diff options
Diffstat (limited to 'base/template_util.h')
| -rw-r--r-- | base/template_util.h | 67 |
1 files changed, 67 insertions, 0 deletions
diff --git a/base/template_util.h b/base/template_util.h index 1bfc1ac814..42552107cf 100644 --- a/base/template_util.h +++ b/base/template_util.h @@ -23,6 +23,28 @@ #define CR_USE_FALLBACKS_FOR_OLD_GLIBCXX #endif +// Some versions of libstdc++ have partial support for type_traits, but misses +// a smaller subset while removing some of the older non-standard stuff. Assume +// that all versions below 5.0 fall in this category, along with one 5.0 +// experimental release. Test for this by consulting compiler major version, +// the only reliable option available, so theoretically this could fail should +// you attempt to mix an earlier version of libstdc++ with >= GCC5. But +// that's unlikely to work out, especially as GCC5 changed ABI. +#define CR_GLIBCXX_5_0_0 20150123 +#if (defined(__GNUC__) && __GNUC__ < 5) || \ + (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 +148,53 @@ 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. 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 r239653, with a gcc compiler, 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 libc++'s commit r239653 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_ |