[libc++abi] Create a macro for the 32 bit guard setting

[ldionne]

Summary

The goal is to use a descriptive name for this feature, instead of just
using arm.

[EricWF]

Theres more to the ARM config. The macro should reflect were implementing the ARM C++ ABI and not the plain itanium one.

FYI I've got a design doc and full rework of this file coming yet. I wouldnt spend much time cleaning it up.

[ldionne]

Update the name of the macro per a discussion with Eric

libcxxabi/include/__cxxabi_config.h

 #endif
 
 #if __has_attribute(__no_sanitize__) && defined(_LIBCXXABI_COMPILER_CLANG)
 #define _LIBCXXABI_NO_CFI __attribute__((__no_sanitize__("cfi")))
 #else
 #define _LIBCXXABI_NO_CFI
 #endif
 
+#if defined(__arm__)
+#  define _LIBCXXABI_USE_32BIT_GUARD
+#endif
+
 #endif // ____CXXABI_CONFIG_H

libcxxabi/include/cxxabi.h

 
 // 3.2.6 Pure Virtual Function API
 extern _LIBCXXABI_FUNC_VIS _LIBCXXABI_NORETURN void __cxa_pure_virtual(void);
 
 // 3.2.7 Deleted Virtual Function API
 extern _LIBCXXABI_FUNC_VIS _LIBCXXABI_NORETURN void __cxa_deleted_virtual(void);
 
 // 3.3.2 One-time Construction API
-#ifdef __arm__
+#if defined(_LIBCXXABI_USE_32BIT_GUARD)
 extern _LIBCXXABI_FUNC_VIS int __cxa_guard_acquire(uint32_t *);
 extern _LIBCXXABI_FUNC_VIS void __cxa_guard_release(uint32_t *);
 extern _LIBCXXABI_FUNC_VIS void __cxa_guard_abort(uint32_t *);
 #else
 extern _LIBCXXABI_FUNC_VIS int __cxa_guard_acquire(uint64_t *);
 extern _LIBCXXABI_FUNC_VIS void __cxa_guard_release(uint64_t *);
 extern _LIBCXXABI_FUNC_VIS void __cxa_guard_abort(uint64_t *);
 #endif

libcxxabi/src/cxa_guard.cpp

 namespace
 {
 
 enum InitializationResult {
   INIT_COMPLETE,
   INIT_NOT_COMPLETE,
 };
 
-#ifdef __arm__
+#if defined(_LIBCXXABI_USE_32BIT_GUARD)
 // A 32-bit, 4-byte-aligned static data value. The least significant 2 bits must
 // be statically initialized to 0.
 typedef uint32_t guard_type;
 #else
 typedef uint64_t guard_type;
 #endif
 
 #if !defined(_LIBCXXABI_HAS_NO_THREADS) && defined(__APPLE__) &&               \
-    !defined(__arm__)
+    !defined(_LIBCXXABI_USE_32BIT_GUARD)
 // This is a special-case pthread dependency for Mac. We can't pull this
 // out into libcxx's threading API (__threading_support) because not all
 // supported Mac environments provide this function (in pthread.h). To
 // make it possible to build/use libcxx in those environments, we have to
 // keep this pthread dependency local to libcxxabi. If there is some
 // convenient way to detect precisely when pthread_mach_thread_np is
 // available in a given Mac environment, it might still be possible to
 // bury this dependency in __threading_support.

 //===----------------------------------------------------------------------===//
 //                        GuardValue Definitions
 //===----------------------------------------------------------------------===//
 
 GuardValue GuardValue::ZERO() { return GuardValue(0); }
 
 GuardValue GuardValue::INIT_COMPLETE() {
   guard_type value = {0};
-#ifdef __arm__
+#if defined(_LIBCXXABI_USE_32BIT_GUARD)
   value |= 1;
 #else
   char* init_bit = (char*)&value;
   *init_bit = 1;
 #endif
   return GuardValue(value);
 }
 
 GuardValue GuardValue::INIT_PENDING() {
   return GuardValue(guard_value_from_lock(LOCK_ID_FOR_THREAD()));
 }
 
 bool GuardValue::is_initialization_complete() const {
-#ifdef __arm__
+#if defined(_LIBCXXABI_USE_32BIT_GUARD)
   return value & 1;
 #else
   const char* init_bit = (const char*)&value;
   return *init_bit;
 #endif
 }
 
 bool GuardValue::is_initialization_pending() const {
   return lock_value_from_guard(value) != 0;
 }
 
 lock_type GuardValue::get_lock_value() const {
   return lock_value_from_guard(value);
 }
 
 // Create a guard object with the lock set to the specified value.
 guard_type GuardValue::guard_value_from_lock(lock_type l) {
-#if defined(__APPLE__) && !defined(__arm__)
+#if defined(__APPLE__) && !defined(_LIBCXXABI_USE_32BIT_GUARD)
 #if __LITTLE_ENDIAN__
   return static_cast<guard_type>(l) << 32;
 #else
   return static_cast<guard_type>(l);
 #endif
-#else  // defined(__APPLE__) && !defined(__arm__)
+#else  // defined(__APPLE__) && !defined(_LIBCXXABI_USE_32BIT_GUARD)
   guard_type f = {0};
   memcpy(static_cast<char*>(static_cast<void*>(&f)) + 1, &l, sizeof(lock_type));
   return f;
-#endif // defined(__APPLE__) && !defined(__arm__)
+#endif // defined(__APPLE__) && !defined(_LIBCXXABI_USE_32BIT_GUARD)
 }
 
 lock_type GuardValue::lock_value_from_guard(guard_type g) {
-#if defined(__APPLE__) && !defined(__arm__)
+#if defined(__APPLE__) && !defined(_LIBCXXABI_USE_32BIT_GUARD)
 #if __LITTLE_ENDIAN__
   return static_cast<lock_type>(g >> 32);
 #else
   return static_cast<lock_type>(g);
 #endif
-#else  // defined(__APPLE__) && !defined(__arm__)
+#else  // defined(__APPLE__) && !defined(_LIBCXXABI_USE_32BIT_GUARD)
   uint8_t guard_bytes[sizeof(guard_type)];
   memcpy(&guard_bytes, &g, sizeof(guard_type));
   return guard_bytes[1] != 0;
-#endif // defined(__APPLE__) && !defined(__arm__)
+#endif // defined(__APPLE__) && !defined(_LIBCXXABI_USE_32BIT_GUARD)
 }
 
 }  // __cxxabiv1