Force stack alignment on x86 devices

Summary:
public

JSC enforces a stack-alignment that wasn't always being provided on x86 builds. See D2886997.

Reviewed By: foghina

Differential Revision: D2900982

fb-gh-sync-id: 3f8dffcc1c528a4c21d7ed0b17e49d87761b1df7

ReactAndroid/src/main/jni/react/JSCHelpers.h

@@ -20,6 +20,25 @@ JSValueRef makeJSCException(
     JSContextRef ctx,
     const char* exception_text);
 
-JSValueRef evaluateScript(JSContextRef context, JSStringRef script, JSStringRef source, const char *cachePath = nullptr);
+#ifdef __i386__
+// The Android x86 ABI states that the NDK toolchain assumes 16 byte stack
+// alignment: http://developer.android.com/ndk/guides/x86.html JSC checks for
+// stack alignment, and fails with SIGTRAP if it is not.  Empirically, the
+// google android x86 emulator does not provide this alignment, and so JSC
+// calls may crash.  All checked calls go through here, so the attribute here
+// is added to force alignment and prevent crashes.
+
+JSValueRef evaluateScript(
+    JSContextRef ctx,
+    JSStringRef script,
+    JSStringRef sourceURL,
+    const char* cachePath = nullptr) __attribute__((force_align_arg_pointer));
+#else
+JSValueRef evaluateScript(
+    JSContextRef ctx,
+    JSStringRef script,
+    JSStringRef sourceURL,
+    const char* cachePath = nullptr);
+#endif
 
 } }