[libcxx] implement <simd> ABI for Clang/GCC vector extension, constru…

…ctors, copy_from and copy_to.

Summary:
This patch adds a new macro _LIBCPP_HAS_NO_VECTOR_EXTENSION for detecting
whether a vector extension (\_\_attribute\_\_((vector_size(num_bytes)))) is
available.

On the top of that, this patch implements the following API:
* all constructors
* operator[]
* copy_from
* copy_to

It also defines simd_abi::native to use vector extension, if available.
In GCC and Clang, certain values with vector extension are passed by registers,
instead of memory.

Based on D41148.

Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits, MaskRay, lichray, sanjoy

Differential Revision: https://reviews.llvm.org/D41376

llvm-svn: 338309

libcxx/include/__config

@@ -620,6 +620,8 @@ namespace std {
 
 #define _LIBCPP_ALWAYS_INLINE __forceinline
 
+#define _LIBCPP_HAS_NO_VECTOR_EXTENSION
+
 #elif defined(_LIBCPP_COMPILER_IBM)
 
 #define _ALIGNAS(x) __attribute__((__aligned__(x)))
@@ -652,6 +654,8 @@ namespace std {
 
 #define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
 
+#define _LIBCPP_HAS_NO_VECTOR_EXTENSION
+
 #endif // _LIBCPP_COMPILER_[CLANG|GCC|MSVC|IBM]
 
 #if _LIBCPP_STD_VER >= 17

libcxx/include/experimental/__config

@@ -64,4 +64,11 @@
 #define _LIBCPP_END_NAMESPACE_EXPERIMENTAL_SIMD_ABI \
     } _LIBCPP_END_NAMESPACE_EXPERIMENTAL_SIMD
 
+// TODO: support more targets
+#if defined(__AVX__)
+#define _LIBCPP_NATIVE_SIMD_WIDTH_IN_BYTES 32
+#else
+#define _LIBCPP_NATIVE_SIMD_WIDTH_IN_BYTES 16
+#endif
+
 #endif