This doesn't have the effect I expected: loop xs = case blah of One thing -> ... loop The other -> ... realloc ... where {-# NOINLINE realloc #-} realloc = do something loop ... My intention here was that the loop would not contain the code for realloc and that it'd be done as a call at the cmm level. My intention is to take the slow and rarely taken realloc path out of the code for the fast path. It seems the {-# NOINLINE realloc #-} pagma did not have the effect I intended. Looking at the -ddump-simpl and -ddump-cmm, the code for the realloc gets expanded in place in a branch of a case statement. In the cmm code we end up with just what I didn't want: loop_info: if (offset != 4096) goto later; ... ... lots of realloc code taking up space ... in the instruction / trace cache ... later: .. do the fast bits, read a byte, write a byte jump loop_info; Not only does the slow path take up space but it's in the location favoured by the hardware's static branch prediction. Reversing the test doesn't help because either way ghc turns it into: case thing of _DEFAULT -> 4096 -> and from that generates CMM: if (thing != 4096) goto much_later; ... much_later: ... The reason I was looking at this is because I've been trying to figure out why our lazy byte string fusion primitives are much slower than the strict versions. It's improving though, it's now only half the speed rather than a tenth of the speed. :-) The ByteString.Lazy code is an interesting mixture of strict and lazy. We must strictly read/write the chunks but lazily generate/consume the list of chunks. I just discovered that I should have been reading STG all along rather than core from the simplifier or CMM. STG takes out all the type annotations which tend to make things quite verbose. Mind you, seeing the types can be handy too to see if/how things are unboxed. Even so, I kind of wish there were a stage between STG and CMM that showed the imperative model of STG with linear layout, control flow and notes to indicate thunk/closure allocations. I expect most of my problem is that I do not understand the STG evaluation model sufficiently well to see how it maps to basic blocks, jumps/calls etc. Duncan