Re: A testcase anad a patch for the __gmon_start__ problem on PPC.

[Geoff Keating <geoffk at cygnus dot com>]

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> From: Franz Sirl <Franz.Sirl-kernel@lauterbach.com>
> Date: Wed, 11 Oct 2000 22:10:03 +0200
> Cc: Ulrich Drepper <drepper@redhat.com>, Jack Howarth <howarth@fuse.net>,
>         libc-alpha@sourceware.cygnus.com, aj@suse.de
> 
> On Wed, 11 Oct 2000, H . J . Lu wrote:
> > On Wed, Oct 11, 2000 at 09:34:44PM +0200, Franz Sirl wrote:
> > > > I don't think so. You can call it an ABI bug if you want.
> > >
> > > Hmm. Anyway, in what way is your patch different from the previous
> > > situation when WEAK_GMON_START was still honored? Isn't it better to just
> > > revert the responsible patches? Why maintain 2 files if one was enough
> > > before?
> >
> > The difference is if you do nothing, you will get the clean code. If
> > you want the binary compatibility, you can use the old one.
> 
> What if I want both the clean code and backwards compatibility? Go with my 
> suggested solution? I tested that (one minor mod) with my testcase and it 
> works fine. It depends on the fact that space is allocated for __gmon_start__ 
> and initialized to 0. The first 2 words maybe touched by relocation, but 
> starting with the 3rd word it stays at 0, which is an illegal opcode for PPC 
> and thus no valid __gmon_start__ routine is there.
> I wonder though how the size of the __gmon_start__ function gets calculated 
> for allocation.

Aah, that's what the code was doing!

OK, this is slightly wrong.  The right test would be that the first
instruction at __gmon_start__ is 'bl,a 0'.

Now that I know what it's doing, I think this is an OK workaround.
It's probably better than falling back to the old way of doing
__gmon_start__, since that has problems with shared object unloading.

(However, I'm now working on a proper fix, which is to make weak
symbols work correctly in executables in the first place, at least in
-fpic code.)

-- 
- Geoffrey Keating <geoffk@cygnus.com>