Re: STABS records for 64-bit function arguments via registers in i386

[Mark Kettenis <mark dot kettenis at xs4all dot nl>]

> Date: Wed, 13 Oct 2010 11:08:19 -0400
> From: kew@emc.com
> 
> Hi
> 
> In i386 architecture (32 bit) gcc can optimize a static function to pass 
> function arguments via registers (instead of in the stack). For 64-bit 
> arguments, it uses 2 registers, but gdb reports only 1 (via "info 
> scope") when the binary being debugged is compiled using the STABS 
> debugging format.
> 
> I checked using objdump and could see indeed only 1 N_RSYM record being 
> given (so no wonder gdb would only report 1 register).
> 
> How does the STABS debugging format indicate the 2 registers used for 
> passing 64 function arguments in i386 architecture? Is this supported? 

Historically the convention has been that a 64-bit value is stored in
register N and N + 1, where N is the register number that's stored in
the STABS debugging information.

If your compiler doesn't store 64-bit values in that way (and I think
modern versions of GCC no longer do this) things will break.

> If not, what's the process to add support for it?

There isn't really.  STABS is a historic debugging format that isn't
expressive enough to support modern compilers and languages like C++.
Use DWARF 2/3/4 instead.