[commit: ghc] ghc-8.2: Broaden demand analysis IO hack notes (4ff1374)

[git at git.haskell.org]

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

On branch  : ghc-8.2
Link       : http://ghc.haskell.org/trac/ghc/changeset/4ff1374074e6fac48512c62f5baf9e1863883a23/ghc

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

commit 4ff1374074e6fac48512c62f5baf9e1863883a23
Author: David Feuer <david.feuer at gmail.com>
Date:   Mon Mar 13 15:18:49 2017 -0400

    Broaden demand analysis IO hack notes
    
    The I/O hack for demand analysis has broader and arguably more
    important implications than the note expressed. Broaden it.
    
    [skip ci]
    
    Reviewers: austin, bgamari
    
    Reviewed By: bgamari
    
    Subscribers: carter, rwbarton, thomie
    
    Differential Revision: https://phabricator.haskell.org/D3307
    
    (cherry picked from commit dd3b06aa2d44fa160e36097ce90c4574160bd4eb)


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4ff1374074e6fac48512c62f5baf9e1863883a23
 compiler/stranal/DmdAnal.hs | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/compiler/stranal/DmdAnal.hs b/compiler/stranal/DmdAnal.hs
index 9271eda..25a4f8b 100644
--- a/compiler/stranal/DmdAnal.hs
+++ b/compiler/stranal/DmdAnal.hs
@@ -344,10 +344,14 @@ dmdAnalAlt env dmd case_bndr (con,bndrs,rhs)
 {- Note [IO hack in the demand analyser]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 There's a hack here for I/O operations.  Consider
-     case foo x s of { (# s, r #) -> y }
-Is this strict in 'y'?  Normally yes, but what if 'foo' is an I/O
-operation that simply terminates the program (not in an erroneous way)?
-In that case we should not evaluate 'y' before the call to 'foo'.
+
+     case foo x s of { (# s', r #) -> y }
+
+Is this strict in 'y'? Often not! If foo x s performs some observable action
+(including raising an exception with raiseIO#, modifying a mutable variable, or
+even ending the program normally), then we must not force 'y' (which may fail
+to terminate) until we have performed foo x s.
+
 Hackish solution: spot the IO-like situation and add a virtual branch,
 as if we had
      case foo x s of