LangRef documentation for the stackmap and patchpoint intrinsics.

These still have "experimental" status, meaning we don't guarantee
backward compatibility. However, they are already actively used by the
open source WebKit project, and have started to be adopted by other
projects.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197930 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LangRef.rst

@@ -366,6 +366,18 @@ added in the future:
     accessed runtime components pinned to specific hardware registers.
     At the moment only X86 supports this convention (both 32 and 64
     bit).
+"``webkit_jscc``" - WebKit's JavaScript calling convention
+    This calling convention has been implemented for `WebKit FTL JIT
+    <https://trac.webkit.org/wiki/FTLJIT>`_. It passes arguments on the
+    stack right to left (as cdecl does), and returns a value in the
+    platform's customary return register.
+"``anyregcc``" - Dynamic calling convention for code patching
+    This is a special convention that supports patching an arbitrary code
+    sequence in place of a call site. This convention forces the call
+    arguments into registers but allows them to be dynamcially
+    allocated. This can currently only be used with calls to
+    llvm.experimental.patchpoint because only this intrinsic records
+    the location of its arguments in a side table. See :doc:`StackMaps`.
 "``cc <n>``" - Numbered convention
     Any calling convention may be specified by number, allowing
     target-specific calling conventions to be used. Target specific
@@ -8912,3 +8924,10 @@ Semantics:
 
 This intrinsic does nothing, and it's removed by optimizers and ignored
 by codegen.
+
+Stack Map Intrinsics
+--------------------
+
+LLVM provides experimental intrinsics to support runtime patching
+mechanisms commonly desired in dynamic language JITs. These intrinsics
+are described in :doc:`StackMaps`.