On Mon, Aug 13, 2007 at 07:35:31PM +0100, Andrew Coppin wrote:
Not related to optimisation, but... is there some switch to warn you if something gets removed? (Presumably this means you forgot to export something, or you haven't coded the bit that calls it yet or something.)
-fwarn-unused-binds (included in -Wall)
(I once compiled a program that used the GHC API. The final binary was several times larger than ghc.exe...)
GHC is a particularly bad case because what it does is determined by the settings of a bunch of switches in the configuration data. Of course, GHC isn't smart enough to perform inter-module control flow analysis, so even with -split-objs you'd probably still link most of GHC.
I read somewhere that if one funtion returns a tuple, and the caller immediately extracts the values from the tuple, GHC tries to optimise away the tuple - so it's as if the function can just return multiple values at once. Is this true? Does it apply only to tuples, or to all types?
This is called the Constructed Product Return (CPR) analysis, and it applies to all types with one constructor (in archaic jargon, product types).
Right. So it doesn't have to have strict fields or anything? Just has to have exactly one constructor?
Yep, CPR is completely independent of strictness, however the returned product must be *new*, since returning an old object by value risks losing sharing (and thus creating large memory leaks). Stefan