Hi,

The attached patch adds a new sh-specific command:

set backtrace abi-sniffer

This command allows the ABI frame sniffer to be disabled. The default 
state remains 'on', as it is now.

It is useful to disable this sniffer because it has an irritating habit 
of causing (non-fatal) errors when it runs out of frames. Specifically 
it often says 'Previous frame identical to this frame (corrupt stack?)'.

This message is worrying if you do not know what it means, but it is 
normally easily ignored. Unfortunately, the problem is more serious when 
used in conjunction with 'thread apply' as the first error kills the 
output from the rest of the threads. This command is used by some GUIs 
to populate their displays.

It is difficult to see how the ABI sniffer can be made more robust 
(although I have not looked into it in great detail), and it certainly 
can't be made totally reliable, so the best option would seem to be to 
disable it altogether.

With the ABI sniffer 'off' it is not possible to backtrace through code 
that does not have CFI. On sh-elf, at least, there is always CFI when 
there is debug info, so it is relatively rare that the ABI sniffer is 
actually required.

I have not set the default to 'off' because that causes quite a number 
of FAILs in the testsuite.

Is this all OK?

Thanks

Andrew Stubbs