Bug 1590907 - Determine 64-bit in public JS header without configure macro. r=froydnj,sfink

If only relying on JS_64BIT to determine whether the system is 64-bits,
then the result will be incorrect when the header is installed as a
public header for use by embedders, and since JS_BITS_PER_WORD affects
the layout of structs in header files, things will go badly wrong.

This uses two other ways of determining pointer width, hopefully
cross-platform enough. __SIZEOF_POINTER__ is a GCC-ism and probably
works in Clang as well. UINTPTR_MAX is hopefully sufficiently
cross-platform as a last resort.

Differential Revision: https://phabricator.services.mozilla.com/D52458

--HG--
extra : moz-landing-system : lando

js/src/jsapi.cpp

@@ -124,6 +124,14 @@ using JS::SourceText;
 #  define JS_ADDRESSOF_VA_LIST(ap) (&(ap))
 #endif
 
+// See preprocessor definition of JS_BITS_PER_WORD in jstypes.h; make sure
+// JS_64BIT (used internally) agrees with it
+#ifdef JS_64BIT
+static_assert(JS_BITS_PER_WORD == 64, "values must be in sync");
+#else
+static_assert(JS_BITS_PER_WORD == 32, "values must be in sync");
+#endif
+
 JS_PUBLIC_API void JS::CallArgs::reportMoreArgsNeeded(JSContext* cx,
                                                       const char* fnname,
                                                       unsigned required,

js/src/jstypes.h

@@ -109,12 +109,17 @@ constexpr size_t RoundDown(size_t x, size_t y) { return (x / y) * y; }
 constexpr size_t Round(size_t x, size_t y) { return ((x + y / 2) / y) * y; }
 }  // namespace js
 
-#if defined(JS_64BIT)
+#if (defined(__SIZEOF_POINTER__) && __SIZEOF_POINTER__ == 8) || \
+    (defined(UINTPTR_MAX) && UINTPTR_MAX == 0xFFFFFFFFFFFFFFFFu)
 #  define JS_BITS_PER_WORD 64
 #else
 #  define JS_BITS_PER_WORD 32
 #endif
 
+static_assert(sizeof(void*) == 8 ? JS_BITS_PER_WORD == 64
+                                 : JS_BITS_PER_WORD == 32,
+              "preprocessor and compiler must agree");
+
 /***********************************************************************
 ** MACROS:      JS_FUNC_TO_DATA_PTR
 **              JS_DATA_TO_FUNC_PTR