[commit: ghc] master: Mark retry# as returning bottom. (e2b72ca)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/e2b72cadce3ae799a74981f9957b40bb201f1ba1/ghc

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

commit e2b72cadce3ae799a74981f9957b40bb201f1ba1
Author: Austin Seipp <aseipp at pobox.com>
Date:   Sat Aug 10 22:39:15 2013 -0500

    Mark retry# as returning bottom.
    
    This change helps the simplifier eliminate unreachable code, since
    retry# technically doesn't return.
    
    This closes ticket #8091.
    
    Authored-by: Patrick Palka <patrick at parcs.ath.cx>
    Signed-off-by: Austin Seipp <aseipp at pobox.com>


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

e2b72cadce3ae799a74981f9957b40bb201f1ba1
 compiler/prelude/primops.txt.pp |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/compiler/prelude/primops.txt.pp b/compiler/prelude/primops.txt.pp
index 3d3518b..0c325a6 100644
--- a/compiler/prelude/primops.txt.pp
+++ b/compiler/prelude/primops.txt.pp
@@ -1611,9 +1611,24 @@ primop	AtomicallyOp "atomically#" GenPrimOp
    out_of_line = True
    has_side_effects = True
 
+-- NB: retry#'s strictness information specifies it to return bottom.
+-- This lets the compiler perform some extra simplifications, since retry#
+-- will technically never return.
+--
+-- This allows the simplifier to replace things like:
+--
+--   case retry# s1
+--     (# s2, a #) -> e
+--
+-- with:
+--
+--   retry# s1
+--
+-- where 'e' would be unreachable anyway.
 primop  RetryOp "retry#" GenPrimOp
    State# RealWorld -> (# State# RealWorld, a #)
    with 
+   strictness  = { \ _arity -> mkStrictSig (mkTopDmdType [topDmd] botRes) }
    out_of_line = True
    has_side_effects = True