__builtin_va_list bug ir feature

["Gisle Vanem" <gvanem at eunet dot no>]

gcc -v
Reading specs from e:/djgpp/lib/gcc-lib/djgpp/3.22/specs
Configured with: /dj204/gnu/gcc-3.22/configure i586-pc-msdosdjgpp 
  --prefix=/dev/env/DJDIR --disable-nls
Thread model: single
gcc version 3.2.2

I'm porting some code from Watcom to gcc and encountered tons 
of warnings on illegal use of va_arg(). So I cooked up a little test:

#include <stdarg.h>
char x;
void foo (va_list arg)
{
  x = va_arg (arg, char);
}

This gives a warning "`char' is promoted to `int' when passed through `...'"
Fair enough, but when I produce an asm-listing, here's what I get:
        .file   "test.c"
        .section .text
.globl _foo
_foo:
        pushl   %ebp
        movl    %esp, %ebp
        int     $5               # <<<<
.comm _x,16
        .ident  "GCC: (GNU) 3.2.2"

What's the "int $5" doing there?

Is this a gcc bug or feature? I found absolutely nothing in the
gcc online docs about this. If it's a feature, what's the rationale
behind it? IMHO if gcc should treat this as a fatal error instead
of emitting a bound trap (int $5). That is just too nasty. I spent
a full day tracking this down.

Using "va_arg (arg,int)" of-course produces the correct code.

--gv