Re: Help with cfi markup for MIPS16 hard-float stubs

[Richard Sandiford <rdsandiford at googlemail dot com>]

Richard Henderson <rth@redhat.com> writes:
> On 02/15/2012 11:53 AM, Richard Sandiford wrote:
>> We then trip:
>> 
>>       /* Don't let us unwind past the handler context.  */
>>       gcc_assert (!match_handler);
>> 
>> in _Unwind_RaiseException_Phase2.  What's the right thing to do here?
>> 
>
> Ug.  The Right Thing is to fix the unwinder so that it identifies
> frames by pc+cfa, like gdb does, rather than by cfa alone.  The
> assumption has been that a function without a frame can't perform
> a call, and can only by unwound to via signals.
>
> As a workaround for 4.7, you can try this hack:
>
> 	.cfi_startproc simple
> 	.cfi_def_cfa	29, -1		# fake cfa one byte below sp
> 	.cfi_register	29, 29		# "save" sp in itself so we don't use the fake cfa
> 	move	$18,$31
> 	.cfi_register 31, 18
> 	...

Ooh, nice (if that's the word).  It certainly fixes the testcase,
although I had to use -4 rather than -1 in order to defeat
DWARF2_CIE_DATA_ALIGNMENT.  That should still be OK, since the
stack is 8-byte aligned.

GDB doesn't seem to be able to backtrace through this, but that
has to come second to correctness.  I'll aim to get a tested fix
in this weekend.

Thanks,
Richard