Re: ELKS memcpy_fromfs() failing. Wrong variable offsets

[Eduardo Pereira Habkost <ehabkost@conectiva.com.br>]

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On Wed, Jun 02, 2004 at 09:47:25AM +0100, Robert de Bath wrote:
<snip>
> 
> Oh my, confusion reigns.

I guess so.  8)

> 
> Okay first the si and di variables are usually defined as callee saves
> unless you're compiling with the '-Mc' option.
> 
> The "memcpy_fromfs.off" stuff is generated for optimising the push/pop
> of the si and di variables. It's a reasonably recent addition so all
> accesses of si and si can be eliminated if they are unused. However,
> it's only turned on if you use '-O'.

I've seen that this .off stuff was added between 0.16.0 and 0.16.15. That
is why the (wrong) ELKS code used to work.

> 
> Ie: this is working as it should.
> 
> You have highlighted a bug though, the definition some of the 'set'
> variables is actually wrong, they should have memcpy_fromfs.off added
> when it might be non-zero.
> 
> BTW: You should really be using those variables to access C arguments
> and locals, the '_memcpy_fromfs.ds' is for stack pointer relative the
> '.memcpy_fromfs.ds' is for 'bp' relative.

Hey, nice! I've never looked at that code before, and I didn't know what
the author have assumed about the offsets of the variables, but
these [._]<function>.<variable> macros are what I was looking for
fixing the code. Thanks. :)

BTW: these [._]<function>.<variable>macros are available since old
versions, right? I will change ELKS code to use them.

> 
> I can fix the bug in one of two ways; either actually add the
> 'memcpy_fromfs.off' variable to the 'set' variables or simply
> have the compiler assume that any '#asm' will use si and di.
> (The asm("...") function will not however.)
> 
> Actually I think assuming that #asm uses si/di is best because as this
> example has shown it is quite likely to be true, plus, it's the failsafe
> option.
> 
> (PS: It's also very easy to do :-) see attached. )
> 
> So do you want me to make it like it was before? :-)

No, I just wanted to understand what happened. Now I understand the
Right Way to access variables and parameters inside #asm. :)

I think that forcing the user to save DI and SI if he wants to use the
registers is ok, unless it would break some existing software. It's up
to you. Adding a way to make #asm tell the compiler which registers it
will use (and maybe assume by default that it will change all registers)
would be interesting, too.

> 
> -- 
> Rob.                          (Robert de Bath <robert$ @ debath.co.uk>)
>                                        <http://www.cix.co.uk/~mayday>
> 

> diff -Nurd linux86.old/bcc/table.c linux86/bcc/table.c
> --- linux86.old/bcc/table.c	Sun Jul 28 08:43:13 2002
> +++ linux86/bcc/table.c	Wed Jun  2 09:37:56 2004
> @@ -400,6 +400,11 @@
>      register struct symstruct *symptr;
>      int i;
>  
> +    if (framep && optimise && !callersaves) {
> +       regfuse |= callee1mask;
> +       outnstr("! Assuming #asm uses all callee saves registers");
> +    }
> +
>      for (i = 0; i < HASHTABSIZE; ++i)
>  	for (symptr = hashtab[i]; symptr != NULL; symptr = symptr->next)
>  	    if (symptr->storage == LOCAL)


-- 
Eduardo

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