Make bit_cast fail if the source or dest are not trivially copyable

base/template_util.h

Index: base/template_util.h
diff --git a/base/template_util.h b/base/template_util.h
index 1bfc1ac814a23ceb186a9fcf6fa5924281d92783..100697534846f2295242f56d6c9fdb9b84909c6e 100644
--- a/base/template_util.h
+++ b/base/template_util.h
@@ -19,10 +19,27 @@
 #define CR_GLIBCXX_4_6_4 20121127
 #if defined(__GLIBCXX__) &&                                               \
     (__GLIBCXX__ < CR_GLIBCXX_4_7_0 || __GLIBCXX__ == CR_GLIBCXX_4_5_4 || \
-     __GLIBCXX__ == CR_GLIBCXX_4_6_4)
+     __GLIBCXX__ == CR_GLIBCXX_4_6_4 || __GLIBCXX__ == CR_GLIBCXX_5_0_0)
 #define CR_USE_FALLBACKS_FOR_OLD_GLIBCXX
 #endif
 
+// Some chromeos bots are using experimental 5.0 for some reason
+// which has partial support for type_traits, but misses a smaller subset
+// while removing some of the older non-standard stuff.
+#define CR_GLIBCXX_5_0_0 20150123

[dcheng, 2016/12/21 20:50:18]

Doesn't line 22 reference this? Should we delete addition at line 22, since we
want to use a different fallback path?

[danakj, 2016/12/21 20:58:38]

Oops, ya removing the 22 one.

+#if defined(__GLIBCXX__) && (__GLIBCXX__ == CR_GLIBCXX_5_0_0)
+#define CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX
+#endif
+
+// This hacks around using gcc with libc++ which has some incompatibilies.
+// - is_trivially_* doesn't work: https://llvm.org/bugs/show_bug.cgi?id=27538
+// TODO(danakj): Remove this when android builders are all using a newer version
+// of gcc, or the android ndk is updated to a newer libc++ that works with older
+// gcc versions.
+#if !defined(__clang__) && defined(_LIBCPP_VERSION)
+#define CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX
+#endif
+
 namespace base {
 
 template <class T> struct is_non_const_reference : std::false_type {};
@@ -126,8 +143,53 @@ template <class T>
 using is_trivially_destructible = std::is_trivially_destructible<T>;
 #endif
 
+// is_trivially_copyable is especially hard to get right.
+// - Older versions of libstdc++ will fail to have it like they do for other
+// type traits. In this case we should provide it based on compiler intrinsics.

[dcheng, 2016/12/21 20:50:18]

Nit: I find bulleted lists in comments to be easier to read when the list item
is aligned to the right of the bullets.

[danakj, 2016/12/21 20:58:38]

k

+// This is covered by the CR_USE_FALLBACKS_FOR_OLD_GLIBCXX define.
+// - An experimental release of gcc includes most of type_traits but misses
+// is_trivially_copyable, so we still have to avoid using libstdc++ in this
+// case, which is covered by CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX.
+// - When compiling libc++ from before git commit eeeada1c7, with gcc compiler,

[dcheng, 2016/12/21 20:50:18]

I'm guessing this is not a Chromium commit hash, so perhaps mention which
project this is (I guess libc++? But I think libc++ still uses svn?)

[danakj, 2016/12/21 20:58:38]

Youre right, I got it from the bug, but it is r239653 for libc++ thats better
then.

+// the std::is_trivially_copyable can fail. So we need to work around that by
+// not using the one in libc++ in this case. This is covered by the
+// CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX define, and is discussed in
+// https://llvm.org/bugs/show_bug.cgi?id=27538#c1 where they point out that
+// in eeeada1c7 this is fixed by libc++ checking for gcc 5.1.
+// - In both of the above cases we are using the gcc compiler. When defining
+// this ourselves on compiler intrinsics, the __is_trivially_copyable()
+// intrinsic is not available on gcc before version 5.1 (see the discussion in
+// https://llvm.org/bugs/show_bug.cgi?id=27538#c1 again), so we must check for
+// that version.
+// - When __is_trivially_copyable() is not available because we are on gcc older
+// than 5.1, we need to fall back to something, so we use __has_trivial_copy()
+// instead based on what was done one-off in bit_cast() previously.
+
+// TODO(crbug.com/554293): Remove this when all platforms have this in the std
+// namespace and it works with gcc as needed.
+#if defined(CR_USE_FALLBACKS_FOR_OLD_GLIBCXX) ||              \
+    defined(CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX) || \
+    defined(CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX)
+template <typename T>
+struct is_trivially_copyable {
+// TODO(danakj): Remove this when android builders are all using a newer version
+// of gcc, or the android ndk is updated to a newer libc++ that does this for
+// us.
+#if _GNUC_VER >= 501
+  static constexpr bool value = __is_trivially_copyable(T);
+#else
+  static constexpr bool value = __has_trivial_copy(T);
+#endif
+};
+#else
+template <class T>
+using is_trivially_copyable = std::is_trivially_copyable<T>;
+#endif
+
 }  // namespace base
 
 #undef CR_USE_FALLBACKS_FOR_OLD_GLIBCXX
+#undef CR_USE_FALLBACKS_FOR_GCC_WITH_LIBCXX
+#undef CR_USE_FALLBACKS_FOR_OLD_EXPERIMENTAL_GLIBCXX
 
 #endif  // BASE_TEMPLATE_UTIL_H_