Copyright (c) 2001 Hans-Peter Nilsson.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1
or any later version published by the Free Software Foundation;
with the Invariant Sections being just "Location of the original",
with no Front-Cover Texts, and with no Back-Cover Texts.
A copy of the license is included in the section entitled "GNU
Free Documentation License".
The programs in the slides are put in the public domain; the slides
themselves are covered by the GNU Free Documentation License as above.
This document is a webbification of some slides and notes presented at
the MMIXfest
2001, October 6 in Munich, Germany, hosted by the Computer Science and
Mathematics Department, Munich University of Applied Sciences.
You are encouraged to report errors in the original document or
opinions about it to
hp@bitrange.com.
Contents
The original of this document is located at
http://bitrange.com/mmix/mmixfest-2001/mmixabi.html.
(No changes since the original webbification on
2001-10-10.)
I'll present an ABI based on Knuth's documents. I want your
opinions about some details where I'm undecided. I'll also
sketch a few details of an alternative ABI.
Performance of programs for the MMIX architecture relies on
keeping as many variables as possible in registers, to minimize
memory accesses by other means than through the register stack
and instruction fetches. So first hand, registers should be
used rather than a traditional memory stack.
Another goal is for the assembly code to be usable as input
for further modification by a human. The generated code should
as far as possible be compatible with mmixal and conventions in
Knuth's texts. There are currently (2001-10-10) some
non-conformance issues, which will be fixed wherever
possible.
It is described (in [ref 1]) that
parameters are usually passed in registers $0 and up, as seen by
the called function. Further, the return address is passed in
rJ, using PUSHJ and PUSHGO for function calls and POP for
returning ([ref 3]). Return values are put
in $0 and up, as seen by the returning (called) function.
In [ref 2], it is mentioned that $255 is
used by mmixal for instruction expansion under control of the
-x option. This happens when loading out-of-range addresses,
making register $255 unsuitable for literal use in assembly
programs. At the same place, it is suggested that functions
that need a memory stack can use $254 as a stack pointer and
when necessary, $253 as a frame-pointer.
The number of local registers (the value of special register
rG), is defined to never be lower than 32. It is also
recommended (in [ref 1]) that a program
should not assume more than 32 available local registers. This
number is higher than most human-written C-functions will use
anyway. Therefore, the number of incoming or outgoing registers
used for parameters cannot reasonably be higher than 32. In an
attempt to keep things simple (and to work around restrictions
in GCC), it seemed best to split that number even for incoming
and outgoing parameters; the number of registers used for
parameters is 16.
Hopefully, this should also minimize the number of register
moves necessary to get incoming registers "out-of-the-way" when
calling other functions.
Please note that register $16 being an outgoing parameter
register is just a first approximation. I'll explain briefly
the GCC-related issues later. The
17:th parameter and beyond are passed on the stack, with $254
(the stack-pointer) pointing to the first parameter in the
called function.
Just as mmixal does, the GNU binutils uses register $255 for
its own purposes when loading addresses. GCC also uses it for
temporary purposes in some cases, but only for instructions that
the assembler and linker will not expand.
The stack pointer $254 is set up in the code between Main and
main, and must have the same value when a function returns as
when it was entered.
If register $253, the suggested frame-pointer, is used in a
function, it must likewise be saved so it has the same value at
function exit as at function entry.
The GCC implementation reserves a few registers.
There's a GCC extension for nested functions, that for some
codes circumstances need to pass a context pointer; a pointer to
the local variables of the enclosing function when a pointer to
a nested function is passed to another function[1]. That pointer is passed in register
$252.
Register $251 is used for the structure return pointer.
Registers $247..$250 are used for C++ exception handling, and
registers $231.. $246 are reserved for the GNU ABI. There is no
need for a function to preserve these register values, but it
must be prepared that a called function may change them.
This is just the current (2001-10-10) work-in-progress
mapping. These allocations are not final, and some of the uses
can certainly share the same register. For the imaginary
typical MMIX programmer these allocations should not matter, as
they do not collide with GREG-allocated registers (unless the
program is short on registers).
MMIX can handle 8, 16, 32 and 64-bit data. A reasonable
mapping of C types exposes all these sizes so e.g. a C
programmer wanting to write for MMIX can do it simply by using
standard types[2]. The fundamental address
unit type in C is char (sizeof char is always 1), so BYTE maps
naturally to it. The C "char" type is by default signed,
because this is the case for most gcc ports and therefore would
presumably lead to the fewest compatibility problems. The rest
of the normal C integer types are "short int", "int" and "long
int". They naturally map to wyde, tetrabyte and octabyte.
Incidentally, this is the same mapping as the alpha gcc port
uses. The gcc type "long long"[3] has to
map to octabyte, otherwise cross-compilation from a 32-bit host
is not possible. This is a gcc restriction, but might not
matter too much, since on a "long long" is usually also 64 bits,
this being natural for existing 32-bit hosts.
The normal C floating point types are float, double and long
double. MMIX has 64-bit IEEE floating point quantities and
limited operations on 32-bit quantities, "short floats". They
naturally map to double and float, with long double an alias for
double.
Each of the fundamental types must be mapped to an address at
least a multiple of its size, so called "naturally aligned". To
be theoretically addressable with a GETA, individual variables
are aligned to tetrabyte alignment.
Similarly for structures, the structure layout has members
naturally aligned, with padding inserted where necessary. The
alignment of a structure is that of its largest contained type.
The size is a multiple of that type, with padding inserted at
the end of the structure.
If a parameter fits in a register, it is passed by-value,
with integer types extended to 64 bits by the caller.
Otherwise, it is passed by reference (as a pointer to the
original), and the called function has to copy it, in case it
needs a local modifiable copy.
The same goes for parameters passed on the stack; integer
types are promoted, passed as 64 bits, with larger-than-64-bit
types passed by reference.
Scalar values are returned in $0 as seen by the calling
function. We usually don't have to worry about the register
hole.
There is an exception: complex values together larger than 64
bits are returned in $0 and $1 as seen by the calling function.
For these multi-register return values, the called function
compensates for the hole, so the calling function sees the
register with $0 being the natural first part; like the order
allocated for registers.
Structures are returned specially. The caller passes a pointer
to an area where the structure contents is to be stored by the
called function, regardless of the structure size. The
structure-return pointer is passed in register $251.
While it's certainly possible to implement special calling
conventions for functions declared to take a variable number of
parameters, like printf, there is no need to. The called stdarg
function will arrange to map parameter registers to be
accessible as a va_list (usually an array). The called function
will have to push the parameters that were passed in registers
onto the stack before processing them. There's an implicit
assumption that stdarg functions don't have to be optimal in
terms of speed. Also, this yields simple, easy-to-understand
code.
When accessing memory, the address has to get into a register
somehow. A common convention is to allocate a register with
GREG at the start of a number of memory variables, and use a
base-plus-offset expression to translate the address into a
register plus offset. That method is restricted to about two
hundred such variables (or areas of variables, 256 bytes long)
per program.
The ARM, Hitachi SH and others use "constant pools", where
each far-away address is kept nearby the code; its
address loaded with a GETA equivalent, then using a LDOU to load
the real address. On the other hand, these memory accesses
appear out of sequence compared to instruction fetches and the
"real" memory accesses, a reason to avoid that solution, at
least when modeling real hardware. Also, that would mean a
GETA and LDOU with the address taking up an octabyte:
disregarding possible sharing of addresses, this equals a
four-instruction sequence, at least in size.
I chose to use GETA for address loading, leaving it to the
assembler and linker to expand it when necessary. This is
counter to the goal of using the same conventions as for
manually written assembly code. Therefore I'll implement the
GREG/base-plus-offset approach as an option, default for
standard applications. Anyway, it seems good to at least
optionally be lean on GREG allocations, using the GETA
approach.
Some ports prepend symbols with underscores to disambiguate
register names, but there's no need for that with MMIX. Some
use dollar signs ($) or dots (.) to protect compiler-generated
symbols; the MMIX port uses colon (:) at strategic places. (See
the static chain example.)
A few properties and limitations of gcc affect the MMIX port
badly.
As a rule, GCC considers the set of registers that can hold
parameters and the set of return-value registers constant. Two
other important sets, are the set of call-saved registers and
call-clobbered registers. While GCC is written for these sets
being constant, they should be chosen dynamically for best MMIX
code (even per-call within a function), to e.g. avoid setting L
higher than necessary. Unfortunately the possibility of making
these properties dynamic, or one a function of the other, is
limited at best. For the GCC MMIX port, these properties *are*
currently (2001-10-10) constant: registers $0..$14 are
call-saved, and $16..$31 are call-clobbered.
Changing these parts of gcc would be a major rewrite.
It is reasonably simple, that at a final pass "rename"
registers, for example to close an unused gap between the end of
the call-saved registers and the outgoing parameter registers.
There's hope that the end result will be reasonably near an
optimal register allocation, like the one a skilled human would
do.
After mentioning the register allocation limitations, I think
I should cheer you up by mentioning that GCC has some provisions
for allocating global variables in registers. It doesn't do it
automatically, but it should be reasonably simple to interface
global register definitions with GREG allocations. I have
however not investigated this thoroughly.
To simplify development of the port, I started with a
traditional ABI, one where parameters are passed in
call-clobbered global registers (same for called and calling
function) and where the return value is returned in primarily
the first parameter registers. I call this the GNU ABI.
At the moment I don't have any performance figures. Though in
my experience, the number of register moves are kept to a
minimum if incoming and outgoing parameter register and the
primary return value register are the same. On the other hand
MMIX instructions being three-operand lessens the importance of
that.
Should structures be passed by-value in multiple registers?
If so, there should preferably be a size-threshold. What
should be the structure-size threshold for that?
Who should extend passed integers, caller or called function?
The 64-bit extension of passed integer types is mainly for ease
of use; the programmer who interfaces assembly code to
compiler-generated code does not have to remember the exact type
passed, "it's an integer, extend or truncate it and pass it in a
register" is sufficient for most use. I haven't showed you
actual examples of this, because there are some related bugs
that cause the code to be quite ugly. That's why I've used
"long" in most examples.
Is it useful that the generated code is as mmixal-compatible
as possible, even if it results in non-optimal code?
What are your thoughts about the GNU ABI?
The sources for these references can be found at
http://www-cs-staff.Stanford.EDU/~knuth/mmix-news.html and
(probably, I have not checked) in the MMIX
book.
- mmix-doc.ps, section 29, page 23
- mmixal-intro.ps, section 18, page 8
- mmix-doc.ps, section 18, page 13
The static chain register is necessary when a
function pointer to the nested function is passed to another function, the
nested function accessing variables in the enclosed function. The passed
function pointer for the nested function is actually points to another
piece of code, a trampoline, where the static chain register is
loaded. The trampoline then jumps to the nested function, as can be seen
in the figure below:
In the "new" C99 standard, there can be
names for types of specific sizes, like int64_t, accessible
through the <stdint.h> header file. Mapping ordinary C
types to specific sizes may therefore not strictly be
necessary.
The type long long and its
unsigned variant are standard types in the C99
standard.
Version 1.1, March 2000
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