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authorSimon Peyton Jones <simonpj@microsoft.com>2017-02-16 09:41:55 +0000
committerSimon Peyton Jones <simonpj@microsoft.com>2017-02-16 09:41:55 +0000
commitfc9d152b058f21ab03986ea722d0c94688b9969f (patch)
tree3995139e513088c99094253525d35473c890be5a
parente28fbbb7c3d5904a88b4743d0d10f212d61d8293 (diff)
downloadhaskell-fc9d152b058f21ab03986ea722d0c94688b9969f.tar.gz
Comments and tiny refactor only
-rw-r--r--compiler/simplCore/OccurAnal.hs28
1 files changed, 17 insertions, 11 deletions
diff --git a/compiler/simplCore/OccurAnal.hs b/compiler/simplCore/OccurAnal.hs
index 6217dc8575..f2f7da636a 100644
--- a/compiler/simplCore/OccurAnal.hs
+++ b/compiler/simplCore/OccurAnal.hs
@@ -1867,17 +1867,17 @@ occAnalApp env (Var fun, args, ticks)
-- This is the *whole point* of the isRhsEnv predicate
-- See Note [Arguments of let-bound constructors]
- n_val_args = valArgCount args + length (takeWhile isOneShotInfo (occ_one_shots env))
- -- See Note [Sources of one-shot information], bullet point A'
-
+ n_val_args = valArgCount args
n_args = length args
fun_uds = mkOneOcc env fun (n_val_args > 0) n_args
is_exp = isExpandableApp fun n_val_args
-- See Note [CONLIKE pragma] in BasicTypes
-- The definition of is_exp should match that in Simplify.prepareRhs
- one_shots = argsOneShots (idStrictness fun) n_val_args
- -- See Note [Sources of one-shot information]
+ one_shots = argsOneShots (idStrictness fun) guaranteed_val_args
+ guaranteed_val_args = n_val_args + length (takeWhile isOneShotInfo
+ (occ_one_shots env))
+ -- See Note [Sources of one-shot information], bullet point A']
occAnalApp env (fun, args, ticks)
= (markAllNonTailCalled (fun_uds +++ args_uds),
@@ -1909,33 +1909,39 @@ A: Saturated applications: eg f e1 .. en
f's strictness signature into e1 .. en, but /only/ if n is enough to
saturate the strictness signature. A strictness signature like
- f :: C1(C1(L))LS
+ f :: C1(C1(L))LS
means that *if f is applied to three arguments* then it will guarantee to
call its first argument at most once, and to call the result of that at
most once. But if f has fewer than three arguments, all bets are off; e.g.
- map (f (\x y. expensive) e2) xs
+ map (f (\x y. expensive) e2) xs
Here the \x y abstraction may be called many times (once for each element of
xs) so we should not mark x and y as one-shot. But if it was
- map (f (\x y. expensive) 3 2) xs
+ map (f (\x y. expensive) 3 2) xs
then the first argument of f will be called at most once.
+ The one-shot info, derived from f's strictness signature, is
+ computed by 'argsOneShots', called in occAnalApp.
+
A': Non-obviously saturated applications: eg build (f (\x y -> expensive))
+ where f is as above.
In this case, f is only manifestly applied to one argument, so it does not
look saturated. So by the previous point, we should not use its strictness
signature to learn about the one-shotness of \x y. But in this case we can:
-
- build is fully applied, so we may use its strictness signature. From that
- we learn that build calls its argument with two arguments *at most once*.
+ build is fully applied, so we may use its strictness signature; and from
+ that we learn that build calls its argument with two arguments *at most once*.
So there is really only one call to f, and it will have three arguments. In
that sense, f is saturated, and we may proceed as described above.
+ Hence the computation of 'guaranteed_val_args' in occAnalApp, using
+ '(occ_one_shots env)'. See also Trac #13227, comment:9
+
B: Let-bindings: eg let f = \c. let ... in \n -> blah
in (build f, build f)