Add Function multiversion to the release notes.

[erichkeane]

Summary

Richard suggested that I add this feature to the release notes,
so I was hoping someone (anyone willing:) ) could do a quick read
through for me.

Thanks!
-Erich

[echristo]

I think you're missing that right now it's x86 only yes? :)

-eric

[erichkeane]

Ah, right :) Slipped my mind that ELF != x86-linux.

[rsmith]

Looks fine. Can you also update include/clang/Basic/AttrDocs.td to mention multiversioning in TargetDocs? Then you can add a few words here to say "consult the documentation for the target attribute for more information" or similar :)

docs/ReleaseNotes.rst

 -----------------------
 
 Clang now supports the ...
 
 
 Attribute Changes in Clang
 --------------------------
 
+- Clang now supports function multiversioning with attribute 'target' on ELF
+  based x86/x86-64 environments by using indirect functions. This implementation
+  has a few minor limitations over the GCC implementation for the sake of AST
+  sanity, however it is otherwise compatible with existing code using this
+  feature for GCC. Consult the documentation for the target attribute for more
+  information.
 - ...
 
 Windows Support
 ---------------
 
 Clang's support for building native Windows programs ...
 
 

include/clang/Basic/AttrDocs.td

 The current set of options correspond to the existing "subtarget features" for
 the target with or without a "-mno-" in front corresponding to the absence
 of the feature, as well as ``arch="CPU"`` which will change the default "CPU"
 for the function.
 
 Example "subtarget features" from the x86 backend include: "mmx", "sse", "sse4.2",
 "avx", "xop" and largely correspond to the machine specific options handled by
 the front end.
+
+Additionally, this attribute supports function multiversioning for ELF based
+x86/x86-64 targets, which can be used to create multiple implementations of the
+same function that will be resolved at runtime based on the priority of their
+``target`` attribute strings. A function is considered a multiversioned function
+if either two declarations of the function have different ``target`` attribute
+  strings, or if it has a ``target`` attribute string of ``default``.  For
+  example:
+
+  .. code-block:: c++
+  __attribute__((target("arch=atom")))
+  void foo() {} // will be called on 'atom' processors.
+  __attribute__((target("default")))
+  void foo() {} // will be called on any other processors.
+
+All multiversioned functions must contain a ``default`` (fallback)
+implementation, otherwise usages of the function are considered invalid.
+Additionally, a function may not become multiversioned after its first use.
 }];
 }
 
 def DocCatAMDGPUAttributes : DocumentationCategory<"AMD GPU Attributes">;
 
 def AMDGPUFlatWorkGroupSizeDocs : Documentation {
   let Category = DocCatAMDGPUAttributes;
   let Content = [{