gcc/config/m32c/m32c.abi

   Target Definitions for R8C/M16C/M32C
   Copyright (C) 2005-2021 Free Software Foundation, Inc.
   Contributed by Red Hat.

   This file is part of GCC.

   GCC is free software; you can redistribute it and/or modify it
   under the terms of the GNU General Public License as published
   by the Free Software Foundation; either version 3, or (at your
   option) any later version.

   GCC is distributed in the hope that it will be useful, but WITHOUT
   ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
   or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
   License for more details.

   You should have received a copy of the GNU General Public License
   along with GCC; see the file COPYING3.  If not see
   <http://www.gnu.org/licenses/>.


These are just some random notes I used during development of this
port.  Please don't consider these to be "official" specifications,
just additional information to help make the code easier to
understand.


Frame
=====

	+--------------------
	| incoming args
	+--------------------
	| return Address
osp ->	+--------------------
	| saved fp
fp ->	+--------------------
	| local data
	+--------------------
	| saved regs
	+--------------------
	| outgoing args (opt)
sp ->	+--------------------

Argument Passing
================

r8c, m16c
---------

First arg may be passed in r1l or r1 if it (1) fits (QImode or
HImode), (2) is named, and (3) is an integer or pointer type (no
structs, floats, etc).  Otherwise, it's passed on the stack.

Second arg may be passed in r2, same restrictions (but not QImode),
even if the first arg is passed on the stack.

Third and further args are passed on the stack.  No padding is used,
stack "alignment" is 8 bits.

m32cm, m32c
-----------
First arg may be passed in r0l or r0, same restrictions as above.

Second and further args are passed on the stack.  Padding is used
after QImode parameters (i.e. lower-addressed byte is the value,
higher-addressed byte is the padding), stack "alignment" is 16 bits.


Return Value
============

r8c, m16c
---------

QImode in r0l
HImode in r0
near pointer in r0
(desired)
SImode in r2r0
far pointer in r2r0
(actual)
Anything bigger than 16 bits is returned in memory, at mem0 (mem0
through mem15 are provided by libgcc.a)

Aggregate values (regardless of size) are returned by pushing a
pointer to a temporary area on the stack after the args are pushed.
The function fills in this area with the value.  Note that this
pointer on the stack does not affect how register arguments, if any,
are configured.

m32cm, m32c
-----------
Same.


Registers Preserved Across Calls
================================

r8c, m16c
---------
sb, fb, sp (i.e. nearly all registers are call clobbered)

m32cm, m32c
-----------
r1, r2, r3, a0, a1, sb, fb, sp
(except when used for return values)


Interrupt Handlers
==================

The stack frame is slightly different for interrupt handlers, because
(1) we don't have a usable parent frame, and (2) we have to use
special instructions to return and thus must save/restore everything
differently.

	+--------------------
	| program state
osp ->	+--------------------
	| return address
	+--------------------
	| saved r0..fp (pushm)
fp ->	+--------------------
	| local data
	+--------------------
	| saved regs mem0..mem15
	+--------------------
	| outgoing args (opt)
sp ->	+--------------------