[Dwarf-Discuss] compilers generating ABI non-compliant function calls?

[Michael Eager]

On 3/9/21 7:13 AM, Frank Ch. Eigler via Dwarf-Discuss wrote:
> As I understand it, the location of*function return values*  is
> however a gap in DWARF, and a consumer tool must resort to ABI specs.
> (Thus the elfutils dwfl_module_return_value_location() function.)  I'm
> sure there's a Reason for this, but having worked on a consumer, it'd
> be handy if DWARF did explicitly identify the return value location
> too.

DWARF does not duplicate information which is documented in the ABI
or in other information which is shared by compilers and debuggers.
For example, DWARF does not describe the calling convention for a
function.  Producers and consumers are expected to know this info.

The rationale is that duplicating shared ABI knowledge would greatly
increase the size of a debug file, while not improving the ability
to debug a program.

For example, DWARF describes the arguments to a function.  It doesn't
describe the calling convention, which registers are preserved, which
are clobbered, or anything else which is specified by the ABI.  In
most ABIs, the location of a function return value is similarly
constrained.  If there are multiple calling conventions, this is
identified to allow a debugger to generate a valid call to a function.

DWARF only contains information which describes what a compiler
generates which cannot be unambiguous determined by knowledge of the
ABI.  A limited exception is the CFI, which in many cases mirrors the
ABI.

If there are occasions when a compiler might place a function return
value in a variety of different locations, not constrained by the
ABI, a DWARF attribute might be useful.

-- 
Michael Eager