Re: [PATCH] "tfind" across unavailable-stack frames.

[Pedro Alves <palves at redhat dot com>]

On 12/17/2013 09:02 AM, Yao Qi wrote:
> On 12/17/2013 12:19 AM, Pedro Alves wrote:
>>>> What does the last sentence mean in the comments?
>>>>
>> The same comment is in frame_id_build.  Re. what is means, see struct frame_id:
>>
>>   /* The frame's special address.  This shall be constant through out the
>>      lifetime of the frame.  This is used for architectures that may have
>>      frames that do not change the stack but are still distinct and have
>>      some form of distinct identifier (e.g. the ia64 which uses a 2nd
>>      stack for registers).  This field is treated as unordered - i.e. will
>>      not be used in frame ordering comparisons.
>>
>>      This field is valid only if special_addr_p is true.  Otherwise, this
>>      frame is considered to have a wildcard special address, i.e. one that
>>                                                              ^^^^^^^^^^^^^
>>      matches every address value in frame comparisons.  */
>>      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>   CORE_ADDR special_addr;
> 
> The last sentence, especially "wildcard" stuff, is still confusing to
> me.  Go through the usage of "special_addr", looks "special_addr" is
> used only if "special_addr_p" is true.  Comment "This field is valid
> only if special_addr_p is true" is clear and sufficient.  

It is clear, but it is not as precise or sufficient.  A wild
card means that given these frame ids:

 fid1: {code_p,stack_p,special_p}
 fid2: {!code_p,stack_p,!special_p}

 fid3: {code_p,stack_p,special_p}
 fid4: {code_p,stack_p,!special_p}

{fid1, fid2} with same stack addresses,
and {fid3, fid4} with same code and stack addresses,
both:

 frame_id_eq(fid1, fid2)
 frame_id_eq(fid3, fid4)

return true.

> I don't see
> any extra information the last sentence "Otherwise, xxxxx" delivered
> except confusion.

The extra information indicates that e.g., frame_id_eq(fid3, fid4)
above returns true, not false, as one might at first expect.

Whether this whole wildcarding business is a good idea, is
another story.

-- 
Pedro Alves