Fix or suppress GCC warnings during build.

Summary:
Currently a number of GCC warnings are emitted when building libc++. This patch fixes or ignores all of them. The primary changes are:

* Work around strict aliasing issues in `typeinfo::hash_code()` by using __attribute__((may_alias)). However I think a non-aliasing `hash_code()` implementation is possible. Further investigation needed.
* Add `_LIBCPP_UNREACHABLE()` to switch in `strstream.cpp` to avoid -Wpotentially-uninitialized.
* Fix -Wunused-value warning in `__all` by adding a void cast.
* Ignore -Wattributes for now. There are a number of real attribute issues when using GCC but enabling the warning is too noisy.
* Ignore -Wliteral-suffix since it warns about the use of reserved identifiers. Note Only GCC 7.0 supports disabling this warning.
* Ignore -Wc++14-compat since it warns about the sized new/delete overloads.



Reviewers: EricWF

Differential Revision: https://reviews.llvm.org/D24003

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@280007 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 0 deletions

diff --git a/include/cstdlib b/include/cstdlib
index 10ed23107..9a775b844 100644
--- a/include/cstdlib
+++ b/include/cstdlib
@@ -89,6 +89,12 @@ void *aligned_alloc(size_t alignment, size_t size);                       // C11
 #pragma GCC system_header
 #endif
 
+#ifdef __GNUC__
+#define _LIBCPP_UNREACHABLE() __builtin_unreachable()
+#else
+#define _LIBCPP_UNREACHABLE() _VSTD::abort()
+#endif
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 using ::size_t;