[Bug gdb/16157] the function get_pc_function_start (CORE_ADDR pc) maybe inaccurate

["palves at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16157

--- Comment #4 from Pedro Alves <palves at redhat dot com> ---
> the code address are different. So GDB thinking the program stepped into a new 
> function

That's not sufficient, the frame that was frame #0 before the step must be
frame #1 after the step for GDB to consider this was a subroutine call.  That's
this part of the condition:

      && (frame_id_eq (frame_unwind_caller_id (get_current_frame ()),
               ecs->event_thread->control.step_stack_frame_id)

If before the stepi you have:

 #0  _start () at crt0.S:93

and then after you have:

 #0  lop2 () at crt0.S:95
 #1  0xfffffffe in ?? ()

Then I don't understand how that frame_id_eq returned true.  Well, unless both
were outer_frame_id.  Please check that.

I also don't understand why GDB thinks the function is _start just before the
stepi, instead of lop3.  What's different between lop3 and lop2?  You need to
step through  lookup_minimal_symbol_by_pc_section_1 and understand that.

> the label lop2 and lop3 hava adress values, if the pc value is equal to the 
> address of lop2 or lop3, the msymbol returned from
> lookup_minimal_symbol_by_pc() must be lop2 or lop3, then uses 
> SYMBOL_VALUE_ADDRESS (msymbol) to get the address, and treats the address as 
> function start address.
> I Think this is the problem, is it accurate?

Not exactly.  lookup_minimal_symbol_by_pc, if not returning the "real"
function, then should be returning the closes label.  That is, for all
instructions between lop3 and lop2, it should return lop3, etc.

But that shouldn't be a problem on its own, the other checks in the
"Check for subroutine calls" bit should catch that.  Unless, again, this is 
really the outer_frame_id bits triggering.  outer_frame_id really should die...

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