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Re: multi-arch and CALL_DUMMY_BREAKPOINT_OFFSET


> I guess I'm going to find several things like this...


> Well it appears that in a multi-arch gdb (even at level 1), 
> CALL_DUMMY_BREAKPOINT_OFFSET can only be a constant for any particular 
> architecture.  This is a problem, because on the ARM it is currently a 
> function that returns one of two values depending on whether the 
> call-dummy stub has to be ARM code or Thumb code.  Note that both types of 
> code can exist within a single application and it is not always safe to 
> assume that every function is interworking safe.


Oops :-(  People keep finding things I thought would be constant but are 
not.


> I guess I could re-write the whole of the call-dummy stuff so that 
> appropriate breakpoints are built in, but that is certainly going to be 
> non-trivial.
> 
> Any suggestions?  Can I diddle with the gdbarch setting dynamically -- eg 
> by calling gdbarch_set_call_dummy_breakpoint_offset() from within 
> arm_fix_call_dummy()?  It's quite gross, but it might work.


Two suggestions:

Replace CALL_DUMMY_BREAKPOINT_OFFSET and CALL_DUMMY_BREAKPOINT_OFFSET_P 
with a predicate function (``F:'')?  The predicate mechanism was only 
added recently.  I have a feeling that while this looks good, it isn't 
as easy as it seems :-(

Introduce a new method (``f:'') that, for legacy code, uses 
CALL_DUMMY_BREAKPOINT_OFFSET?  Deprecate (ARI / bug report) the old 
CALL_DUMMY_BREAKPOINT_OFFSET variable.


> Long term it would probably be better to rewrite the call-dummy handling 
> to remove the covert variable that is used to communicate between the 
> various call-dummy stubs, but I'd rather not do that now.


   /* CALL_DUMMY is an array of words (REGISTER_SIZE), but each word
      is in host byte order.  Before calling FIX_CALL_DUMMY, we byteswap it
      and remove any extra bytes which might exist because ULONGEST is
      bigger than REGISTER_SIZE.

      NOTE: This is pretty wierd, as the call dummy is actually a
      sequence of instructions.  But CISC machines will have
      to pack the instructions into REGISTER_SIZE units (and
      so will RISC machines for which INSTRUCTION_SIZE is not
      REGISTER_SIZE).

      NOTE: This is pretty stupid.  CALL_DUMMY should be in strict
      target byte order. */

You would not be alone.

Andrew




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