Re: get_frame_func() VS get_frame_id().code

[Andrew Cagney <cagney@gnu.org>]

> On Fri, Mar 05, 2004 at 11:05:54AM -0500, Andrew Cagney wrote:
> 
>>> Hello,
>>> 
>>> The current get_frame_func() is implemented as roughly:
>>> 
>>>       fi->prev_func.addr = get_pc_function_start (addr_in_block);
>>> 
>>> Unfortunatly this isn't valid for a signal trampoline (or at least the 
>>> evil ones that consist of random bytes in a random memory location). 
>>> For such trampolines, get_pc_function_start [rightly] fails and "func" 
>>> ends up as zero -- not good -- a properly constructed frame ID requires 
>>> non-zero code and stack addresses.
>>> 
>>> Fortunatly, with a bit of extra instruction pattern matching, it is 
>>> possible to identify the first instruction of a signal trampoline and 
>>> hence correctly compute the trampolines "func" address.  Similarly, more 
>>> normal frames can determine the function start using the symbol table's 
>>> get_pc_function_start.
>>> 
>>> Consequently, I think there should be mechanism for obtaining both the 
>>> symbol table and frame's idea of a function's start address.  This would 
>>> mean introducing:
>>> 
>>> - get_frame_func_by_symtab
>>> Returns the function start according to the symbol table.  Much of the 
>>> existing code (especially unwinders) would need to be updated to use this.
> 
> 
> How about just leaving this as frame_func_unwind?

Because the func, based on the frame ID, is more likely to be correct. 
The current get_frame_func and frame_func_unwind can return 0 or the 
wrong function :-(

BTW, there is get_frame_function() which returns the symbol for the 
function.

 >  The only current use
 > of frame_func_unwind that I see outside of unwinders is the
 > implementation of get_frame_func.

... get_frame_func is hardly called ...

 >  So we could define frame_func_unwind
 > to attempt to find the beginning of the function at the unwound PC
 > using the symtab.

... so it may even be possible to simply pull get_frame_func from the 
frame interface.

> It is already the unwinder's duty to propogate the function address
> into the ID.

Andrew