[commit: ghc] : Note about GCing before match, fixes a termination bug (6972dc8)
Max Bolingbroke <[email protected]>
| Newsgroups | gmane.comp.lang.haskell.cvs.ghc |
|---|---|
| Message-ID | <[email protected]> |
Repository : ssh://darcs.haskell.org//srv/darcs/ghc On branch : http://hackage.haskell.org/trac/ghc/changeset/6972dc818589311eebfde5eb1e10cef0157dc688 >--------------------------------------------------------------- commit 6972dc818589311eebfde5eb1e10cef0157dc688 Author: Max Bolingbroke <[email protected]> Date: Thu Mar 15 09:58:10 2012 +0000 Note about GCing before match, fixes a termination bug >--------------------------------------------------------------- .../supercompile/Supercompile/Drive/Process3.hs | 22 +++++++++++++++++++- 1 files changed, 21 insertions(+), 1 deletions(-) diff --git a/compiler/supercompile/Supercompile/Drive/Process3.hs b/compiler/supercompile/Supercompile/Drive/Process3.hs index d1553bb..b32fd7d 100644 --- a/compiler/supercompile/Supercompile/Drive/Process3.hs +++ b/compiler/supercompile/Supercompile/Drive/Process3.hs @@ -452,8 +452,28 @@ memo opt init_state = {-# SCC "memo'" #-} memo_opt init_state data MemoHow = Skip | CheckOnly | CheckAndRemember +-- NB: don't garbage collect when reducing for a match! +-- +-- If you do then you can start with this term: +-- [1] let $dNum = ww3 in * a $dNum +-- +-- Looks like this after reduction+GC: +-- [2] case ww3 of Num ... +-- +-- And if we reduce+split [1] instead we get: +-- [3] case $dNum of Num ... +-- +-- Reducing+GCing [3] term gives us [3] again, and that is alpha equivalent to [2], +-- so we tie back to it rather than continuing. But that means our code is: +-- let h @a ww3 = let $dNum = ww3 +-- in h a $dNum +-- +-- Which is a loop. So we need to do one of: +-- 1. Not GC before matching +-- 2. GC *after* reduction in the main codepath. +-- 3. Not eliminate dead update frames when GCing reduceForMatch :: State -> (Bool, State) -reduceForMatch state = second gc $ reduceWithFlag (case state of (_, h, k, e) -> (maxBound, h, k, e)) -- Reduce ignoring deeds for better normalisation +reduceForMatch state = {- second gc -} $ reduceWithFlag (case state of (_, h, k, e) -> (maxBound, h, k, e)) -- Reduce ignoring deeds for better normalisation supercompile :: M.Map Var Term -> Term -> Term supercompile unfoldings e = fVedTermToTerm $ start (liftM snd . sc)