DOC-ONLY: improve multi-arch documentation
Document CLIB_MARCH_FN and CLIB_MARCH_FN_SELECT
Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I661ba2af79db8205d70fa87203d40a2e31b795e4
diff --git a/docs/gettingstarted/developers/multiarch/arbfns.rst b/docs/gettingstarted/developers/multiarch/arbfns.rst
new file mode 100644
index 0000000..d469bd8
--- /dev/null
+++ b/docs/gettingstarted/developers/multiarch/arbfns.rst
@@ -0,0 +1,87 @@
+Multi-Architecture Arbitrary Function Cookbook
+==============================================
+
+Optimizing arbitrary functions for multiple architectures is simple
+enough, and very similar to process used to produce multi-architecture
+graph node dispatch functions.
+
+As with multi-architecture graph nodes, we compile source files
+multiple times, generating multiple implementations of the original
+function, and a public selector function.
+
+Details
+-------
+
+Decorate function definitions with CLIB_MARCH_FN macros. For example:
+
+Change the original function prototype...
+
+::
+
+ u32 vlib_frame_alloc_to_node (vlib_main_t * vm, u32 to_node_index,
+ u32 frame_flags)
+
+...by recasting the function name and return type as the first two
+arguments to the CLIB_MARCH_FN macro:
+
+::
+
+ CLIB_MARCH_FN (vlib_frame_alloc_to_node, u32, vlib_main_t * vm,
+ u32 to_node_index, u32 frame_flags)
+
+In the actual vpp image, several versions of vlib_frame_alloc_to_node
+will appear: vlib_frame_alloc_to_node_avx2,
+vlib_frame_alloc_to_node_avx512, and so forth.
+
+
+For each multi-architecture function, use the CLIB_MARCH_FN_SELECT
+macro to help generate the one-and-only multi-architecture selector
+function:
+
+::
+
+ #ifndef CLIB_MARCH_VARIANT
+ u32
+ vlib_frame_alloc_to_node (vlib_main_t * vm, u32 to_node_index,
+ u32 frame_flags)
+ {
+ return CLIB_MARCH_FN_SELECT (vlib_frame_alloc_to_node)
+ (vm, to_node_index, frame_flags);
+ }
+ #endif /* CLIB_MARCH_VARIANT */
+
+Once bound, the multi-architecture selector function is about as
+expensive as an indirect function call; which is to say: not very
+expensive.
+
+Modify CMakeLists.txt
+---------------------
+
+If the component in question already lists "MULTIARCH_SOURCES", simply
+add the indicated .c file to the list. Otherwise, add as shown
+below. Note that the added file "new_multiarch_node.c" should appear in
+*both* SOURCES and MULTIARCH_SOURCES:
+
+::
+
+ add_vpp_plugin(myplugin
+ SOURCES
+ multiarch_code.c
+ ...
+
+ MULTIARCH_SOURCES
+ multiarch_code.c
+ ...
+ )
+
+A Word to the Wise
+------------------
+
+A file which liberally mixes functions worth compiling for multiple
+architectures and functions which are not will end up full of
+#ifndef CLIB_MARCH_VARIANT conditionals. This won't do a thing to make
+the code look any better.
+
+Depending on requirements, it may make sense to move functions to
+(new) files to reduce complexity and/or improve legibility of the
+resulting code.
diff --git a/docs/gettingstarted/developers/multiarch/index.rst b/docs/gettingstarted/developers/multiarch/index.rst
index 48065b8..a1dfe98 100644
--- a/docs/gettingstarted/developers/multiarch/index.rst
+++ b/docs/gettingstarted/developers/multiarch/index.rst
@@ -9,3 +9,4 @@
:maxdepth: 1
nodefns
+ arbfns