[commit: ghc] wip/T7994-calledArity: Some very prelimary explanations (f33d231)

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Repository : ssh://git@git.haskell.org/ghc

On branch  : wip/T7994-calledArity
Link       : http://ghc.haskell.org/trac/ghc/changeset/f33d2313c0999e4da43b385fbcc1b36f2cf096f3/ghc

>---------------------------------------------------------------

commit f33d2313c0999e4da43b385fbcc1b36f2cf096f3
Author: Joachim Breitner <mail at joachim-breitner.de>
Date:   Tue Jan 28 11:26:09 2014 +0000

    Some very prelimary explanations


>---------------------------------------------------------------

f33d2313c0999e4da43b385fbcc1b36f2cf096f3
 compiler/coreSyn/CoreArity.lhs |   36 +++++++++++++++++++++++++++++++-----
 1 file changed, 31 insertions(+), 5 deletions(-)

diff --git a/compiler/coreSyn/CoreArity.lhs b/compiler/coreSyn/CoreArity.lhs
index d4c0a6b..287087b 100644
--- a/compiler/coreSyn/CoreArity.lhs
+++ b/compiler/coreSyn/CoreArity.lhs
@@ -1031,12 +1031,23 @@ freshEtaId n subst ty
 \end{code}
 
   
-Call arity anlysis: For let-bound things, find out the minimum arity it is
-called with, so that the value can be eta-expanded to that arity, creating
-better code (main goal: good foldl as foldr).
+%************************************************************************
+%*									*
+              Called Arity Analyis
+%*									*
+%************************************************************************
+
+
+For let-bound things, find out the minimum arity it is called with, so that the
+value can be eta-expanded to that arity, creating better code (main goal: good
+foldl as foldr).
+
+Main problem: Thunks! We must not lose sharing. So one needs to consider how
+often something is called. We use the following approximation:
 
-Be sure to only consider tail calls. This allows us to detect that there is at
-most one call to n in
+    Only consider tail calls!
+
+This allows us to detect that there is at most one call to n in
 
  let n = case .. of .. -- A thunk!
  in case .. of
@@ -1045,6 +1056,21 @@ most one call to n in
                               False > n
     False -> n
 
+despite a call from a recursive function, becuase
+
+    A tail-recursive function can make at most one tail-call somewhere else.
+
+
+The following code is not yet very quick. Possible improvements:
+ * Re-use existing information in nested fixed-points.
+ * Upon failure in fixed-pointing, go directly to 0 (or the manifestArity).
+
+Possibe improvements in precision:
+ * Always start with at least the manifestArity.
+ * When analysing a recursive RHS, starting with its manifestArity,
+   even if it calls itself at a lower arity, that is still good enough to stop.
+ * In the above, use exprArity instead of manifestArity.
+
 \begin{code}
 
 callArityAnalProgram :: DynFlags -> CoreProgram -> CoreProgram