[Bug debug/108996] Proposal for adding DWARF call site information in GCC with -O0

["uweigand at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108996

--- Comment #4 from Ulrich Weigand <uweigand at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #3)
> What is done on other arches?

That depends on the platform ABI.  On some arches, including x86/x86_64 and
arm/aarch64, the ABI requires the generated code reloads the return buffer
pointer into a defined register at function exit (either the same it was in on
function entry, or some other ABI-defined register).  On those arches, GDB can
at least inspect the return value at the point the function return happens.

On a few arches, in particular SPARC and RISC-V, the ABI even guarantees that
the return buffer pointer register remains valid throughout execution of the
function, so that GDB can inspect and/or modify the return value at any point.

But on most other arches, including s390x and ppc/ppc64, the ABI does not
guarantee anything, so GDB simply cannot access the function return value at
all (after the point the return buffer pointer register is no longer needed by
generated code and the register has been reused).

However, *if* the debug info contains an entry-value record for that register
at the call site in the current caller, then the return buffer can be accessed
at any time, on all arches.   Given that in this specific case, most callers
will actually just point the return buffer register to a local stack buffer
(i.e. set the register to "stack pointer plus some constant"), generating an
entry-value record for these special cases should actually be quite
straightforward for the compiler, without requiring a lot of value-tracking
machinery.