Implement function prefix data as an IR feature.

Previous discussion:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-July/063909.html

Differential Revision: http://llvm-reviews.chandlerc.com/D1191

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190773 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LangRef.rst

@@ -552,16 +552,16 @@ an optional ``unnamed_addr`` attribute, a return type, an optional
 name, a (possibly empty) argument list (each with optional :ref:`parameter
 attributes <paramattrs>`), optional :ref:`function attributes <fnattrs>`,
 an optional section, an optional alignment, an optional :ref:`garbage
-collector name <gc>`, an opening curly brace, a list of basic blocks,
-and a closing curly brace.
+collector name <gc>`, an optional :ref:`prefix <prefixdata>`, an opening
+curly brace, a list of basic blocks, and a closing curly brace.
 
 LLVM function declarations consist of the "``declare``" keyword, an
 optional :ref:`linkage type <linkage>`, an optional :ref:`visibility
 style <visibility>`, an optional :ref:`calling convention <callingconv>`,
 an optional ``unnamed_addr`` attribute, a return type, an optional
 :ref:`parameter attribute <paramattrs>` for the return type, a function
-name, a possibly empty list of arguments, an optional alignment, and an
-optional :ref:`garbage collector name <gc>`.
+name, a possibly empty list of arguments, an optional alignment, an optional
+:ref:`garbage collector name <gc>` and an optional :ref:`prefix <prefixdata>`.
 
 A function definition contains a list of basic blocks, forming the CFG
 (Control Flow Graph) for the function. Each basic block may optionally
@@ -598,7 +598,7 @@ Syntax::
            [cconv] [ret attrs]
            <ResultType> @<FunctionName> ([argument list])
            [fn Attrs] [section "name"] [align N]
-           [gc] { ... }
+           [gc] [prefix Constant] { ... }
 
 .. _langref_aliases:
 
@@ -757,6 +757,50 @@ The compiler declares the supported values of *name*. Specifying a
 collector which will cause the compiler to alter its output in order to
 support the named garbage collection algorithm.
 
+.. _prefixdata:
+
+Prefix Data
+-----------
+
+Prefix data is data associated with a function which the code generator
+will emit immediately before the function body.  The purpose of this feature
+is to allow frontends to associate language-specific runtime metadata with
+specific functions and make it available through the function pointer while
+still allowing the function pointer to be called.  To access the data for a
+given function, a program may bitcast the function pointer to a pointer to
+the constant's type.  This implies that the IR symbol points to the start
+of the prefix data.
+
+To maintain the semantics of ordinary function calls, the prefix data must
+have a particular format.  Specifically, it must begin with a sequence of
+bytes which decode to a sequence of machine instructions, valid for the
+module's target, which transfer control to the point immediately succeeding
+the prefix data, without performing any other visible action.  This allows
+the inliner and other passes to reason about the semantics of the function
+definition without needing to reason about the prefix data.  Obviously this
+makes the format of the prefix data highly target dependent.
+
+A trivial example of valid prefix data for the x86 architecture is ``i8 144``,
+which encodes the ``nop`` instruction:
+
+.. code-block:: llvm
+
+    define void @f() prefix i8 144 { ... }
+
+Generally prefix data can be formed by encoding a relative branch instruction
+which skips the metadata, as in this example of valid prefix data for the
+x86_64 architecture, where the first two bytes encode ``jmp .+10``:
+
+.. code-block:: llvm
+
+    %0 = type <{ i8, i8, i8* }>
+
+    define void @f() prefix %0 <{ i8 235, i8 8, i8* @md}> { ... }
+
+A function may have prefix data but no body.  This has similar semantics
+to the ``available_externally`` linkage in that the data may be used by the
+optimizers but will not be emitted in the object file.
+
 .. _attrgrp:
 
 Attribute Groups