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| author | Simon Peyton Jones <simonpj@microsoft.com> | 2013-03-27 14:24:51 +0000 |
|---|---|---|
| committer | Simon Peyton Jones <simonpj@microsoft.com> | 2013-03-27 14:24:51 +0000 |
| commit | 54bb2f83f5c292f952bf7479f0c3277831fa987d (patch) | |
| tree | 0c26b1af8ef02e9d08d2b594b824d8f38703f623 | |
| parent | 1b37a386d310278096e2697fd8df425587560aee (diff) | |
| download | haskell-54bb2f83f5c292f952bf7479f0c3277831fa987d.tar.gz | |
Improve comments about dead code (thanks to Nick Frisby)
| -rw-r--r-- | compiler/simplCore/OccurAnal.lhs | 35 |
1 files changed, 13 insertions, 22 deletions
diff --git a/compiler/simplCore/OccurAnal.lhs b/compiler/simplCore/OccurAnal.lhs index 3f09cd621d..dccbabcbdb 100644 --- a/compiler/simplCore/OccurAnal.lhs +++ b/compiler/simplCore/OccurAnal.lhs @@ -156,35 +156,26 @@ occAnalBind _ env imp_rules_edges (Rec pairs) body_usage Note [Dead code] ~~~~~~~~~~~~~~~~ -Dropping dead code for recursive bindings is done in a very simple way: +Dropping dead code for a cyclic Strongly Connected Component is done +in a very simple way: - the entire set of bindings is dropped if none of its binders are - mentioned in its body; otherwise none are. + the entire SCC is dropped if none of its binders are mentioned + in the body; otherwise the whole thing is kept. -This seems to miss an obvious improvement. +The key observation is that dead code elimination happens after +dependency analysis: so 'occAnalBind' processes SCCs instead of the +original term's binding groups. - letrec f = ...g... - g = ...f... - in - ...g... -===> - letrec f = ...g... - g = ...(...g...)... - in - ...g... - -Now 'f' is unused! But it's OK! Dependency analysis will sort this -out into a letrec for 'g' and a 'let' for 'f', and then 'f' will get -dropped. It isn't easy to do a perfect job in one blow. Consider +Thus 'occAnalBind' does indeed drop 'f' in an example like letrec f = ...g... - g = ...h... - h = ...k... - k = ...m... - m = ...m... + g = ...(...g...)... in - ...m... + ...g... +when 'g' no longer uses 'f' at all (eg 'f' does not occur in a RULE in +'g'). 'occAnalBind' first consumes 'CyclicSCC g' and then it consumes +'AcyclicSCC f', where 'body_usage' won't contain 'f'. ------------------------------------------------------------ Note [Forming Rec groups] |