Re: [PATCH v2] Add Guile frame unwinder interface

[Pedro Alves <palves at redhat dot com>]

On 03/09/2015 06:54 PM, Andy Wingo wrote:
> Hi,
> 
> On Mon 09 Mar 2015 16:41, Pedro Alves <palves@redhat.com> writes:
> 
>> On 03/09/2015 10:34 AM, Andy Wingo wrote:
>>> +@var{register} names a register, and should be a string, for example
>>> +@samp{rip}.  @var{value} is the register value, as a @value{GDBN}
>>> +value.  Alternately, passing @code{#f} as the value will mark the
>>> +register as unavailable.
>>
>> From a glimpse over the code, I think this actually marks it as
>> "<not saved>" (optimized out), right?  That would be the correct
>> thing to do.  Marking a register as "<unavailable>" is also possible,
>> but it is a different thing -- it means the value exists, but gdb
>> couln't get to it, because e.g., the core file is trimmed, or the
>> ptrace interface is missing access to some registers.
>>
>> That said, you may want to consider how you'd expand the API
>> to allow marking registers as unavailable.
> 
> I didn't realize that "unavailable" and "not saved" were different
> things, thanks for the pointer.  I guess given that the default is a
> "not saved" result, I can just document this default state, and that
> ephemeral-frame-add-saved-value! adds a value.  We remove the #f case.

I wonder whether that's the best default though.  That forces the
unwinder to always handle all registers, even random i/o registers, etc
the machine may happen to have/expose?  An alternative would be
assume the register is found unmodified in this_frame.  You'd need
a ephemeral-frame-mark-not-saved! then, of course.

> 
> If we need to support other states like "unavailable", we can add other
> API like ephemeral-frame-mark-unavailable! or similar.

Yeah.  Note that GDB can mark _parts_ of registers not saved
or unavailable, down to the bit level (mark_value_bits_optimized_out /
mark_value_bits_unavailable).

Thanks,
Pedro Alves