support atomicCompareAndSwap on powerpc for GCC versions prior to 4.1

src/powerpc.h

@@ -91,16 +91,33 @@ syncInstructionCache(const void* start, unsigned size)
 }
 
 #ifdef USE_ATOMIC_OPERATIONS
-#  if (__GNUC__ >= 4) && (__GNUC_MINOR__ >= 1)
 inline bool
 atomicCompareAndSwap(uintptr_t* p, uintptr_t old, uintptr_t new_)
 {
+#if (__GNUC__ >= 4) && (__GNUC_MINOR__ >= 1)
   return __sync_bool_compare_and_swap(p, old, new_);
+#else // not GCC >= 4.1
+  bool result;
+
+  __asm__ __volatile__("  sync\n"
+                       "1:\n"
+                       "  lwarx  %0,0,%2\n"
+                       "  cmpw   %0,%3\n"
+                       "  bne-   2f\n"
+                       "  stwcx. %4,0,%2\n"
+                       "  bne-   1b\n"
+                       "  isync  \n"
+                       "2:\n"
+                       "  xor    %0,%0,%3\n"
+                       "  cntlzw %0,%0\n"
+                       "  srwi   %0,%0,5\n"
+                       : "=&r"(result), "+m"(*p)
+                       : "r"(p), "r"(old), "r"(new_)
+                       : "cc", "memory");
+ 
+  return result;
+#endif // not GCC >= 4.1
 }
-#  else // not GCC >= 4.1
-// todo: implement using inline assembly
-#    undef USE_ATOMIC_OPERATIONS
-#  endif // not GCC >= 4.1
 #endif // USE_ATOMIC_OPERATIONS
 
 inline uint64_t