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From: allan@smsc.sony.com (Allan McNaughton)
Newsgroups: comp.sys.mips
Subject: Varargs is broken
Message-ID: <1990Jun8.000213.5271@smsc.sony.com>
Date: 8 Jun 90 00:02:13 GMT
Reply-To: allan@smsc.Sony.COM (Allan McNaughton)
Organization: Sony Microsystems Corp.
Lines: 112

Mister Mips:

Varargs appears to be broken.  We are running a M2000 with RISC/OS 4.5b
and the 2.0 version of the C compiler.

Your call/return standard says that if the first argument is a float, then 
the argument will be passed in one of the fp registers.  Works just fine,
except when you want to pass it to a function that is expecting a varargs
list.  Varargs, assumes all arguments are on the stack and forces them to
be by saving a0...a3 to the stack.  However, it knows nothing about the fp
registers at all.  

How could this have ever worked?  What has been done to fix the problem?  
This failure was found because System V.4 SVVS tests use it all over the 
place.

Allan Mc Naughton
Sony Microsystem Company
San Jose, CA

allan@sony.com
(408)434-6644 x4069

-------------------------------------------------------------------------------
#include <varargs.h>
double a;
main()
{
	func(a);
}

func(va_alist)
va_dcl
{
	va_list	ap; 	
	double  x;

	va_start(ap);
	x = va_arg(ap, double);
	va_end(ap);
}	

-------------------------------------------------------------------------------

	.verstamp	2 0
	.comm	a 8
	.text	
	.align	2
	.file	2 "var.c"
	.globl	main
	.loc	2 6
 #   1	#include <varargs.h>
 #   2	
 #   3	double a;
 #   4	
 #   5	main()
 #   6	{
	.ent	main 2
main:
	.option	O1
	subu	$sp, 24
	sw	$31, 20($sp)
	.mask	0x80000000, -4
	.frame	$sp, 24, $31
	.loc	2 7
 #   7		func(a);
	l.d	$f12, a		>>>> pass "a" in f12,f13
	jal	func		>>>> call func
	.loc	2 8
 #   8	}
	lw	$31, 20($sp)
	addu	$sp, 24
	j	$31
	.end	main
	.text	
	.align	2
	.file	2 "var.c"
	.globl	func
	.loc	2 12
 #   9	
 #  10	func(va_alist)
 #  11	va_dcl
 #  12	{
	.ent	func 2
func:
	.option	O1
	subu	$sp, 16
	sd	$4, 16($sp)	>>>> function prologue for varargs routines
	sd	$6, 24($sp)	>>>> saves the arg registers on the stack so
	.frame	$sp, 16, $31	>>>> the varargs macros can access them
	.loc	2 16
 #  13		va_list	ap; 	
 #  14		double  x;
 #  15	
 #  16		va_start(ap);
	addu	$14, $sp, 16	>>>> get address of arg list
	sw	$14, 12($sp)
	.loc	2 18
 #  17	
 #  18		x = va_arg(ap, double);
	addu	$15, $14, 15	>>>> how would this ever work????  my argument
	and	$24, $15, -8	>>>> is in a fp register, not on the stack!
	sw	$24, 12($sp)	
	l.d	$f4, -8($24)
	s.d	$f4, 4($sp)
	.loc	2 21
 #  19	
 #  20		va_end(ap);
 #  21	}	
	addu	$sp, 16
	j	$31
	.end	func