Re: 1.3.4 status?

[Christopher Faylor <cgf at redhat dot com>]

On Tue, Oct 23, 2001 at 12:18:05PM +1000, Robert Collins wrote:
>on every call, it checks the stack depth. _everything_ allocated from a
>call point deeper in the stack is reclaimed, and freed (alloca only uses
>the stack to identify when to do stuff, not for storage).

That's what libiberty's alloca does.  That is for systems for which a
real alloca is difficult or impossible.  That isn't what the builtin
alloca does.

The alloca that cygwin uses is below.  This is from
gcc/config/i386/cygwin.asm.

It is possible that a function would call __chkstk when it needed to
allocate space beyond a certain limit.  Not every function begins with a
call to __chkstk/alloca, though.

cgf

/* stuff needed for libgcc on win32. */

#ifdef L_chkstk

        .global ___chkstk
        .global __alloca
___chkstk:
__alloca:
        pushl  %ecx             /* save temp */
        movl   %esp,%ecx        /* get sp */
        addl   $0x8,%ecx        /* and point to return addr */

probe:  cmpl   $0x1000,%eax     /* > 4k ?*/
        jb    done              

        subl   $0x1000,%ecx             /* yes, move pointer down 4k*/
        orl    $0x0,(%ecx)              /* probe there */
        subl   $0x1000,%eax             /* decrement count */
        jmp    probe                    /* and do it again */

done:   subl   %eax,%ecx           
        orl    $0x0,(%ecx)      /* less that 4k, just peek here */

        movl   %esp,%eax
        movl   %ecx,%esp        /* decrement stack */

        movl   (%eax),%ecx      /* recover saved temp */
        movl   4(%eax),%eax     /* get return address */
        jmp    *%eax    


#endif