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From: "uweigand at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug debug/108996] Proposal for adding DWARF call site information
 in GCC with -O0
Date: Fri, 03 Mar 2023 13:14:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108996

--- 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 th=
at
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 poi=
nt.

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 regist=
er
at the call site in the current caller, then the return buffer can be acces=
sed
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.=