Make sure that ScratchBuffer::Allocate() always return 8-byte aligned address.

mojo/public/cpp/bindings/tests/buffer_unittest.cc

Index: mojo/public/cpp/bindings/tests/buffer_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/buffer_unittest.cc b/mojo/public/cpp/bindings/tests/buffer_unittest.cc
index 7ef7520f5aec054eea41cdebbb3c9e37e7c222d5..1487ced7937d6e7e71c9fb181edaf661408677b8 100644
--- a/mojo/public/cpp/bindings/tests/buffer_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/buffer_unittest.cc
@@ -53,6 +53,23 @@ TEST(ScratchBufferTest, Basic) {
   EXPECT_TRUE(!overflow);
 }
 
+TEST(ScratchBufferTest, Alignment) {
+  Environment env;
+
+  internal::ScratchBuffer buf;
+  // Test that small allocations on the stack are aligned properly.
+  void* small = buf.Allocate(1);
+  EXPECT_EQ(0, reinterpret_cast<ptrdiff_t>(small) % 8);
+  small = buf.Allocate(2);
+  EXPECT_EQ(0, reinterpret_cast<ptrdiff_t>(small) % 8);
+
+  // Test that large allocations on the heap are aligned properly.
+  void* large = buf.Allocate(10*1024);
+  EXPECT_EQ(0, reinterpret_cast<ptrdiff_t>(large) % 8);
+  large = buf.Allocate(100*1024);
+  EXPECT_EQ(0, reinterpret_cast<ptrdiff_t>(large) % 8);
+}
+
 // Tests that Buffer::current() returns the correct value.
 TEST(ScratchBufferTest, Stacked) {
   Environment env;