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In the CVS version there's a -xt RTS flag that enables inclusion of thread stacks in a heap profile. It's crude, because thread stacks grow by doubling their size every time they run out of space, but it should be enough to show the difference between O(n) stack usage and O(1).

Hmm. That sounds like what I need, I think. Is it in HEAD or in the 5.04 (now 5.04.3?) branch?

It's only in the HEAD. Cheers, Simon