[commit: ghc] wip/T13227: Improve the Occurrence Analyzer’s handling of one-shot functions (4f92864)

[git at git.haskell.org]

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

On branch  : wip/T13227
Link       : http://ghc.haskell.org/trac/ghc/changeset/4f92864d04045b9ad58c6ee95616631f53c817c3/ghc

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

commit 4f92864d04045b9ad58c6ee95616631f53c817c3
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


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

4f92864d04045b9ad58c6ee95616631f53c817c3
 compiler/simplCore/OccurAnal.hs | 42 +++++++++++++++++++++++++++++------------
 1 file changed, 30 insertions(+), 12 deletions(-)

diff --git a/compiler/simplCore/OccurAnal.hs b/compiler/simplCore/OccurAnal.hs
index b02ddc9..1438ac6 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 info]
     rhs_env = env1 { occ_one_shots = argOneShots dmd }
 
 
@@ -1867,7 +1867,13 @@ 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))
+       -- n_val_args is used to determine if this call is saturated. We want to consider
+       -- the lambda expression in 'build (\x y -> …)' to be considered
+       -- saturated, so we count one-shot arguments from the context.
+       -- But 'map (\x -> …)' is not saturated, so we only count the one-shot
+       -- arguments.
+
     n_args     = length args
     fun_uds    = mkOneOcc env fun (n_val_args > 0) n_args
     is_exp     = isExpandableApp fun n_val_args
@@ -1876,7 +1882,7 @@ occAnalApp env (Var fun, args, ticks)
            -- 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),
@@ -1898,10 +1904,9 @@ zapDetailsIf True  uds = zapDetails uds
 zapDetailsIf False uds = uds
 
 {-
-Note [Use one-shot information]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The occurrrence 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:
 
   * Applications:  eg   build (\c n -> blah)
 
@@ -1924,6 +1929,22 @@ 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 +2029,7 @@ 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 +2058,8 @@ 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