[commit: ghc] wip/T13227: Improve the Occurrence Analyzer’s handling of one-shot functions (917f43a)
git at git.haskell.org
git at git.haskell.org
Sat Feb 11 16:49:29 UTC 2017
Repository : ssh://git@git.haskell.org/ghc
On branch : wip/T13227
Link : http://ghc.haskell.org/trac/ghc/changeset/917f43acb4a1b315108f8bafe0cfd61fe56e7688/ghc
>---------------------------------------------------------------
commit 917f43acb4a1b315108f8bafe0cfd61fe56e7688
Author: Joachim Breitner <mail at joachim-breitner.de>
Date: Sun Feb 5 10:52:12 2017 -0500
Improve the Occurrence Analyzer’s handling of one-shot functions
when determining whether an expression is used saturatedly, count the
number of value arguments that the occurrence analyser sees, and add
the number of one-shot arguments that we know (from the strictness
analyser) are passed from the context. This was suggested by Simon PJ in
ticket #13227.
perf results suggest no noticable change in allocations, reduction of
code sizes, and performance regression possibliy due to loss of join
points.
Differential Revision: https://phabricator.haskell.org/D3089
>---------------------------------------------------------------
917f43acb4a1b315108f8bafe0cfd61fe56e7688
compiler/simplCore/OccurAnal.hs | 80 ++++++++++++++++++++++++++++++-----------
1 file changed, 59 insertions(+), 21 deletions(-)
diff --git a/compiler/simplCore/OccurAnal.hs b/compiler/simplCore/OccurAnal.hs
index 728e472..92c21ad 100644
--- a/compiler/simplCore/OccurAnal.hs
+++ b/compiler/simplCore/OccurAnal.hs
@@ -1547,7 +1547,7 @@ occAnalNonRecRhs env bndr bndrs body
env1 | certainly_inline = env
| otherwise = rhsCtxt env
- -- See Note [Use one-shot info]
+ -- See Note [Sources of one-shot information]
rhs_env = env1 { occ_one_shots = argOneShots dmd }
@@ -1867,16 +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
+ n_val_args = valArgCount args + length (takeWhile isOneShotInfo (occ_one_shots env))
+ -- See Note [Sources of one-shot information], bullet point A'
+
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
+ -- 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 [Use one-shot info]
+ -- See Note [Sources of one-shot information]
occAnalApp env (fun, args, ticks)
= (markAllNonTailCalled (fun_uds +++ args_uds),
@@ -1898,20 +1899,44 @@ zapDetailsIf True uds = zapDetails uds
zapDetailsIf False uds = uds
{-
-Note [Use one-shot information]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The occurrence analyser propagates one-shot-lambda information in two
-situations:
+Note [Sources of one-shot information]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The occurrence analyser obtains one-shot-lambda information from two sources:
+
+A: Saturated applications: eg f e1 .. en
+
+ In general, given a call (f e1 .. en) we can propagate one-shot info from
+ f's strictness signature into e1 .. en, but /only/ if n is enough to
+ saturate the strictness signature. A stricteness signature like
+
+ 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
+
+ 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
- * Applications: eg build (\c n -> blah)
+ map (f (\x y. expensive) 3 2) xs
- Propagate one-shot info from the strictness signature of 'build' to
- the \c n.
+ then the first argument of f will be called at most once.
- This strictness signature can come from a module interface, in the case of
- an imported function, or from a previous run of the demand analyser.
+A': Non-obviously satuated applications: eg build (f (\x y -> expensive))
- * Let-bindings: eg let f = \c. let ... in \n -> blah
+ In this case, f is only manifestly applied to one argument, so it does not
+ look saturated. So bye 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*.
+
+ 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.
+
+B: Let-bindings: eg let f = \c. let ... in \n -> blah
in (build f, build f)
Propagate one-shot info from the demanand-info on 'f' to the
@@ -1924,6 +1949,22 @@ Previously, the demand analyser would *also* set the one-shot information, but
that code was buggy (see #11770), so doing it only in on place, namely here, is
saner.
+Note [OneShots]
+~~~~~~~~~~~~~~~
+When analysing an expression, the occ_one_shots argument contains information
+about how the function is being used. The length of the list indicates
+how many arguments will eventually be passed to the analysed expression,
+and the OneShotInfo indicates whether this application is once or multiple times.
+
+Example:
+
+ Context of f occ_one_shots when analysing f
+
+ f 1 2 [OneShot, OneShot]
+ map (f 1) [OneShot, NoOneShotInfo]
+ build f [OneShot, OneShot]
+ f 1 2 `seq` f 2 1 [NoOneShotInfo, OneShot]
+
Note [Binders in case alternatives]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
@@ -2008,7 +2049,7 @@ wrapAltRHS _ _ alt_usg _ alt_rhs
data OccEnv
= OccEnv { occ_encl :: !OccEncl -- Enclosing context information
- , occ_one_shots :: !OneShots -- Tells about linearity
+ , occ_one_shots :: !OneShots -- See Note [OneShots]
, occ_gbl_scrut :: GlobalScruts
, occ_rule_act :: Activation -> Bool -- Which rules are active
-- See Note [Finding rule RHS free vars]
@@ -2037,11 +2078,8 @@ instance Outputable OccEncl where
ppr OccRhs = text "occRhs"
ppr OccVanilla = text "occVanilla"
+-- See note [OneShots]
type OneShots = [OneShotInfo]
- -- [] No info
- --
- -- one_shot_info:ctxt Analysing a function-valued expression that
- -- will be applied as described by one_shot_info
initOccEnv :: (Activation -> Bool) -> OccEnv
initOccEnv active_rule
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