From: Ian Lynagh <igloo@earth.li>
Date: Tue, 4 Sep 2007 13:41:40 +0000 (+0000)
Subject: Remove some incorrect rules; fixes #1658: CSE [of Doubles] changes semantics
X-Git-Tag: 2007-09-13~2
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=2ec02ba43467f2ca34378c7f62f78bd0626cad33;p=ghc-base.git

Remove some incorrect rules; fixes #1658: CSE [of Doubles] changes semantics
---

diff --git a/GHC/Base.lhs b/GHC/Base.lhs
index baf2694..5c33176 100644
--- a/GHC/Base.lhs
+++ b/GHC/Base.lhs
@@ -919,14 +919,26 @@ gtInt, geInt, eqInt, neInt, ltInt, leInt :: Int -> Int -> Bool
 "plusDouble x 0.0"   forall x#. (+##) x#    0.0## = x#
 "plusDouble 0.0 x"   forall x#. (+##) 0.0## x#    = x#
 "minusDouble x 0.0"  forall x#. (-##) x#    0.0## = x#
-"minusDouble x x"    forall x#. (-##) x#    x#    = 0.0##
-"timesDouble x 0.0"  forall x#. (*##) x#    0.0## = 0.0##
-"timesDouble 0.0 x"  forall x#. (*##) 0.0## x#    = 0.0##
 "timesDouble x 1.0"  forall x#. (*##) x#    1.0## = x#
 "timesDouble 1.0 x"  forall x#. (*##) 1.0## x#    = x#
 "divideDouble x 1.0" forall x#. (/##) x#    1.0## = x#
   #-}
 
+{-
+We'd like to have more rules, but for example:
+
+This gives wrong answer (0) for NaN - NaN (should be NaN):
+    "minusDouble x x"    forall x#. (-##) x#    x#    = 0.0##
+
+This gives wrong answer (0) for 0 * NaN (should be NaN):
+    "timesDouble 0.0 x"  forall x#. (*##) 0.0## x#    = 0.0##
+
+This gives wrong answer (0) for NaN * 0 (should be NaN):
+    "timesDouble x 0.0"  forall x#. (*##) x#    0.0## = 0.0##
+
+These are tested by num014.
+-}
+
 -- Wrappers for the shift operations.  The uncheckedShift# family are
 -- undefined when the amount being shifted by is greater than the size
 -- in bits of Int#, so these wrappers perform a check and return