Use C++11 alignment primitives

base/compiler_specific.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_COMPILER_SPECIFIC_H_
 #define BASE_COMPILER_SPECIFIC_H_
 
 #include "build/build_config.h"
 
 #if defined(COMPILER_MSVC)
@@ skipping 93 matching lines @@
 #elif COMPILER_MSVC && defined(NDEBUG)
 #define ALWAYS_INLINE __forceinline
 #else
 #define ALWAYS_INLINE inline
 #endif
 
 // Specify memory alignment for structs, classes, etc.
 // Use like:
 //   class ALIGNAS(16) MyClass { ... }
 //   ALIGNAS(16) int array[4];
+//
+// In most places you can use the C++11 keyword "alignas", which is preferred.
+//
+// But compilers have trouble mixing __attribute__((...)) syntax with
+// alignas(...) syntax.
+//
+// Doesn't work in clang or gcc:
+//   struct alignas(16) __attribute__((packed)) S { char c; };
+// Works in clang but not gcc:
+//   struct __attribute__((packed)) alignas(16) S2 { char c; };
+// Works in clang and gcc:
+//   struct alignas(16) S3 { char c; } __attribute__((packed));
+//
+// There are also some attributes that must be specified *before* a class
+// definition: visibility (used for exporting functions/classes) is one of
+// these attributes. This means that it is not possible to have use alignas()

[danakj, 2017/06/12 18:32:10]

typo: "to have use"

[brettw, 2017/06/12 22:22:15]

I did this but haven't uploaded yet since I assume there will be another issue
and I'll have to do a new patchset.

+// with a class that is marked as exported.
 #if defined(COMPILER_MSVC)
 #define ALIGNAS(byte_alignment) __declspec(align(byte_alignment))
 #elif defined(COMPILER_GCC)
 #define ALIGNAS(byte_alignment) __attribute__((aligned(byte_alignment)))
 #endif
 
-// Return the byte alignment of the given type (available at compile time).
-// Use like:
-//   ALIGNOF(int32_t)  // this would be 4
-#if defined(COMPILER_MSVC)
-#define ALIGNOF(type) __alignof(type)
-#elif defined(COMPILER_GCC)
-#define ALIGNOF(type) __alignof__(type)
-#endif
-
 // Annotate a function indicating the caller must examine the return value.
 // Use like:
 //   int foo() WARN_UNUSED_RESULT;
 // To explicitly ignore a result, see |ignore_result()| in base/macros.h.
 #undef WARN_UNUSED_RESULT
 #if defined(COMPILER_GCC) || defined(__clang__)
 #define WARN_UNUSED_RESULT __attribute__((warn_unused_result))
 #else
 #define WARN_UNUSED_RESULT
 #endif
@@ skipping 84 matching lines @@
 
 // Compiler feature-detection.
 // clang.llvm.org/docs/LanguageExtensions.html#has-feature-and-has-extension
 #if defined(__has_feature)
 #define HAS_FEATURE(FEATURE) __has_feature(FEATURE)
 #else
 #define HAS_FEATURE(FEATURE) 0
 #endif
 
 #endif  // BASE_COMPILER_SPECIFIC_H_