Re: [Haskell-cafe] Re: does the compiler optimize repeated calls?

On Wed, Sep 06, 2006 at 03:26:29PM +0200, Stephane Bortzmeyer wrote:

On Wed, Sep 06, 2006 at 02:12:28PM +0100, Sebastian Sylvan wrote a message of 36 lines which said:

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I think most compilers actually do CSE

And automatic memoization? Because common subexpressions can be difficult to recognize but, at run-time, it is much easier to recognize a function call that has already been done. Any common compiler / interpreter which automatically memoizes? In theory, it is also a huge advantage of "pure" functional programming languages.

Have you even considered the space costs of this? For almost any non-trivial bit of code I've written, automatic memoization would result in a completely useless binary, as it almost guarantees horrific space leaks. In order to do this, you'd need to have some rather sophisticated analysis to figure out the memory costs of both the output of a function, and its input, both of which would need to be stored in order to memoize the thing. Before something like this were to be implemented, I'd rather see automatic strictifying for avoidence of stack overflows (i.e. when the stack starts filling up, start evaluating things strictly), which is most likely another insanely dificult bit of compiler code, which would also involve figuring out which code paths would save memory, and which would increase the memory use. -- David Roundy