[clang] e38be70 - [Clang] Clarify __builtin_memcpy_inline documentation

[Guillaume Chatelet via cfe-commits]

Author: Guillaume Chatelet
Date: 2020-09-11T07:20:39Z
New Revision: e38be7091ee3d00430652aaa7b66ba3fc8394916

URL: https://github.com/llvm/llvm-project/commit/e38be7091ee3d00430652aaa7b66ba3fc8394916
DIFF: https://github.com/llvm/llvm-project/commit/e38be7091ee3d00430652aaa7b66ba3fc8394916.diff

LOG: [Clang] Clarify __builtin_memcpy_inline documentation

This patch updates the documentation about `__builtin_memcpy_inline` and reorders the sections so it is more consitent and understandable.

Differential Revision: https://reviews.llvm.org/D87458

Added: 
    

Modified: 
    clang/docs/LanguageExtensions.rst

Removed: 
    


################################################################################
diff  --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
index 60b3f21b3e50..073d9c86e22f 100644
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -2408,20 +2408,6 @@ with ``__has_feature(cxx_constexpr_string_builtins)``.
 Memory builtins
 ---------------
 
- * ``__builtin_memcpy_inline``
-
-.. code-block:: c
-
-  void __builtin_memcpy_inline(void *dst, const void *src, size_t size);
-
-``__builtin_memcpy_inline(dst, src, size)`` is identical to
-``__builtin_memcpy(dst, src, size)`` except that the generated code is
-guaranteed not to call any external functions. See LLVM IR `llvm.memcpy.inline
-<https://llvm.org/docs/LangRef.html#llvm-memcpy-inline-intrinsic>`_ Intrinsic 
-for more information.
-
-Note that the `size` argument must be a compile time constant.
-
 Clang provides constant expression evaluation support for builtin forms of the
 following functions from the C standard library headers
 ``<string.h>`` and ``<wchar.h>``:
@@ -2439,7 +2425,27 @@ are pointers to arrays with the same trivially copyable element type, and the
 given size is an exact multiple of the element size that is no greater than
 the number of elements accessible through the source and destination operands.
 
-Constant evaluation support is not yet provided for ``__builtin_memcpy_inline``.
+Guaranteed inlined copy
+^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: c
+
+  void __builtin_memcpy_inline(void *dst, const void *src, size_t size);
+
+
+``__builtin_memcpy_inline`` has been designed as a building block for efficient
+``memcpy`` implementations. It is identical to ``__builtin_memcpy`` but also
+guarantees not to call any external functions. See LLVM IR `llvm.memcpy.inline
+<https://llvm.org/docs/LangRef.html#llvm-memcpy-inline-intrinsic>`_ Intrinsic 
+for more information.
+
+This is useful to implement a custom version of ``memcpy``, implemement a
+``libc`` memcpy or work around the absence of a ``libc``.
+
+Note that the `size` argument must be a compile time constant.
+
+Note that this intrinsic cannot yet be called in a ``constexpr`` context.
+
 
 Atomic Min/Max builtins with memory ordering
 --------------------------------------------