b=429387, add --with-arm-kuser; use it in JS, and pass it down to NSPR's configure; r=shaver,r=ted

configure.in

@@ -4255,6 +4255,18 @@ MOZ_ARG_WITH_STRING(java-bin-path,
 [  --with-java-bin-path=dir   Location of Java binaries (java, javac, jar)],
     JAVA_BIN_PATH=$withval)
 
+dnl ========================================================
+dnl Use ARM userspace kernel helpers; tell NSPR to enable
+dnl their usage and use them in spidermonkey.
+dnl ========================================================
+MOZ_ARG_WITH_BOOL(arm-kuser,
+[  --with-arm-kuser         Use kuser helpers (Linux/ARM only -- requires kernel 2.6.13 or later)],
+    USE_ARM_KUSER=1,
+    USE_ARM_KUSER=)
+if test -n "$USE_ARM_KUSER"; then
+   AC_DEFINE(USE_ARM_KUSER)
+fi
+
 dnl ========================================================
 dnl =
 dnl = Application
@@ -8264,6 +8276,9 @@ if test -z "$MOZ_NATIVE_NSPR" || test "$MOZ_LDAP_XPCOM"; then
     if test -n "$HAVE_64BIT_OS"; then
         ac_configure_args="$ac_configure_args --enable-64bit"
     fi
+    if test -n "$USE_ARM_KUSER"; then
+        ac_configure_args="$ac_configure_args --with-arm-kuser"
+    fi
     AC_OUTPUT_SUBDIRS(nsprpub)
     ac_configure_args="$_SUBDIR_CONFIG_ARGS"
 fi

js/src/jslock.cpp

@@ -178,6 +178,24 @@ js_CompareAndSwap(jsword *w, jsword ov, jsword nv)
     return !_check_lock((atomic_p)w, ov, nv);
 }
 
+#elif defined(USE_ARM_KUSER)
+
+/* See https://bugzilla.mozilla.org/show_bug.cgi?id=429387 for a
+ * description of this ABI; this is a function provided at a fixed
+ * location by the kernel in the memory space of each process.
+ */
+typedef int (__kernel_cmpxchg_t)(int oldval, int newval, volatile int *ptr);
+#define __kernel_cmpxchg (*(__kernel_cmpxchg_t *)0xffff0fc0)
+
+JS_STATIC_ASSERT(sizeof(jsword) == sizeof(int));
+
+static JS_INLINE int
+js_CompareAndSwap(jsword *w, jsword ov, jsword nv)
+{
+    volatile int *vp = (volatile int*)w;
+    return !__kernel_cmpxchg(ov, nv, vp);
+}
+
 #else
 
 #error "Define NSPR_LOCK if your platform lacks a compare-and-swap instruction."

js/src/jslock.h

@@ -226,7 +226,7 @@ extern void js_SetScopeInfo(JSScope *scope, const char *file, int line);
     !( (defined(_WIN32) && defined(_M_IX86)) ||                               \
        (defined(__GNUC__) && defined(__i386__)) ||                            \
        (defined(SOLARIS) && defined(sparc) && defined(ULTRA_SPARC)) ||        \
-       defined(AIX) )
+       defined(AIX) || defined(USE_ARM_KUSER))
 
 #define NSPR_LOCK 1