* generic/tclExecute.c (INST_SUB): fix usage of the new macro for

	overflow detection in sums, adapt to subtraction. Lenghty comment
	added.

1 files changed, 10 insertions, 3 deletions

diff --git a/generic/tclExecute.c b/generic/tclExecute.c
index 6bab480..3a870e0 100644
--- a/generic/tclExecute.c
+++ b/generic/tclExecute.c
@@ -12,7 +12,7 @@
  * See the file "license.terms" for information on usage and redistribution of
  * this file, and for a DISCLAIMER OF ALL WARRANTIES.
  *
- * RCS: @(#) $Id: tclExecute.c,v 1.318 2007/08/19 22:50:03 msofer Exp $
+ * RCS: @(#) $Id: tclExecute.c,v 1.319 2007/08/20 16:52:57 msofer Exp $
  */
 
 #include "tclInt.h"
@@ -5300,10 +5300,17 @@ TclExecuteByteCode(
 #endif
 		{
 		    /*
-		     * Must check for overflow.
+		     * Must check for overflow. The macro tests for overflows
+		     * in sums by looking at the sign bits. As we have a
+		     * subtraction here, we are adding -w2. As -w2 could in turn
+		     * overflow, we test with ~w2 instead: it has the opposite
+		     * sign bit to w2 so it does the job. Note that the only
+		     * "bad" case (w2==0) is irrelevant for this macro, as in
+		     * that case w1 and wResult have the same sign and there
+		     * is no overflow anyway.
 		     */
 
-		    if (Overflowing(w1, w2, wResult)) {
+		    if (Overflowing(w1, ~w2, wResult)) {
 			goto overflow;
 		    }
 		}