Bools are not unboxed

Simon Peyton-Jones simonpj at
Mon Oct 4 08:13:20 EDT 2004

| >     for :: Int -> IO () -> IO ()
| >     for 0 _ = return ()
| >     for n x = x >> for (n - 1) x

Good example (allocates lots of silly thunks).  I'd come across this
before, and fixed the HEAD, but the 6.2 branch is still doing badly.
We'll try to fix that.  

| Playing with the code generated by ghc is a great way to waste time
| for me.  Wait until you have found the RULES-pragma :-)

In general, GHC's optimiser should behave predictably; small changes in
the source program should not have big effects.  Of course, sometimes
they do, and for good reason.  But not always; I'm keen to identify
cases where the optimiser does a poor/unpredictable job, and fix them.

So please let me know when that happens.  The more you can boil down a
program, and identify the culprit, as Carsten did so accurately in this
case, the more likely I am to fix it.  At the other end of the spectrum
"this 1000 line program goes slower than I expect" is less helpful :-)

Incidentally, GHC does use pre-allocated True and False booleans.


More information about the Glasgow-haskell-users mailing list