[gdb/symtab] Handle DW_AT_string_length with location list

Consider a fortran routine where a string variable s is modified:
...
subroutine f(s)
  character*(*) s
  print *, s
  s(1:3) = 'oof'
  print *, s
end subroutine f
...

When compiling with optimization level -O1 and printing the type of
variable s we get:
...
$ gdb -q -batch outputs/gdb.opt/fortran-string/fortran-string \
  -ex "b f" \
  -ex run \
  -ex "ptype s"
Breakpoint 1 at 0x4006f7: file fortran-string.f90, line 21.

Breakpoint 1, f (s=..., _s=_s@entry=3) at fortran-string.f90:21
21      subroutine f(s)
type = character*1
...
while with -O0 we have instead:
...
type = character (3)
...

The problem is that the type of s is:
...
 <1><2d6>: Abbrev Number: 21 (DW_TAG_string_type)
    <2d7>   DW_AT_string_length: 0xbf (location list)
    <2db>   DW_AT_byte_size   : 4
...
where the DW_AT_string_length is a location list, a case that is not handled
by attr_to_dynamic_prop.

Fix this by handling attr->form_is_section_offset () in attr_to_dynamic_prop.

Tested on x86_64-linux.

The test-case is based on gdb.opt/fortran-string.exp from
https://src.fedoraproject.org/rpms/gdb/raw/f32/f/gdb-archer-vla-tests.patch .
I've updated the copyrights to stretch to 2021.

[ I've tried to create a dwarf assembly test-case for this, but didn't
manage. ]

Co-Authored-By: Jan Kratochvil <jan.kratochvil@redhat.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26910

1 files changed, 27 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.opt/fortran-string.f90 b/gdb/testsuite/gdb.opt/fortran-string.f90
new file mode 100644
index 0000000..71b6bae
--- /dev/null
+++ b/gdb/testsuite/gdb.opt/fortran-string.f90
@@ -0,0 +1,27 @@
+! Copyright 2009-2021 Free Software Foundation, Inc.
+!
+! This program is free software; you can redistribute it and/or modify
+! it under the terms of the GNU General Public License as published by
+! the Free Software Foundation; either version 3 of the License, or
+! (at your option) any later version.
+!
+! This program is distributed in the hope that it will be useful,
+! but WITHOUT ANY WARRANTY; without even the implied warranty of
+! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+! GNU General Public License for more details.
+!
+! You should have received a copy of the GNU General Public License
+! along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+subroutine f(s)
+  character*(*) s
+  print *, s
+  s(1:3) = 'oof'
+  print *, s
+end subroutine f
+
+program main
+  character*3 s;
+  s = 'foo'
+  call f (s)
+end program main