[committed] Add testcase for DW_AT_count referencing a variable

[Gary Benson]

Tom de Vries wrote:
> On 11/16/20 6:52 PM, Gary Benson via Gdb-patches wrote:
> > Clang describes the upper bounds of variable length arrays using
> > a DW_AT_count attribute which references the DIE of a synthetic
> > variable whose value is specified using a DW_AT_location.  In some
> > cases GDB correctly handles these, but in other cases GDB adds an
> > extra dereference causing the test to fail.  This commit adds a
> > new test to gdb.dwarf2/count.exp with the same DWARF as that
> > generated by Clang for gdb.base/vla-optimized-out.exp, one of the
> > failing tests.
> > 
> > Checked on Fedora 32 x86_64, with GCC and Clang.  Ok to commit?
> 
> I wrote a test-case myself here (
> https://sourceware.org/pipermail/gdb-patches/2020-November/173438.html )
> and only after that found your patch.

I hate when that happens!

> I'm committing this patch (with you marked as author obviously),
> because it's more complete, and the kfails allow us to commit it
> before having a fix.

Cool.

> There was one problem: the DWARF expression was missing the
> DW_OP_stack_value, I've added that.

Thanks.  FWIW I hadn't seen the "Implicit Location Descriptions" bit
in the spec, I just constructed an expression to end with the value
I wanted, but having read it now it seems it was a mistake that I'd
omitted it.

Reading that bit of the spec, though, it mentions three operations
that specify a value is an implicit location: DW_OP_stack_value,
as you've mentioned, but also DW_OP_implicit_value and
DW_OP_implicit_pointer.  It *doesn't* mention DW_OP_fbreg, which
Clang uses in the DWARF it generates for gdb.base/vla-ptr.exp
(https://sourceware.org/bugzilla/attachment.cgi?id=12970), but
I haven't checked what's actually at that location, the count
itself, or its address.

> Also, I've updated the commit message to better reflect what the
> actual problem is.

Thank you.  If you'd asked me beforehand I'd have said to put both our
names in there, to reflect the work you've put into this.  It's too
late to change this one now I guess, I'm more commenting in case
something similar comes up in future.

Cheers,
Gary

-- 
Gary Benson - he / him / his
Principal Software Engineer, Red Hat