[PATCH] D87458: [Clang] Clarify __builtin_memcpy_inline documentation

[Guillaume Chatelet via Phabricator via cfe-commits]

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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D87458

Files:
  clang/docs/LanguageExtensions.rst


Index: clang/docs/LanguageExtensions.rst
===================================================================
--- clang/docs/LanguageExtensions.rst
+++ clang/docs/LanguageExtensions.rst
@@ -2408,20 +2408,6 @@
 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 @@
 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
 --------------------------------------------


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