[C2hs] Profiling c2hs

[Duncan Coutts]

All,

I was writing a demo app to add to gtk2hs's small collection; it's a
viewer for ghc's time profile log files, and I needed a large profile
file to test it on so I thought I'd try profiling c2hs. Here's what I
found:

Looking at allocations (which corresponds quite closely with time):
Note that these are % of the total cost, not % of their parent cost

11.7% is taken by the C & chs lexers which is not unreasonable.
77.4% is taken by the CParser

Within the CParser:
72.0% is taken by parseCExtDeclList, within which
65.5% is taken by morphTypeNames (and 58.2% of the runtime)
31.4% alloc & 26.5% time was taken just by calls to Prelude.elem within
morphTypeNames.

In this particular run of c2hs, in which it was processing some large
gtk+ headers, it called morphTypeNames over 55 million times.

I narrowed this down a bit more and found that it is the call to
morphTypeNames from parseCExtDeclList that takes the time, the call from
parseCHeader is insignificant. Furthermore, it is the recursive calls to
morphTypeNames that is expensive; parseCExtDeclList only called
morphTypeNames about 1000 times (compared to 55 million recursive
calls).

So what's going and and what can we do?

If I understand the code correctly, the reason we are doing this
morphTypeNames is because we can't distinguish typedef'ed identifiers in
the parser, we need to remember which identifiers were typedefs and then
treat them differently in the parser later if they were. At the moment
this is done by after parsing each declaration (which might contain
typedefs) modifying the remaining stream of tokens changing all
identifiers that have been discovered to be typedefs from ordinary
identifier tokens to typedef-identifier tokens. This is expensive.

Perhaps it would be possible to add this set of typedef identifiers to
the parser state monad and thread it through to the parsers for cid and
typedefName which could consult this parser state and match
conditionally on the identifier token being in the typedef-identifier
set or not. (cidOrTN would not need to to a lookup) This would avoid
traversing the token stream after parsing each declaration.

Sound reasonable?

Duncan