Trying to fix an efficiency issue noted in a TODO in SAT.hs

[Simon Peyton Jones]

The static argument transformation is currently run only when –fstatic-argument-transformation is specified; i.e almost never.  It has not received any love for some time.  I note the following comment from Max Bolingbroke in DynFlags.hs


--     , ([2],     Opt_StaticArgumentTransformation)

-- Max writes: I think it's probably best not to enable SAT with -O2 for the

-- 6.10 release. The version of SAT in HEAD at the moment doesn't incorporate

-- several improvements to the heuristics, and I'm concerned that without

-- those changes SAT will interfere with some attempts to write "high

-- performance Haskell", as we saw in some posts on Haskell-Cafe earlier

-- this year. In particular, the version in HEAD lacks the tail call

-- criterion, so many things that look like reasonable loops will be

-- turned into functions with extra (unnecessary) thunk creation.

I’m therefore reluctant to commit refactoring to SAT.  They will, in effect, be entirely un-tested.

There is a ticket here: https://ghc.haskell.org/trac/ghc/ticket/9374   Do by all means add your diffs to it, so that if anyone (possibly even you) picks it up they will get the benefit of your work.

Simon

From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of David Feuer
Sent: 07 September 2014 04:06
To: ghc-devs
Subject: Trying to fix an efficiency issue noted in a TODO in SAT.hs

compiler/simplCore/SAT.hs has a TODO comment about the fact that it does a fair bit of appending onto the ends of lists, and that should be done differently. I made an attempt to fix it. The complexity of the recursion, however, leaves me uncertain as to whether I really did or not. I've attached a diff and I hope someone will be able to take a look at it. The only use of Sequence.fromList is source line 172, and the only significant use of Foldable.toList (aside from pretty-printing) is on source line 402. Note that the use of Sequence may be temporary—I want to get the right code structure down before choosing the best data structure.
Thanks,
David Feuer
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/ghc-devs/attachments/20140909/96b487e4/attachment-0001.html>