🚀 go-pugleaf

Or - Subject: Minix 68k: yet another C compiler (long)

Just to add another compiler version to discuss about...

I always wanted to know how big the effect  of  passing  function
arguments  in  registers  vs.  on  the  stack _really_ is. Unfor-
tunately, I never found a C compiler that provides both  options.
Finally  I  decided  to change this. With some help from a friend
(thanks, Raimund Gruber!) I hacked up a  configuration  file  for
GCC  that  allows  for  register arguments as well as stack args.
Maybe someone is interested? 

The new GCC uses up to 5 registers for argument passing:  D0,  D1
and  D2  for  integer  types and A0 and A1 for pointer types. All
other types are passed  on  the  stack.  The  arguments  for  GNU
library  functions  (libcall) are always passed on the stack. For
functions with a varying number of arguments the last  named  and
all  unnamed arguments are passed on the stack. WARNING: only the
stdarg.h method is supported; you will have to change  code  that
still  uses the old method from varargs.h (supporting this method
seemed to be too difficult and not necessary -- the conversion to 
stdarg is very simple).  Return  values  are  passed  in  D0  for
integer and in A0 for pointer types. 

I will post patches against the original version of  GCC-1.40  as
released by the FSF. There are two types of modifications I made: 

(1) Changes to use register arguments on the 68000. This includes 
    patches to config/tm-m68k.h and to loop.c (thanks, Ian  Lance
    Taylor, for this patch!). 

(2) Changes to make GCC compile under Minix68k (Atari  ST  in  my
    case)  and  in  a cross compiling environment (UN*X System V,
    big  endian)  with  and  without  register  arguments.  These
    changes are based on earlier versions of GCC for Minix68k (by 
    J.  Bammi and others), but I had modify a lot of other things
    (mostly adding include files  and  function  prototypes).  In
    fact,  only  the  driver program gcc and the preprocessor cpp
    can be compiled with register args; the compiler itself (cc1) 
    is not enough conforming to ANSI C. 
    
This new version of GCC operates in 4 different modes: 16 (Option 
-mshort) or 32 bit ints( Option -mnoshort, default); passing args 
in registers (Option -mregparm, default) or on the stack  (Option
-mnoregparm).  For  each  mode  you  need  another version of the
startup file and the C library to link with: 

                        -regparm		-mnoregparm
			 (default) 
                        
-mnoshort	/usr/gnu/lib/crtso.o	/usr/gnu/lib/crtso.s.o
 (default)	/usr/gnu/lib/libc.a	/usr/gnu/lib/libc.s.a

-mshort		usr/gnu/lib/crtso16.o	/usr/gnu/lib/crtso16.s.o
                /usr/gnu/lib/libc16.a	/usr/gnu/lib/libc16.s.a 

If you are compiling with  register  arguments  the  preprocessor
symbol  __REGPARM__  will be set so that you can #ifdef your code
where necessary. I have no idea where it _could_ be necessary  in
C, but this avoids to have different versions of assembler source 
code  (for  instance  when  compiling  the  kernel/*.cpp files in
Minix). 

So what do I have to do?
========================

Take the sources of GCC-1.40, copy Makefile to Makefile.cross and 
Makefile.native; then apply the patches. If you are compliling on 
a  Minix  system:  copy  Makefile.native  to  Makefile  and   run
"config.gcc  minix68k" [the standard minix sh will not do; either
you use ksh (probably you will have to give it some  more  memory
:-( ) or the simple script xconfig). Then type "make" and prepare 
to  do  some  other  things (it runs for about 3 hours on a 8 MHz
Atari  ST).  If  you  are  compiling  on  another  machine:  copy
Makefile.cross to Makfile and edit the following files: 

	Makefile
	gcc.c
	config/tm-minix68k.h

Replace the strings "/user1/wd/..." with pathnames  that  reflect
your cross compiling environment. 

Problems when compiling GCC: 
----------------------------

Possibly a lot, but they should be minor. On a minix  system  you
will  probably  have to change some of the header files used with
GCC (don't try to compile the new  GCC  with  another  compiler).
Your  header  files should contain prototypes for all routines of
the C library. On a cross system: I don't know what problems will 
spring up if you compile not under System V (for BSD you may have 
to change some #includes) or not on a big endian system (no  idea
what this could mean). 

Problems when using the new GCC:
--------------------------------


First, remember to rename your old startup file  and  C  library.
Then  recompile  your library. There are a few files that have to
be changed for use with register args. I will post the versions I 
am using now, but the changes are all simple. 

For all compilations with the new  GCC  with  register  args:  Be
_EXTREMELY_ careful to prototype _ALL_ your functions. I strongly 
recommend  that  you  _ALWAYS_  use the options "-pedantic -Wall"
with the new GCC. Then you will see where  to  add  prototypes  -
each compiler warning 

    foo.c:1234: warning: implicit declaration of function `foobar'

means that you are missing a prototype. If  this  happens  it  is
likely that some parameters are put in the wrong registers or the 
return  value  is  taken from the wrong register (especially true
for all functions returning pointers). The usual effect is a core 
dump :-( 

Programs using varargs.h will have to be changed to use stdarg.h. 
I recommend you to _remove_ varargs.h from  your  system  (or  to
rename it to varargs.NOT_USABLE or so) so that any attempt to use 
it will be flagged as an error. At least insert lines like these:

	#ifdef	__REGPARM__
	#error Can't use varargs.h with register arguments
	#endif

into your varargs.h.

But can't you just give us the binaries?
========================================

Of course I can. What shall do? If I put them on any ftp  archive
(where  should  I put them?) I will get lots of messages "I don't
have ftp"; if I post them I will receive flames for posting  such
an amout of pretty useless stuff. What do you want me to do?

Resume   or   What will I get if I use the new GCC?
===================================================

First of all: You will have lots of trouble if you try to compile 
code that was written without keeping the Ten Commandments for  C
Programmers in mind (GCC itself is such a thing :-( ); or if your 
header files are not set up for ANSI C. 

You will have 4 different libraries  -  maintainance becomes more
difficult.

The executables produced by  the  new  GCC  are  usually  5...10%
smaller  in  size and about 5% faster. No, this is not very much.
If I had known before that the effect is so small I don't know if 
I would have done all this work :-(. But even small  improvements
sum  up. (For myself: I learned a lot about how GCC is configured
and how it works.) 

You may have noted that the method for chosing registers to  hold
arguments  is  the  same as used in Turbo-C (Pure-C?). As long as
there are no references to libcalls or name length problems it is 
very simple to convert object files  from  one  compiler  to  the
other and vice versa. (No, I have not done this and I do not plan 
to do it; I am quite lucky with GCC.) 

I think all of you who want "portable" code  to  compile  without
looking  at  it  should simply ignore this thingy. All the others
are welcome to comment on it. Flames - as usual -  to  /dev/null,
please. 

BTW: If you want  to  reduce  the  size  of  your  object  files,
libraries  and executables: compile (after debugging) with option
"-fomit-frame-pointer". And "-fcombine-regs" is a good idea, too.


Wolfgang

==================================================================
Name    : Wolfgang Denk                         (+49)-89-68004-288
Company : PCS GmbH, Pfaelzer-Wald-Str. 36,  8000 Munich,  Germany.
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