[commit: ghc] : implement ARMv6/7 specific xchg function (330dff3)

[Manuel Chakravarty]

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : 

http://hackage.haskell.org/trac/ghc/changeset/330dff34ddc45eb0adc8d098a55f491cce0ea108

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

commit 330dff34ddc45eb0adc8d098a55f491cce0ea108
Author: Karel Gardas <karel.gardas at centrum.cz>
Date:   Tue Jul 5 20:10:18 2011 +0200

    implement ARMv6/7 specific xchg function

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

 includes/stg/SMP.h |   20 ++++++++++++++++++--
 1 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/includes/stg/SMP.h b/includes/stg/SMP.h
index d093439..0b7592d 100644
--- a/includes/stg/SMP.h
+++ b/includes/stg/SMP.h
@@ -1,6 +1,6 @@
 /* ----------------------------------------------------------------------------
  *
- * (c) The GHC Team, 2005-2009
+ * (c) The GHC Team, 2005-2011
  *
  * Macros for multi-CPU support
  *
@@ -136,10 +136,26 @@ xchg(StgPtr p, StgWord w)
 	: "+r" (result), "+m" (*p)
 	: /* no input-only operands */
       );
-#elif arm_HOST_ARCH
+#elif arm_HOST_ARCH && defined(PRE_ARMv6)
     __asm__ __volatile__ ("swp %0, %1, [%2]"
                          : "=&r" (result)
                          : "r" (w), "r" (p) : "memory");
+#elif arm_HOST_ARCH && !defined(PRE_ARMv6)
+    // swp instruction which is used in PRE_ARMv6 code above
+    // is deprecated in AMRv6 and later. ARM, Ltd. *highly* recommends
+    // to use ldrex/strex instruction pair for the same purpose
+    // see chapter: Synchronization and semaphores in ARM Architecture
+    // Reference manual
+    StgWord tmp;
+    __asm__ __volatile__ (
+                          "1:    ldrex  %0, [%3]\n"
+                          "      strex  %1, %2, [%3]\n"
+                          "      teq    %1, #1\n"
+                          "      beq    1b\n"
+                          : "=&r" (result), "=&r" (tmp)
+                          : "r" (w), "r" (p)
+                          : "memory"
+                          );
 #elif !defined(WITHSMP)
 #error xchg() unimplemented on this architecture
     result = *p;