Mojo: Require our public options structs to be aligned like int64_t's.

The structs are meant to be extensible and part of the ABI, and not
requiring such alignment of them would mean that it'd be difficult to
add (u)int64_t's (or anything aligned like them) to them in the future.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/295383012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273508 0039d316-1c4b-4281-b951-d872f2087c98


CrOS-Libchrome-Original-Commit: 54743a5071e673a394501c3da0f3de7409cf3891

4 files changed, 65 insertions, 3 deletions

diff --git a/mojo/public/c/system/buffer.h b/mojo/public/c/system/buffer.h
index 360832150f..ab5734d7eb 100644
--- a/mojo/public/c/system/buffer.h
+++ b/mojo/public/c/system/buffer.h
@@ -38,7 +38,8 @@ const MojoCreateSharedBufferOptionsFlags
     ((MojoCreateSharedBufferOptionsFlags) 0)
 #endif
 
-struct MojoCreateSharedBufferOptions {
+MOJO_COMPILE_ASSERT(MOJO_ALIGNOF(int64_t) == 8, int64_t_has_weird_alignment);
+struct MOJO_ALIGNAS(8) MojoCreateSharedBufferOptions {
   uint32_t struct_size;
   MojoCreateSharedBufferOptionsFlags flags;
 };
diff --git a/mojo/public/c/system/data_pipe.h b/mojo/public/c/system/data_pipe.h
index 511858df76..1313448ebd 100644
--- a/mojo/public/c/system/data_pipe.h
+++ b/mojo/public/c/system/data_pipe.h
@@ -46,7 +46,8 @@ const MojoCreateDataPipeOptionsFlags
     ((MojoCreateDataPipeOptionsFlags) 1 << 0)
 #endif
 
-struct MojoCreateDataPipeOptions {
+MOJO_COMPILE_ASSERT(MOJO_ALIGNOF(int64_t) == 8, int64_t_has_weird_alignment);
+struct MOJO_ALIGNAS(8) MojoCreateDataPipeOptions {
   uint32_t struct_size;
   MojoCreateDataPipeOptionsFlags flags;
   uint32_t element_num_bytes;
diff --git a/mojo/public/c/system/macros.h b/mojo/public/c/system/macros.h
index 322c3be61f..f1e3c7d683 100644
--- a/mojo/public/c/system/macros.h
+++ b/mojo/public/c/system/macros.h
@@ -26,8 +26,10 @@
 #define MOJO_WARN_UNUSED_RESULT
 #endif
 
+// Assert things at compile time. (|msg| should be a valid identifier name.)
 // This macro is currently C++-only, but we want to use it in the C core.h.
-// Used to assert things at compile time.
+// Use like:
+//   MOJO_COMPILE_ASSERT(sizeof(Foo) == 12, Foo_has_invalid_size);
 #if __cplusplus >= 201103L
 #define MOJO_COMPILE_ASSERT(expr, msg) static_assert(expr, #msg)
 #elif defined(__cplusplus)
@@ -38,4 +40,33 @@ namespace mojo { template <bool> struct CompileAssert {}; }
 #define MOJO_COMPILE_ASSERT(expr, msg)
 #endif
 
+// Like the C++11 |alignof| operator.
+#if __cplusplus >= 201103L
+#define MOJO_ALIGNOF(type) alignof(type)
+#elif defined(__GNUC__)
+#define MOJO_ALIGNOF(type) __alignof__(type)
+#elif defined(_MSC_VER)
+// The use of |sizeof| is to work around a bug in MSVC 2010 (see
+// http://goo.gl/isH0C; supposedly fixed since then).
+#define MOJO_ALIGNOF(type) (sizeof(type) - sizeof(type) + __alignof(type))
+#else
+#error "Please define MOJO_ALIGNOF() for your compiler."
+#endif
+
+// Specify the alignment of a |struct|, etc.
+// Use like:
+//   struct MOJO_ALIGNAS(8) Foo { ... };
+// Unlike the C++11 |alignas()|, |alignment| must be an integer. It may not be a
+// type, nor can it be an expression like |MOJO_ALIGNOF(type)| (due to the
+// non-C++11 MSVS version).
+#if __cplusplus >= 201103L
+#define MOJO_ALIGNAS(alignment) alignas(alignment)
+#elif defined(__GNUC__)
+#define MOJO_ALIGNAS(alignment) __attribute__((aligned(alignment)))
+#elif defined(_MSC_VER)
+#define MOJO_ALIGNAS(alignment) __declspec(align(alignment))
+#else
+#error "Please define MOJO_ALIGNAS() for your compiler."
+#endif
+
 #endif  // MOJO_PUBLIC_C_SYSTEM_MACROS_H_
diff --git a/mojo/public/c/system/tests/macros_unittest.cc b/mojo/public/c/system/tests/macros_unittest.cc
index 5225b67cdb..fdc0c87c99 100644
--- a/mojo/public/c/system/tests/macros_unittest.cc
+++ b/mojo/public/c/system/tests/macros_unittest.cc
@@ -44,5 +44,34 @@ TEST(MacrosTest, CompileAssert) {
                       bad_compile_assert_failure);
 }
 
+TEST(MacrosTest, Alignof) {
+  // Strictly speaking, this isn't a portable test, but I think it'll pass on
+  // all the platforms we currently support.
+  EXPECT_EQ(1u, MOJO_ALIGNOF(char));
+  EXPECT_EQ(4u, MOJO_ALIGNOF(int32_t));
+  EXPECT_EQ(8u, MOJO_ALIGNOF(int64_t));
+  EXPECT_EQ(8u, MOJO_ALIGNOF(double));
+}
+
+// These structs are used in the Alignas test. Define them globally to avoid
+// MSVS warnings/errors.
+#if defined(_MSC_VER)
+#pragma warning(push)
+// Disable the warning "structure was padded due to __declspec(align())".
+#pragma warning(disable:4324)
+#endif
+struct MOJO_ALIGNAS(1) StructAlignas1 { char x; };
+struct MOJO_ALIGNAS(4) StructAlignas4 { char x; };
+struct MOJO_ALIGNAS(8) StructAlignas8 { char x; };
+#if defined(_MSC_VER)
+#pragma warning(pop)
+#endif
+
+TEST(MacrosTest, Alignas) {
+  EXPECT_EQ(1u, MOJO_ALIGNOF(StructAlignas1));
+  EXPECT_EQ(4u, MOJO_ALIGNOF(StructAlignas4));
+  EXPECT_EQ(8u, MOJO_ALIGNOF(StructAlignas8));
+}
+
 }  // namespace
 }  // namespace mojo