Re: PATCH: Fix linkonce support with debug

[Daniel Jacobowitz <drow at mvista dot com>]

On Fri, Jun 13, 2003 at 11:27:00AM -0700, H. J. Lu wrote:
> On Fri, Jun 13, 2003 at 01:58:02PM -0400, Daniel Jacobowitz wrote:
> > > > > This patch seems to work for me. We should try to preserve debug
> > > > > information discarded by linkonce as much as we can. It may not be
> > > > > ideal. But it is better than the current one.
> > > > 
> > > > No, I believe it is worse.
> > > > 
> > > > Consider that you now have multiple sections in .debug_info covering
> > > > the same PC range - not necessarily all identical.
> > > 
> > > That is why I said it was not ideal.
> > > >
> > > > Also consider what happens if the multiple copies of the linkonce
> > > > function are compiled with (say) different optimization levels.  You
> > > > will have added a lot of line information which is completely bogus.
> > > > 
> > > 
> > > Isn't is completely bogus in this situation today? We are picking
> > 
> > Here at least the bad line number information gets thrown down at PC 0. 
> > It's still bogus, but on most systems less likely to get in the way.
> > 
> > > one from 2 bad choices. I don't think mine is any worse than the
> > > current one.
> > 
> > I think it's worse.  It definitely isn't any better.
> > 
> 
> There is a testcase which my patch fixes. Do you have a testcase which
> works without my patch and doesn't work with my patch?

Sure.  It was easy to construct one from my previous explanation.  This
exploits one of the possible problems: Support that a function without
line information is placed right after the linkonce section, and
suppose that the copy we keep was built with -O2 -g (like libstdc++
often is) and the copy we discarded was built with -O0 -g (like my
development code often is).  The line number information for the larger
copy will appear to take control of the function without debug
information, causing GDB's prologue analyzer to place a breakpoint in
the completely wrong location.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

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