On Dec 20, 2007 7:42 PM, Sterling Clover
I'm curious how much of the unboxing helped performance and how much didn't. In my experience playing with this stuff, GHC's strictness analyzer has consistently been really excellent, given the right hints. Unboxed tuples are generally a win, but GHC was often smarter at unboxing ints than I was.
It really did help. I started with an implementation that used Ints, and this sped the program up by at least 2x. I think that's because of the bit-manipulation I'm doing. For example, Data.Bits defines the bitwise and operation on Ints as: (I# x#) .&. (I# y#) = I# (word2Int# (int2Word# x# `and#` int2Word# y#)) Which you can see has 3 conversions in it. The I# deconstruction/construction is also suspicious to me, but I don't know if there are performance costs there or not. Regardless, using Word# directly lets me write (assuming w1# and w2# are already words): w1# `and#` w2# Maybe better rewrite rules in the Data.Bits library would eliminate unnecessary conversions and this wouldn't be necessary/
Also, higher-order functions can be used fine, assuming they're not awful recursive, as long as you set GHC's unfolding threshold high enough. You can also manually force their inlining, to really get control. I found there tended to be a "sweet
I didn't know about unfolding, and I'll have to try that sometime. I found that inlining seemed to have negative affects as often as it did positive. I spent most of my time so far coming up with different algorithms - now that I've got a decent one, I'll have to play with inlining.
spot" for any given program, where much higher or lower would degrade performance. As far as removing the word2int# and soforth, you could probably just use words throughout?
I try to. I just couldn't figure out how to write a Word# value literally. For example, eqWord# compares Word# values. But this gives a type error: 1# `eqWord#` 1# I have to write int2word# 1# `eqWord#` int2word# 1#
Also, as we discovered the other week, you may want to be careful with bang patterns, as forcing strictness on already evaluated values takes some time. Although, SPJ did point out that this was significantly improved in the new GHC.
Good point, thanks. Justin