[rfc/wip] More defensive gdbarch initialization

[Andrew Cagney <ac131313@cygnus.com>]

Hello,

This is fallout from the sh-tdep.c bug Elena recently fixed.
I got curious as to how many multi-arch targets might unintentionally be 
refering to the old ``current_gdbarch'' instead of the new gdbarch being 
created.

For instance, code like:

   set_gdbarch_double_bit (gdbarch, 8*TARGET_CHAR_BIT);
   set_gdbarch_long_double_bit (gdbarch, 2*TARGET_DOUBLE_BIT);

would be wrong. TARGET_DOUBLE_BIT refers to the old 
``current_gdbarch'''s double-bit and not ``gdbarch''.  Did anyone 
mention ``Macro's are bad, M'kay''?

So anyway, my idea was to, for the duration of the XXX_gdbarch_init() 
call, invalidate ``current_gdbarch''.  That way the target code couldn't 
use it.  The patch below is work-in-progress on that line.

The thing that makes this funny is that the first bug I found wasn't in 
any target code.  Rather it was in gdbarch.* proper.  gdbarch_alloc() 
contains:

   gdbarch->long_long_bit = 2*TARGET_LONG_BIT;
   gdbarch->long_double_bit = 2*TARGET_DOUBLE_BIT;
   gdbarch->ptr_bit = TARGET_INT_BIT;
   gdbarch->bfd_vma_bit = TARGET_ARCHITECTURE->bits_per_address;

Ulgh ...

You can expect some ``obvious fixes'' related to this over the next ew 
days.  I'll delay any decision to commit something like the change below 
until after 5.1 has branched, it is proving a little too effective in 
finding bugs :-)

Oh for the day when current_gdbarch isn't a global.

	Andrew
From eliz@is.elta.co.il Sat Jul 28 00:31:00 2001
From: "Eli Zaretskii" <eliz@is.elta.co.il>
To: ac131313@cygnus.com
Cc: gdb-patches@sources.redhat.com
Subject: Re: [rfa] libgdb updates to doco
Date: Sat, 28 Jul 2001 00:31:00 -0000
Message-id: <1438-Sat28Jul2001103025+0300-eliz@is.elta.co.il>
References: <Pine.SUN.3.91.1010726095158.28739G-100000@is> <3B606479.2070205@cygnus.com> <5567-Fri27Jul2001123713+0300-eliz@is.elta.co.il> <3B61D09E.1000902@cygnus.com>
X-SW-Source: 2001-07/msg00685.html
Content-length: 635

> Date: Fri, 27 Jul 2001 16:35:42 -0400
> From: Andrew Cagney <ac131313@cygnus.com>
> > 
> > How about documenting multi-arch so that I could understand how to
> > multi-arch a target without reading the code? ;-)
> 
> Something based on:
> 
> http://sources.redhat.com/gdb/papers/multi-arch/howto.html

That's just a cookbook.  It's important to have that, but it is no
less important to understand how multi-arch works internally, to be
able to do something beyond blindly following the cookbook.

> still for the moment I'm just trying to fix bits of the documentation 
> that are out-of-date or wrong :-)

Good plan, no doubt ;-)