Recording of register saves in DWARF2 CFI

James Molloy
Mon Apr 21 12:48:00 GMT 2008

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 

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!)


James Molloy

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