I've been wondering if a nice option would be to be able to feed profiler information in at compile time and have it override the heuristics.

That way, inlining, specialization, SAT, etc., decisions could be made based on how the code actually gets used during a typical run of the program.

Cheers! -Tyson

Yes, that is the ultimate solution to resolving those things that are traditionally unknown by a compiler, and profile directed optimization could certainly be exploited by GHC (imagine having the compiler automatically run Quickcheck/HUnit tests to exercise the code..). See http://msdn.microsoft.com/en-us/library/e7k32f4k.aspx for details of a practical implementation of this strategy. I wouldn't be suprised if industrial virtual machine JIT compilers were making use of this sort of thing too. Another strategy is to having the runtime dynamically adjust how code is executed based on profiling information. See e.g. Adaptive Evalution of Non-Strict Programs by Robert Ennals (http://berkeley.intel-research.net/rennals/pubs/thesis.pdf). In the case of SAT this might involve the runtime switching to the non-SATed version of a function if it looks like most calls to it had a low iteration count. Cheers, Max