Re: Variable "foo" is not available

["Eli Zaretskii" <eliz@gnu.org>]

> Date: Sat, 2 Apr 2005 16:05:42 -0500
> From: Daniel Jacobowitz <drow@false.org>
> Cc: gdb@sources.redhat.com, Reiner.Steib@gmx.de
> 
> > We are talking about function call arguments here, not just about any
> > local variables.  Can you tell what compiler optimizations could cause
> > what Reiner reported: that the first argument is available to GDB, but
> > the second is not?
> 
> Very easily.  Suppose you have two incoming arguments in registers; GCC
> will do this automatically for static functions even on i386, which
> normally uses a stack convention.  The first is used after a function
> call, so it is preserved by saving it to the stack.  The second is not
> used after the function call, so the compiler has no reason to allocate
> a save slot for it, and no reason to store it to memory before the
> function call.

The functions present in Reiner's backtraces are not static, they are
external, with the exception of funcall_lambda.  I don't have access
to an x86_64 machine, but at least on an IA32 x86 architecture the
code produced by GCC 3.4.3 for these function calls is quite
straightforward (see one example below), and with GDB 6.3 I couldn't
reproduce the "arg not available" message.

> With stack-based argument passing, GCC may be claiming an argument is
> unavailable when the function's local copy is dead, when a copy still
> exists on the stack somewhere.  I don't know if it will do that or not.
> GDB can not assume that the argument is available in the incoming stack
> slot, since it could be reused for other data.

What, if any, would be the expression of this in the machine code?

Also, I don't quite understand how can a stack slot of a function call
argument be reused before the function returns.  Isn't that slot
outside the function's frame?  Reusing it would be a violation of the
ABI, right?

Here's the disassembly of one of the frames from Reiner's backtrace:
funcall_lambda calls Fbyte_code.  I disassembled on a 32-bit x86
machine (Reiner, perhaps you could show the disassembly on yours).
The source code is:

      val = Fbyte_code (AREF (fun, COMPILED_BYTECODE),
			AREF (fun, COMPILED_CONSTANTS),
			AREF (fun, COMPILED_STACK_DEPTH));
    }

  return unbind_to (count, val);

Reiner's backtrace is:

#2  0x000000000057a1d4 in Fbyte_code (bytestr=9727377, vector=Variable "vector" is not available.
)
    at [...]/emacs/src/bytecode.c:531
#3  0x000000000054d59d in funcall_lambda (fun=29850740, nargs=1, 
    arg_vector=0x7fbfffb198)
    at [...]/emacs/src/eval.c:2974

Here's the disassembly from my machine, with the PC location marked
by "<<<<<<":

0x000c219e <funcall_lambda+510>:        push   %eax
0x000c219f <funcall_lambda+511>:        mov    0x14(%ebx),%edi
0x000c21a2 <funcall_lambda+514>:        push   %edi
0x000c21a3 <funcall_lambda+515>:        mov    0x10(%ebx),%esi
0x000c21a6 <funcall_lambda+518>:        push   %esi
0x000c21a7 <funcall_lambda+519>:        mov    0xc(%ebx),%ebx
0x000c21aa <funcall_lambda+522>:        push   %ebx
0x000c21ab <funcall_lambda+523>:        call   0xed130 <Fbyte_code>
0x000c21b0 <funcall_lambda+528>:        pop    %edx  <<<<<<
0x000c21b1 <funcall_lambda+529>:        pop    %ecx
0x000c21b2 <funcall_lambda+530>:        push   %eax
0x000c21b3 <funcall_lambda+531>:        mov    0xffffffec(%ebp),%eax
0x000c21b6 <funcall_lambda+534>:        push   %eax
0x000c21b7 <funcall_lambda+535>:        call   0xc0dc0 <unbind_to>
0x000c21bc <funcall_lambda+540>:        jmp    0xc201c <funcall_lambda+124>

This is quite traditional stack-based argument passing, unless I'm
missing something.

The code produced for the call to funcall_lambda (not shown) does pass
some arguments via registers, but I still am able to see all the
arguments in the backtrace.