[WIP] support arm_neon_optional flag in blink.

Source/wtf/CPU.h

Index: Source/wtf/CPU.h
diff --git a/Source/wtf/CPU.h b/Source/wtf/CPU.h
index 061d128afab82d7c4883c04f7e28a4d226e0e4a8..19ce83972f74eafb3b4a8f91741da142afd7b1c9 100644
--- a/Source/wtf/CPU.h
+++ b/Source/wtf/CPU.h
@@ -155,13 +155,33 @@
 #  endif
 #endif /* !defined(WTF_CPU_ARM_THUMB2) */
 
-#if defined(__ARM_NEON__) && !defined(WTF_CPU_ARM_NEON)
-#define WTF_CPU_ARM_NEON 1
+// The isSupportedNEON() function returns true if the target device
+// is higher than ARMv7-A and supports Neon instructions.
+// In OPTIONAL mode, this actually probes the CPU at runtime (and caches the result).
+#if CPU(ARM_NEON)
+static inline bool isSupportedNEON(void) { return true; }
+#elif CPU(ARM_NEON_OPTIONAL)
+extern bool isSupportedNEON(void) WTF_PURE_FUNCTION;
+#else
+static inline bool isSupportedNEON(void) { return false; }
 #endif
 
-#if CPU(ARM_NEON) && (!COMPILER(GCC) || GCC_VERSION_AT_LEAST(4, 7, 0))
-// All NEON intrinsics usage can be disabled by this macro.
-#define HAVE_ARM_NEON_INTRINSICS 1
+// Helper macro
+#define WTF_CPU_ARM_HAS_NEON() isSupportedNEON()
+
+// Use WTF_CPU_ARM_NEON_WRAP(symbol) to map 'symbol' to a NEON-specific symbol
+// when applicable. This will transform 'symbol' differently depending on
+// the current NEON configuration, i.e.:
+//
+//    NONE                             -> 'symbol'
+//    WTF_CPU_ARM_NEON                 -> 'symbolNEON'
+//    WTF_CPU_ARM_NEON_OPTIONAL        -> 'symbol' or 'symbolNEON' depending on runtime check.
+#if CPU(ARM_NEON)
+#  define WTF_CPU_ARM_NEON_WRAP(x) (x ## NEON)
+#elif CPU(ARM_NEON_OPTIONAL)
+#  define WTF_CPU_ARM_NEON_WRAP(x) (WTF_CPU_ARM_HAS_NEON() ? x ## NEON : x)
+#else
+#  define WTF_CPU_ARM_NEON_WRAP(x) (x)
 #endif
 
 #if defined(__ARM_ARCH_7S__)