[GlobalISel] Expand IRTranslator docs. NFC (#89186)

Add some more details about how calls are lowered and what APIs are
available.

llvm/docs/GlobalISel/IRTranslator.rst

@@ -6,7 +6,7 @@ IRTranslator
 .. contents::
   :local:
 
-This pass translates the input LLVM-IR ``Function`` to a GMIR
+This pass translates the input LLVM-IR ``Function`` to a :doc:`GMIR`
 ``MachineFunction``. This is typically a direct translation but does
 occasionally get a bit more involved. For example:
 
@@ -51,8 +51,37 @@ Translating Function Calls
 
 The ``IRTranslator`` also implements the ABI's calling convention by lowering
 calls, returns, and arguments to the appropriate physical register usage and
-instruction sequences. This is achieved using the ``CallLowering``
-implementation,
+instruction sequences. This is achieved using the ``CallLowering`` interface,
+which provides several hooks that targets should implement:
+``lowerFormalArguments``, ``lowerReturn``, ``lowerCall`` etc.
+
+In essence, all of these hooks need to find a way to move the argument/return
+values between the virtual registers used in the rest of the function and either
+physical registers or the stack, as dictated by the ABI. This may involve
+splitting large types into smaller ones, introducing sign/zero extensions etc.
+In order to share as much of this code as possible between the different
+backends, ``CallLowering`` makes available a few helpers and interfaces:
+
+* ``ArgInfo`` - used for formal arguments, but also return values, actual
+  arguments and call results; contains info such as the IR type, the virtual
+  registers etc; large values will likely have to be split into several
+  ``ArgInfo`` objects (``CallLowering::splitToValueTypes`` can help with that);
+
+* ``ValueAssigner`` - uses a ``CCAssignFn``, usually generated by TableGen (see
+  :ref:`backend-calling-convs`), to decide where to put each
+  ``ArgInfo`` (physical register or stack); backends can use the provided
+  ``IncomingValueAssigner`` (for formal arguments and call results) and
+  ``OutgoingValueAssigner`` (for actual arguments and function returns), but
+  it's also possible to subclass them;
+
+* ``ValueHandler`` - inserts the necessary instructions for putting each value
+  where it belongs; it has pure virtual methods for assigning values to
+  registers or to addresses, and a host of other helpers;
+
+* ``determineAndHandleAssignments`` (or for more fine grained control,
+  ``determineAssignments`` and ``handleAssignments``) - contains some boilerplate
+  for invoking a given ``ValueAssigner`` and ``ValueHandler`` on a series of
+  ``ArgInfo`` objects.
 
 .. _irtranslator-aggregates:
 

llvm/docs/WritingAnLLVMBackend.rst

@@ -1503,6 +1503,8 @@ non-v9 SPARC implementations.
   if (TM.getSubtarget<SparcSubtarget>().isV9())
     setOperationAction(ISD::CTPOP, MVT::i32, Legal);
 
+.. _backend-calling-convs:
+
 Calling Conventions
 -------------------