Inliner behaviour - tiny changes lead to huge performance differences

Bryan O'Sullivan bos at
Tue Nov 17 11:34:05 EST 2009

On Tue, Nov 17, 2009 at 12:24 AM, Simon Peyton-Jones
<simonpj at>wrote:

>  To be honest I’m not terribly enthusiastic about trying to nail down
> exactly what’s happening in 6.10 and 6.12 because, although they are indeed
> the compilers people will be using, it’s otherwise wasted work because the
> HEAD is so different.
I don't blame you! That's completely reasonable.

> Can you try with 6.12 and see if you can find a recipe that does well
> enough?  If you get desperate (ie there’s a huge perf bump that you can’t
> eliminate) then I’ll certainly try to help.
Will do, thanks.

> Meanwhile, I don’t know why 6.10 is faster than HEAD (by 25% too) and I’d
> like to understand that.  Can you submit a Trac ticket saying how to
> reproduce?  You might need to bundle up the library too, to make sure we can
> reproduce it precisely.
Certainly. The test program is tiny, but because of all the inlining in the
library, the simplifier output is pretty fearsome.
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