Re: Recording of register saves in DWARF2 CFI

[James Molloy <jamesmol at transitive dot com>]

Andrew Haley wrote:
> James Molloy wrote:
>   
> > Hi,
> >
> > I'm attempting to write a cross-platform debugger for a hobby kernel,
> > and as part of which I use DWARF-2's CFI stack unwinding functionality.
> >
> > This works perfectly, however I would also like to obtain the parameters
> > given to each function call on the stack. I can do this easily in x86 as
> > all the parameters are passed via stack, however on x64 and MIPS I'm
> > having difficulty, as the first X parameters are passed via register.
> >
> > The DWARF-2 specification makes allowance for this - the CFI system is
> > capable of determining the value of any and every register at the start
> > of any stack frame - but I have noticed that GCC doesn't record
> > unwinding rules for many registers (seemingly any register not essential
> > to the finding of the CFA or return address).
> >
> > Is there any flag available to enable recording of every register, or,
> > even better, certain registers, for every stack frame? I've grepped the
> > manual to no avail.
> >     
>
> Surely this is in the debuginfo, not the unwinder data.  Aren't
> you looking in the wrong place?
>
> Andrew.
>   
Although it is indeed possible to pull this information out of the 
debuginfo, the debuginfo section is *massive* and gives far more 
information than I need or want - it describes the entire low level 
structure of the program - all I want is to be able to find where 
parameters are!

The .debug_frames section is far smaller by comparison and seeing as I 
already use it for stack unwinding I felt that using it along with an 
idea of the default ABI for a given architecture would provide a useful 
and compact way of determining function parameters and helping my 
developers.

The DWARF definition version 3.0 section 6.4 describes editing 
"activations" - this section seems to imply that the full register set 
should be available for editing for any unwound stack frame - not just 
select registers.


Has this been implemented in GCC? Or was there an assumption that most 
debuggers would use the .debug_info information anyway so it was 
redundant? (Or, am I interpreting things incorrectly!)

Thanks,

James Molloy