[commit: ghc] wip/rwbarton-simplify: Stamp out space leaks from demand analysis (eaa4a3d)

[git at git.haskell.org]

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

On branch  : wip/rwbarton-simplify
Link       : http://ghc.haskell.org/trac/ghc/changeset/eaa4a3d1f02f3301ed3a198a2a983566f253c317/ghc

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

commit eaa4a3d1f02f3301ed3a198a2a983566f253c317
Author: Reid Barton <rwbarton at gmail.com>
Date:   Wed Mar 29 23:39:44 2017 -0400

    Stamp out space leaks from demand analysis
    
    This reduces peak memory usage by ~30% on my test case (DynFlags),
    and (probably as a result of reduced GC work) decreases compilation
    time by a few percent as well.
    
    Also fix a bug in seqStrDmd so that demeand info is fully evaluated.


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

eaa4a3d1f02f3301ed3a198a2a983566f253c317
 compiler/basicTypes/Demand.hs |  2 +-
 compiler/stranal/DmdAnal.hs   | 22 +++++++++++++++++++++-
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/compiler/basicTypes/Demand.hs b/compiler/basicTypes/Demand.hs
index 377fc3d..95c7b79 100644
--- a/compiler/basicTypes/Demand.hs
+++ b/compiler/basicTypes/Demand.hs
@@ -332,7 +332,7 @@ bothStr (SProd _) (SCall _)    = HyperStr
 -- utility functions to deal with memory leaks
 seqStrDmd :: StrDmd -> ()
 seqStrDmd (SProd ds)   = seqStrDmdList ds
-seqStrDmd (SCall s)     = s `seq` ()
+seqStrDmd (SCall s)    = seqStrDmd s
 seqStrDmd _            = ()
 
 seqStrDmdList :: [ArgStr] -> ()
diff --git a/compiler/stranal/DmdAnal.hs b/compiler/stranal/DmdAnal.hs
index 25a4f8b..2fc33a4 100644
--- a/compiler/stranal/DmdAnal.hs
+++ b/compiler/stranal/DmdAnal.hs
@@ -17,6 +17,7 @@ import DynFlags
 import WwLib            ( findTypeShape, deepSplitProductType_maybe )
 import Demand   -- All of it
 import CoreSyn
+import CoreSeq          ( seqBinds )
 import Outputable
 import VarEnv
 import BasicTypes
@@ -52,7 +53,8 @@ dmdAnalProgram dflags fam_envs binds
         dumpIfSet_dyn dflags Opt_D_dump_str_signatures
                       "Strictness signatures" $
             dumpStrSig binds_plus_dmds ;
-        return binds_plus_dmds
+        -- See Note [Stamp out space leaks in demand analysis]
+        seqBinds binds_plus_dmds `seq` return binds_plus_dmds
     }
   where
     do_prog :: CoreProgram -> CoreProgram
@@ -79,6 +81,24 @@ dmdAnalTopBind sigs (Rec pairs)
                 -- We get two iterations automatically
                 -- c.f. the NonRec case above
 
+{- Note [Stamp out space leaks in demand analysis]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The demand analysis pass outputs a new copy of the Core program in
+which binders have been annotated with demand and strictness
+information. It's tiresome to ensure that this information is fully
+evaluated everywhere that we produce it, so we just run a single
+seqBinds over the output before returning it, to ensure that there are
+no references holding on to the input Core program.
+
+This is particularly important when we are doing late demand analysis,
+since we don't do a seqBinds at any point thereafter. Hence code
+generation would hold on to an extra copy of the Core program, via
+unforced thunks in demand or strictness information; and it is the
+most memory-intensive part of the compilation process, so this added
+seqBinds makes a big difference in peak memory usage.
+-}
+
+
 {-
 ************************************************************************
 *                                                                      *