On Tue, 18 Aug 2020, Richard Earnshaw wrote:
> > Complement commit b932f770f70d ("x86_64 frame unwind info"), SVN r46374,
> > <https://gcc.gnu.org/ml/gcc-patches/2001-10/msg00860.html>, and replace
> > `-fexceptions -fnon-call-exceptions' with `-fasynchronous-unwind-tables'
> > in LIB2_DIVMOD_FUNCS compilation flags so as to provide unwind tables
> > for the affected functions while not pulling the unwinder proper, which
> > is not required here.
> >
> > Remove the ARM overrides accordingly, retaining the hook infrastructure
> > however, and make the ARM test case a generic one.
[...]
> From a quick glance, I'm not convinced this is right for Arm, since the
> Arm unwind format does not support anything other than call-based
> exceptions. How did you test it?
Surely no ARM target has been verified as I have no access to such
configurations; I will appreciate if either you or someone else suitably
equipped does that for the sake of cross-target code unification (= fewer
special cases to maintain). This has been regression-tested with RISC-V
and x86-64 targets, as noted in the two submissions.
Are you trying to say that `-fasynchronous-unwind-tables' has no effect
on ARM? This code does not throw exceptions, so any unwinding would only
happen in contexts such as in GDB poking at this code or from a signal
handler such as SIGALRM or SIGFPE (if ARM does ever send the latter signal
for integer division operations; I don't know offhand). The GCC option is
generic and is supposed to fully support such use cases regardless of the
target chosen, so shouldn't the ARM backend be wired appropriately so as
to use whatever unwind format is required to handle the use cases,
regardless of whether the minimal format usually used is supported by the
psABI or not?
There is indeed a documented provision for not supporting the option:
"Generate unwind table in DWARF format, if supported by target machine."
however I infer that refers to the support of the DWARF format as a whole
rather than specifically minimal unwind tables, that is if the DWARF
format is supported (as opposed to say stabs or mdebug only), then the
option shall generate an unwind table in that format.
That said I'm of course happy to keep the ARM overrides if you consider
them still necessary in the context of the generic change made. Let me
know what you prefer, and if required, I will submit v3 with the ARM
pieces removed.
Maciej
