Re: [SH][PATCH] Disable ABI frame sniffer

[Daniel Jacobowitz <drow@false.org>]

On Thu, Nov 10, 2005 at 02:33:01PM +0000, Andrew STUBBS wrote:
> Hmm, confusing. Well, I can change the name easily enough. It is the 
> unwinder based on the ABI though. How come the unwind function is called 
> sniffer?

It's not.  That doesn't _do_ the unwinding, it _selects_ which unwinder
to use.  Sniffs around for the right one, I suppose.

> >Quite on the contrary, my experience is that in most programs you'll
> >need it at least a couple of times, e.g. to get out of bits of libc or
> >linker-generated code.
> 
> Bare-machine (or OS21) sh-elf doesn't use glibc with all its 
> complications. We use newlib and we always compile it with debug info 
> and CFI so there is it should never need to fall back.

There are assembly routines in newlib too, you know.  I didn't say
anything about glibc.  You may encounter linker generated branch
trampolines in some cases, too.

> >>It may be possible to improve the unwinder. However, it is dependent on 
> >>the values in registers and in memory. It would be impossible to prevent 
> >>it getting occasionally confused.
> >
> >
> >Just like the rest of GDB on both counts.

In case you missed my sarcasm here, I was responding to "dependent on
the values in registers and memory".  Of course it is!

> The problem only occurs in threads (unless 'set backtrace pastmain' is 
> set, in which case it happens in any program) when the backtrace falls 
> off the end of the program. There are no more frames because there's no 
> more stack, but there's no way to know that unless you assume the CFI 
> and program run out at the same time.

See Mark's reply for more on this.  Had you said what your problem
really was, we could have saved a couple of back-and-forth exchanges;
I'm extremely familiar with the problem of finding clean ways to
terminate the backtrace.

> BTW, were you (and others) waiting for them to do something about the SH 
> specific patches I have been sending? I ask because I have had no 
> response on a few.

I review patches as fast as I am able.  I am more likely to review
patches to code I know something about, which does not include the SH
backend.

-- 
Daniel Jacobowitz
CodeSourcery, LLC