Re: Handling language trampoline

[Pedro Alves <palves at redhat dot com>]

On 12/07/2017 02:21 PM, Dmitry Antipov wrote:
> When debugging a program which is definitely in C++:
> 
> (gdb) info source
> [...skipped...]
> Contains 66 lines.
> Source language is c++.
> Producer is clang version 6.0.0 (trunk 319884).
> Compiled with DWARF 2 debugging format.
> Does not include preprocessor macro info.
> 
> I've noticed that 'step' command causes GDB to perform some
> ObjC-specific work:


...

> Why it is so if source language was recognized as C++?

A program is often composed of sources written in different
languages (C++, C, Asm, etc.).  Plus, a trampoline itself has no
symbol/language associated.

> #0  lookup_minimal_symbol (name=0x8459a1 "_objc_msgSend", sfile=sfile@entry=0x0, objf=objf@entry=0x0) at ../../gdb/minsyms.c:313
> #1  0x00000000005bd0f9 in lookup_bound_minimal_symbol (name=<optimized out>) at ../../gdb/minsyms.c:432
> #2  0x00000000005c1666 in find_objc_msgsend () at ../../gdb/objc-lang.c:1282
> #3  find_objc_msgcall (pc=pc@entry=139646390853344, new_pc=0x7ffe10329598) at ../../gdb/objc-lang.c:1340
> #4  0x00000000005c1820 in objc_skip_trampoline (frame=0x16a0eb0, stop_pc=139646390853344) at ../../gdb/objc-lang.c:313
> #5  0x000000000059f1dc in skip_language_trampoline (frame=frame@entry=0x16a0eb0, pc=139646390853344) at ../../gdb/language.c:605
> #6  0x0000000000597129 in process_event_stop_test (ecs=ecs@entry=0x7ffe10329d10) at ../../gdb/infrun.c:6706
> #7  0x0000000000598a10 in handle_signal_stop (ecs=ecs@entry=0x7ffe10329d10) at ../../gdb/infrun.c:6163

Above, frame #4:

/* Iterate through all registered languages looking for and calling
   any non-NULL struct language_defn.skip_trampoline() functions.
   Return the result from the first that returns non-zero, or 0 if all
   `fail'.  */
CORE_ADDR 
skip_language_trampoline (struct frame_info *frame, CORE_ADDR pc)
{
  for (const auto &lang : languages)
    {
      if (lang->skip_trampoline != NULL)
	{
	  CORE_ADDR real_pc = lang->skip_trampoline (frame, pc);

	  if (real_pc)
	    return real_pc;
	}
    }

  return 0;
}

I don't offhand see how can GDB know which is the right
language for the current PC the program just stopped at, and
if the program stopped inside a trampoline.  That's part of
each language's skip_trampoline's job, so seems reasonable
that GDB has to try them all.

I'm guessing those minsym lookups showed up high in profile?
I guess that could be solved with some per-objfile
"minsym-of-_objc_msgSend" caching.  Something like
breakpoint.c:breakpoint_objfile_data.

Thanks,
Pedro Alves