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

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

[kew]

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

If this is not the right mailing list to ask this question, sorry and 
please if you could refer me to the right mailing list, I'd appreciate it.

Thanks
Andrew

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

[Mark Kettenis]

> 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.