelise only the outermost map, while turning the reduce into a low-overhead sequential loop, or to produce a more expensive segmented reduction, but which exploits more parallelism. The core principle is that parallelism is not free, and the compiler should exploit only as much parallelism as necessary to fully exploit the hardware. Unfortunately, this depends on the size of the input data, which is not known at compile-time.

Our solution, which we call incremental flattening (or sometimes, versioned code), generates all possible parallelisations of a piece of code, and at run-time selects the version that exhibits the least amount of parallelism that still saturates the hardware we are running on. It already works pretty well - it can manage our entire test and benchmark suites correctly and without any real performance regressions, and in several cases with performance improvements. It is still not enabled by default, but can be tried out by setting the environment variable FUTHARK_VERSIONED_CODE before running the compiler. Hopefully it will turned on by default by the next release (and it will certainly be the subject of a blog post eventually). The biggest downside is that it increases compile times, because it naturally causes programs under compilation to increase significantly in size.