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Unified Diff: base/compiler_specific.h

Issue 2932053002: Use C++11 alignment primitives (Closed)
Patch Set: Fix merge Created 3 years, 6 months ago
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Index: base/compiler_specific.h
diff --git a/base/compiler_specific.h b/base/compiler_specific.h
index 34b0c2d7a2913f98c9b7faa10cc5ab7d7640b4c0..dbff842981114c5ea94cd0f4330efb816a519442 100644
--- a/base/compiler_specific.h
+++ b/base/compiler_specific.h
@@ -111,21 +111,29 @@
// 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 use alignas() 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;
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