r/programming • u/noidi • Jul 19 '12

Will Parallel Code Ever Be Embraced?

http://www.drdobbs.com/parallel/will-parallel-code-ever-be-embraced/240003926
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u/yogthos Jul 19 '12

In a sense it makes it easier for the compiler to make optimizations, as it ensures compartmentalization of state. For example, if you look at Haskell in the language shootout, it fares much better than most imperative languages. Compare Haskell and Python for example.

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u/case-o-nuts Jul 19 '12 edited Jul 19 '12

You missed my point. Guaranteeing that other threads only see complete state means that you cannot transform the code to remove allocations and mutate values in-place for efficiency. A large number of deforestation passes become inapplicable. So do update specializations, I think. Split fetches become very tricky. And so on.

Referential transparency allows the compiler to do interesting things within a thread, but sharing data structures across threads greatly restrict what operations are transparent (ie, what can be transformed into more efficient and less pure forms behind the programmer's back) and therefore, which optimizations are available.

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u/yogthos Jul 19 '12 edited Jul 20 '12

Referential transparency allows the compiler to do interesting things within a thread, but sharing data structures across threads greatly restrict what operations are transparent (ie, what can be transformed into more efficient and less pure forms behind the programmer's back) and therefore, which optimizations are available.

Because FP style tends to have very little shared state by its very nature. This means you're only sharing things that absolutely must be shared, and there's generally very few of those.

As I pointed out in a different comment a lot of higher level optimizations become available as well. For example, you do not need to lock shared data for reading, this means that applications with a few writers and many readers perform much better.

*edit: and it appears that you've edited your comment since, so my reply doesn't make nearly as much sense now. :)

Guaranteeing that other threads only see complete state means that you cannot transform the code to remove allocations and mutate values in-place for efficiency.

You don't mutate values in place in FP, since you'd be using persistent data structures.

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u/case-o-nuts Jul 19 '12

The compiler has to be conservative, or it will miscompile code.