diff --git a/autoconf/configure.ac b/autoconf/configure.ac
index 56853e9..7f625d4 100644
--- a/autoconf/configure.ac
+++ b/autoconf/configure.ac
@@ -372,6 +372,7 @@
   default) AC_SUBST(ENABLE_THREADS,[1]) ;;
   *) AC_MSG_ERROR([Invalid setting for --enable-threads. Use "yes" or "no"]) ;;
 esac
+AC_DEFINE_UNQUOTED([ENABLE_THREADS],$ENABLE_THREADS,[Define if threads enabled])
 
 dnl Allow building without position independent code
 AC_ARG_ENABLE(pic,
@@ -897,24 +898,22 @@
 fi
 
 dnl atomic builtins are required for threading support.
-if test "$ENABLE_THREADS" -eq 1 ; then
-  AC_MSG_CHECKING(for GCC atomic builtins)
-  AC_LINK_IFELSE(
-	AC_LANG_SOURCE(
-          [[int main() { 
-              volatile unsigned long val = 1;
-              __sync_synchronize();
-              __sync_val_compare_and_swap(&val, 1, 0);
-              return 0;
-            }
-          ]]),
-        AC_MSG_RESULT(yes),
-        AC_MSG_RESULT(no)
-        AC_SUBST(ENABLE_THREADS, [0])
-        AC_MSG_WARN([Threading support disabled because atomic builtins are missing]))
-fi
+AC_MSG_CHECKING(for GCC atomic builtins)
+AC_LINK_IFELSE(
+  AC_LANG_SOURCE(
+    [[int main() { 
+        volatile unsigned long val = 1;
+        __sync_synchronize();
+        __sync_val_compare_and_swap(&val, 1, 0);
+        return 0;
+      }
+    ]]),
+  AC_MSG_RESULT(yes)
+  AC_DEFINE(LLVM_MULTITHREADED, 1, Build multithreading support into LLVM),
+  AC_MSG_RESULT(no)
+  AC_DEFINE(LLVM_MULTITHREADED, 0, Build multithreading support into LLVM)
+  AC_MSG_WARN([LLVM will be built thread-unsafe because atomic builtins are missing]))
 
-AC_DEFINE_UNQUOTED([ENABLE_THREADS],$ENABLE_THREADS,[Define if threads enabled])
 
 dnl===-----------------------------------------------------------------------===
 dnl===
diff --git a/configure b/configure
index 20e3b4f..1179524 100755
--- a/configure
+++ b/configure
@@ -4870,6 +4870,11 @@
    { (exit 1); exit 1; }; } ;;
 esac
 
+cat >>confdefs.h <<_ACEOF
+#define ENABLE_THREADS $ENABLE_THREADS
+_ACEOF
+
+
 # Check whether --enable-pic was given.
 if test "${enable_pic+set}" = set; then
   enableval=$enable_pic;
@@ -10570,7 +10575,7 @@
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<EOF
-#line 10573 "configure"
+#line 10578 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -12714,7 +12719,7 @@
   ;;
 *-*-irix6*)
   # Find out which ABI we are using.
-  echo '#line 12717 "configure"' > conftest.$ac_ext
+  echo '#line 12722 "configure"' > conftest.$ac_ext
   if { (eval echo "$as_me:$LINENO: \"$ac_compile\"") >&5
   (eval $ac_compile) 2>&5
   ac_status=$?
@@ -14432,11 +14437,11 @@
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:14435: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:14440: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>conftest.err)
    ac_status=$?
    cat conftest.err >&5
-   echo "$as_me:14439: \$? = $ac_status" >&5
+   echo "$as_me:14444: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s "$ac_outfile"; then
      # The compiler can only warn and ignore the option if not recognized
      # So say no if there are warnings other than the usual output.
@@ -14700,11 +14705,11 @@
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:14703: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:14708: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>conftest.err)
    ac_status=$?
    cat conftest.err >&5
-   echo "$as_me:14707: \$? = $ac_status" >&5
+   echo "$as_me:14712: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s "$ac_outfile"; then
      # The compiler can only warn and ignore the option if not recognized
      # So say no if there are warnings other than the usual output.
@@ -14804,11 +14809,11 @@
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:14807: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:14812: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>out/conftest.err)
    ac_status=$?
    cat out/conftest.err >&5
-   echo "$as_me:14811: \$? = $ac_status" >&5
+   echo "$as_me:14816: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s out/conftest2.$ac_objext
    then
      # The compiler can only warn and ignore the option if not recognized
@@ -17256,7 +17261,7 @@
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<EOF
-#line 17259 "configure"
+#line 17264 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -17356,7 +17361,7 @@
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<EOF
-#line 17359 "configure"
+#line 17364 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -19724,11 +19729,11 @@
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:19727: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:19732: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>conftest.err)
    ac_status=$?
    cat conftest.err >&5
-   echo "$as_me:19731: \$? = $ac_status" >&5
+   echo "$as_me:19736: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s "$ac_outfile"; then
      # The compiler can only warn and ignore the option if not recognized
      # So say no if there are warnings other than the usual output.
@@ -19828,11 +19833,11 @@
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:19831: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:19836: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>out/conftest.err)
    ac_status=$?
    cat out/conftest.err >&5
-   echo "$as_me:19835: \$? = $ac_status" >&5
+   echo "$as_me:19840: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s out/conftest2.$ac_objext
    then
      # The compiler can only warn and ignore the option if not recognized
@@ -21398,11 +21403,11 @@
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:21401: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:21406: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>conftest.err)
    ac_status=$?
    cat conftest.err >&5
-   echo "$as_me:21405: \$? = $ac_status" >&5
+   echo "$as_me:21410: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s "$ac_outfile"; then
      # The compiler can only warn and ignore the option if not recognized
      # So say no if there are warnings other than the usual output.
@@ -21502,11 +21507,11 @@
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:21505: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:21510: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>out/conftest.err)
    ac_status=$?
    cat out/conftest.err >&5
-   echo "$as_me:21509: \$? = $ac_status" >&5
+   echo "$as_me:21514: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s out/conftest2.$ac_objext
    then
      # The compiler can only warn and ignore the option if not recognized
@@ -23737,11 +23742,11 @@
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:23740: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:23745: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>conftest.err)
    ac_status=$?
    cat conftest.err >&5
-   echo "$as_me:23744: \$? = $ac_status" >&5
+   echo "$as_me:23749: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s "$ac_outfile"; then
      # The compiler can only warn and ignore the option if not recognized
      # So say no if there are warnings other than the usual output.
@@ -24005,11 +24010,11 @@
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:24008: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:24013: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>conftest.err)
    ac_status=$?
    cat conftest.err >&5
-   echo "$as_me:24012: \$? = $ac_status" >&5
+   echo "$as_me:24017: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s "$ac_outfile"; then
      # The compiler can only warn and ignore the option if not recognized
      # So say no if there are warnings other than the usual output.
@@ -24109,11 +24114,11 @@
    -e 's:.*FLAGS}\{0,1\} :&$lt_compiler_flag :; t' \
    -e 's: [^ ]*conftest\.: $lt_compiler_flag&:; t' \
    -e 's:$: $lt_compiler_flag:'`
-   (eval echo "\"\$as_me:24112: $lt_compile\"" >&5)
+   (eval echo "\"\$as_me:24117: $lt_compile\"" >&5)
    (eval "$lt_compile" 2>out/conftest.err)
    ac_status=$?
    cat out/conftest.err >&5
-   echo "$as_me:24116: \$? = $ac_status" >&5
+   echo "$as_me:24121: \$? = $ac_status" >&5
    if (exit $ac_status) && test -s out/conftest2.$ac_objext
    then
      # The compiler can only warn and ignore the option if not recognized
@@ -33602,21 +33607,20 @@
   fi
 fi
 
-if test "$ENABLE_THREADS" -eq 1 ; then
-  { echo "$as_me:$LINENO: checking for GCC atomic builtins" >&5
+{ echo "$as_me:$LINENO: checking for GCC atomic builtins" >&5
 echo $ECHO_N "checking for GCC atomic builtins... $ECHO_C" >&6; }
-  cat >conftest.$ac_ext <<_ACEOF
+cat >conftest.$ac_ext <<_ACEOF
 /* confdefs.h.  */
 _ACEOF
 cat confdefs.h >>conftest.$ac_ext
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
 int main() {
-              volatile unsigned long val = 1;
-              __sync_synchronize();
-              __sync_val_compare_and_swap(&val, 1, 0);
-              return 0;
-            }
+        volatile unsigned long val = 1;
+        __sync_synchronize();
+        __sync_val_compare_and_swap(&val, 1, 0);
+        return 0;
+      }
 
 _ACEOF
 rm -f conftest.$ac_objext conftest$ac_exeext
@@ -33655,26 +33659,28 @@
   (exit $ac_status); }; }; then
   { echo "$as_me:$LINENO: result: yes" >&5
 echo "${ECHO_T}yes" >&6; }
+
+cat >>confdefs.h <<\_ACEOF
+#define LLVM_MULTITHREADED 1
+_ACEOF
+
 else
   echo "$as_me: failed program was:" >&5
 sed 's/^/| /' conftest.$ac_ext >&5
 
 	{ echo "$as_me:$LINENO: result: no" >&5
 echo "${ECHO_T}no" >&6; }
-        ENABLE_THREADS=0
 
-        { echo "$as_me:$LINENO: WARNING: Threading support disabled because atomic builtins are missing" >&5
-echo "$as_me: WARNING: Threading support disabled because atomic builtins are missing" >&2;}
+cat >>confdefs.h <<\_ACEOF
+#define LLVM_MULTITHREADED 0
+_ACEOF
+
+  { echo "$as_me:$LINENO: WARNING: LLVM will be built thread-unsafe because atomic builtins are missing" >&5
+echo "$as_me: WARNING: LLVM will be built thread-unsafe because atomic builtins are missing" >&2;}
 fi
 
 rm -f core conftest.err conftest.$ac_objext \
       conftest$ac_exeext conftest.$ac_ext
-fi
-
-
-cat >>confdefs.h <<_ACEOF
-#define ENABLE_THREADS $ENABLE_THREADS
-_ACEOF
 
 
 
diff --git a/include/llvm/System/Atomic.h b/include/llvm/System/Atomic.h
index 5830b37..38388a4 100644
--- a/include/llvm/System/Atomic.h
+++ b/include/llvm/System/Atomic.h
@@ -23,14 +23,8 @@
   namespace sys {
     
     inline void MemoryFence() {
-#if !defined(ENABLE_THREADS) || ENABLE_THREADS == 0
-#  if defined(__GNUC__)
-      __asm__ __volatile__("" : : : "memory");
-#  elif defined(_MSC_VER)
-      __asm { };
-#  else
-#    error No memory fence implementation for your platform!
-#  endif
+#if LLVM_MULTITHREADED==0
+      return;
 #else
 #  if defined(__GNUC__)
       __sync_synchronize();
@@ -42,7 +36,7 @@
 #endif
 }
 
-#if !defined(ENABLE_THREADS) || ENABLE_THREADS == 0
+#if LLVM_MULTITHREADED==0
     typedef unsigned long cas_flag;
     template<typename T>
     inline T CompareAndSwap(volatile T* dest,