On February 18, 2009 12:42:02 Tyson Whitehead wrote:
On February 18, 2009 04:29:42 Max Bolingbroke wrote:
Yes - GHC wants to share the work of (maxBound-x)`div`10 between several partial applications of "digit". This is usually a good idea, but in this case it sucks because it's resulted in a massively increased arity. IMHO GHC should fix this by: * Marking divInt# INLINE in the base library. This would result in your code would just containing uses of quotInt# * Making some operations cheap even if they may fail (PrimOp.primpOpIsCheap should change). Though this might mean that we turn non-terminating programs into terminating ones (such operations get pushed inside lambdas) but this is consistent with our treatment of lambdas generally.
I am guess this is because quot maps directly onto the x86 idiv instruction due to both of them truncating towards zero, while div, with its truncation towards negative infinity, does not. Running with -ddump-asm seems to back this up as quotInt# compiles down to an idiv instruction and divInt# to a call through base_GHCziBase_divIntzh_info. Unfortunately for me, I always seem to instinctively go with div and mod ahead of quot and rem.
I see what you mean about div as it is defined through divMod which is in turn defined through quotRem in Prelude. n `div` d = q where (q,_) = divMod n d divMod n d = if signum r == negate (signum d) then (q-1, r+d) else qr where qr@(q,r) = quotRem n d However, GHC almost seems to be doing its own thing as, as I mentioned above, it turns it into a divInt# (which is not in GHC.Prim) in the tidy core, which then turns into a call through base_GHCziBase_divIntzh_info in the assembler. (note that I'm just looking at the source from the haskell.org libraries link) Cheers! -Tyson