GCC: fix __has_builtin defines for non-clang compilers

Defining __has_builtin to 0 for non-clang compilers in base headers can
lead to wrong detection of features. For example in base/location.h
checking for __has_builtin macros succeeds for non-clang compilers,
because base/check_op.h defines __has_builtin to 0 and is included
before base/location.h. Instead of defining __has_builtin to 0 for
non-clang compilers, define an independent preprocessor symbol that
reflects support for requested feature. Undefine the symbol to avoid
collision.

While we're at it fix base/memory/aligned_memory.h too.

Bug: 819294
Change-Id: Iac40dc44e7356b600e7d06aa4ccd1294bb09ebce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2224252Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773784}

base/check_op.h

@@ -48,8 +48,10 @@ BASE_EXPORT char* StreamValToStr(const void* v,
                                  void (*stream_func)(std::ostream&,
                                                      const void*));
 
-#ifndef __has_builtin
-#define __has_builtin(x) 0  // Compatibility with non-clang compilers.
+#ifdef __has_builtin
+#define SUPPORTS_BUILTIN_ADDRESSOF (__has_builtin(__builtin_addressof))
+#else
+#define SUPPORTS_BUILTIN_ADDRESSOF 0
 #endif
 
 template <typename T>
@@ -65,7 +67,7 @@ CheckOpValueStr(const T& v) {
   // operator& might be overloaded, so do the std::addressof dance.
   // __builtin_addressof is preferred since it also handles Obj-C ARC pointers.
   // Some casting is still needed, because T might be volatile.
-#if __has_builtin(__builtin_addressof)
+#if SUPPORTS_BUILTIN_ADDRESSOF
   const void* vp = const_cast<const void*>(
       reinterpret_cast<const volatile void*>(__builtin_addressof(v)));
 #else
@@ -75,6 +77,8 @@ CheckOpValueStr(const T& v) {
   return StreamValToStr(vp, f);
 }
 
+#undef SUPPORTS_BUILTIN_ADDRESSOF
+
 // Overload for types that have no operator<< but do have .ToString() defined.
 template <typename T>
 inline typename std::enable_if<

base/memory/aligned_memory.h

@@ -57,13 +57,15 @@ struct AlignedFreeDeleter {
   }
 };
 
-#ifndef __has_builtin
-#define __has_builtin(x) 0  // Compatibility with non-clang compilers.
+#ifdef __has_builtin
+#define SUPPORTS_BUILTIN_IS_ALIGNED (__has_builtin(__builtin_is_aligned))
+#else
+#define SUPPORTS_BUILTIN_IS_ALIGNED 0
 #endif
 
 inline bool IsAligned(uintptr_t val, size_t alignment) {
   // If the compiler supports builtin alignment checks prefer them.
-#if __has_builtin(__builtin_is_aligned)
+#if SUPPORTS_BUILTIN_IS_ALIGNED
   return __builtin_is_aligned(val, alignment);
 #else
   DCHECK(!((alignment - 1) & alignment))
@@ -72,6 +74,8 @@ inline bool IsAligned(uintptr_t val, size_t alignment) {
 #endif
 }
 
+#undef SUPPORTS_BUILTIN_IS_ALIGNED
+
 inline bool IsAligned(void* val, size_t alignment) {
   return IsAligned(reinterpret_cast<uintptr_t>(val), alignment);
 }