From 5a79ae9405bfc2b531b9a7c36855eb01f29b9cd2 Mon Sep 17 00:00:00 2001
From: schnetter <schnetter@a491c6a4-70bf-4b89-8b36-d6c0cb1f094e>
Date: Sun, 18 Dec 2005 21:40:52 +0000
Subject: Do not use the topmost 2 bits in a CCTK_INT, since this leads to
 undefined behaviour in C.  We could use more clever code and then use the
 second-topmost bit as well, omitting only 1 bit.

git-svn-id: http://svn.cactuscode.org/arrangements/CactusNumerical/SpaceMask/trunk@54 a491c6a4-70bf-4b89-8b36-d6c0cb1f094e
---
 src/MaskUtils.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/MaskUtils.c b/src/MaskUtils.c
index 2e9bf8e..1cb2eea 100644
--- a/src/MaskUtils.c
+++ b/src/MaskUtils.c
@@ -109,7 +109,11 @@ SpaceMask_get_free_bits(int nbits)
   n=1;
   new_bits = 0;
   j = 0;
-  for (i=0; i < (int) sizeof(CCTK_INT)*8 && j<nbits; ++i)
+  /* Do not use the 2 top bits.  Signed integer datatypes in C do not
+     have the overflow semantics that many people expect.  With care,
+     we could use the second topmost bit as well, but this doesn't
+     seem important at the moment.  */
+  for (i=0; i < (int) sizeof(CCTK_INT)*8-2 && j<nbits; ++i)
     {
       if (!(n & used_bits))
         {
@@ -159,7 +163,11 @@ SpaceMask_determine_state_mask(CCTK_INT allocated_bits, int nstates)
   n=1;
   bit=1;
 
-  for (i=0; i < (int) sizeof(CCTK_INT)*8; ++i)
+  /* Do not use the 2 top bits.  Signed integer datatypes in C do not
+     have the overflow semantics that many people expect.  With care,
+     we could use the second topmost bit as well, but this doesn't
+     seem important at the moment.  */
+  for (i=0; i < (int) sizeof(CCTK_INT)*8-2; ++i)
     {
       if (n & allocated_bits)
         {
-- 
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