[commit: ghc] master: Make raiseIO# produce topRes (7b087ae)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/7b087aeba45a7a70a5553ef4c116ee67660423e8/ghc

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

commit 7b087aeba45a7a70a5553ef4c116ee67660423e8
Author: David Feuer <david.feuer at gmail.com>
Date:   Thu Mar 9 10:34:42 2017 -0500

    Make raiseIO# produce topRes
    
    Make `raiseIO#` produce `topRes` instead of `ExnRes`. `ExnRes` leads to
    demand analysis being too aggressive, IMO, allowing imprecise exceptions
    produced by `throw` to replace exceptions thrown by `throwIO` that
    would like to think of as precise.
    
    This fixes that, but is certanly much more conservative than we would
    ideally like. Let's see how bad it is.
    
    Fixes Trac #13380
    
    Reviewers: austin, bgamari
    
    Subscribers: rwbarton, thomie
    
    Differential Revision: https://phabricator.haskell.org/D3301


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7b087aeba45a7a70a5553ef4c116ee67660423e8
 compiler/prelude/primops.txt.pp | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/compiler/prelude/primops.txt.pp b/compiler/prelude/primops.txt.pp
index 1d10223..64971a3 100644
--- a/compiler/prelude/primops.txt.pp
+++ b/compiler/prelude/primops.txt.pp
@@ -2005,11 +2005,9 @@ primop  RaiseOp "raise#" GenPrimOp
 -- must be *precise* - we don't want the strictness analyser turning
 -- one kind of bottom into another, as it is allowed to do in pure code.
 --
--- But we *do* want to know that it returns bottom after
--- being applied to two arguments, so that this function is strict in y
---     f x y | x>0       = raiseIO blah
---           | y>0       = return 1
---           | otherwise = return 2
+-- We currently produce topRes, which is much too conservative (interfering
+-- with dead code elimination, unfortunately), but nothing else we currently
+-- have on tap is actually correct.
 --
 -- TODO Check that the above notes on @f@ are valid. The function successfully
 -- produces an IO exception when compiled without optimization. If we analyze
@@ -2021,7 +2019,7 @@ primop  RaiseOp "raise#" GenPrimOp
 primop  RaiseIOOp "raiseIO#" GenPrimOp
    a -> State# RealWorld -> (# State# RealWorld, b #)
    with
-   strictness  = { \ _arity -> mkClosedStrictSig [topDmd, topDmd] exnRes }
+   strictness  = { \ _arity -> mkClosedStrictSig [topDmd, topDmd] topRes }
    out_of_line = True
    has_side_effects = True