1   Target Definitions for R8C/M16C/M32C
2   Copyright (C) 2005-2022 Free Software Foundation, Inc.
3   Contributed by Red Hat.
4
5   This file is part of GCC.
6
7   GCC is free software; you can redistribute it and/or modify it
8   under the terms of the GNU General Public License as published
9   by the Free Software Foundation; either version 3, or (at your
10   option) any later version.
11
12   GCC is distributed in the hope that it will be useful, but WITHOUT
13   ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
14   or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
15   License for more details.
16
17   You should have received a copy of the GNU General Public License
18   along with GCC; see the file COPYING3.  If not see
19   <http://www.gnu.org/licenses/>.
20
21
22These are just some random notes I used during development of this
23port.  Please don't consider these to be "official" specifications,
24just additional information to help make the code easier to
25understand.
26
27
28Frame
29=====
30
31	+--------------------
32	| incoming args
33	+--------------------
34	| return Address
35osp ->	+--------------------
36	| saved fp
37fp ->	+--------------------
38	| local data
39	+--------------------
40	| saved regs
41	+--------------------
42	| outgoing args (opt)
43sp ->	+--------------------
44
45Argument Passing
46================
47
48r8c, m16c
49---------
50
51First arg may be passed in r1l or r1 if it (1) fits (QImode or
52HImode), (2) is named, and (3) is an integer or pointer type (no
53structs, floats, etc).  Otherwise, it's passed on the stack.
54
55Second arg may be passed in r2, same restrictions (but not QImode),
56even if the first arg is passed on the stack.
57
58Third and further args are passed on the stack.  No padding is used,
59stack "alignment" is 8 bits.
60
61m32cm, m32c
62-----------
63First arg may be passed in r0l or r0, same restrictions as above.
64
65Second and further args are passed on the stack.  Padding is used
66after QImode parameters (i.e. lower-addressed byte is the value,
67higher-addressed byte is the padding), stack "alignment" is 16 bits.
68
69
70Return Value
71============
72
73r8c, m16c
74---------
75
76QImode in r0l
77HImode in r0
78near pointer in r0
79(desired)
80SImode in r2r0
81far pointer in r2r0
82(actual)
83Anything bigger than 16 bits is returned in memory, at mem0 (mem0
84through mem15 are provided by libgcc.a)
85
86Aggregate values (regardless of size) are returned by pushing a
87pointer to a temporary area on the stack after the args are pushed.
88The function fills in this area with the value.  Note that this
89pointer on the stack does not affect how register arguments, if any,
90are configured.
91
92m32cm, m32c
93-----------
94Same.
95
96
97Registers Preserved Across Calls
98================================
99
100r8c, m16c
101---------
102sb, fb, sp (i.e. nearly all registers are call clobbered)
103
104m32cm, m32c
105-----------
106r1, r2, r3, a0, a1, sb, fb, sp
107(except when used for return values)
108
109
110Interrupt Handlers
111==================
112
113The stack frame is slightly different for interrupt handlers, because
114(1) we don't have a usable parent frame, and (2) we have to use
115special instructions to return and thus must save/restore everything
116differently.
117
118	+--------------------
119	| program state
120osp ->	+--------------------
121	| return address
122	+--------------------
123	| saved r0..fp (pushm)
124fp ->	+--------------------
125	| local data
126	+--------------------
127	| saved regs mem0..mem15
128	+--------------------
129	| outgoing args (opt)
130sp ->	+--------------------
131
132