Document source_filename in LangRef.

[tejohnson]

Summary

Add documentation for new LLVM IR source_filename identifier.

[tejohnson]

Ping - let me know if the wording looks ok. Thanks

[tejohnson]

Ping - David, Mehdi, any comments on the wording or should I submit this change?

[mehdi_amini]

LGTM (I was expecting David to chime when you posted, since he was asking for it)

docs/LangRef.rst

::
     }
 
     ; At global scope.
     uselistorder i32* @global, { 1, 2, 0 }
     uselistorder i32 7, { 1, 0 }
     uselistorder i32 (i32) @bar, { 1, 0 }
     uselistorder_bb @foo, %bb, { 5, 1, 3, 2, 0, 4 }
 
+.. _source_filename:
+
+Source Filename
+---------------
+
+The *source filename* string is set to the original module identifier,
+which will be the name of the compiled source file when compiling from
+source through the clang front end, for example. It is then preserved through
+the IR and bitcode.
+
+This is currently necessary to generate a consistent unique global
+identifier for local functions used in profile data, which prepends the
+source file name to the local function name.
+
+The syntax for the source file name is simply:
+
+.. code-block:: llvm
+
+    source_filename = "/path/to/source.c"
+
 .. _typesystem:
 
 Type System
 ===========
 
 The LLVM type system is one of the most important features of the
 intermediate representation. Being typed enables a number of
 optimizations to be performed on the intermediate representation