DWARF: Read constant-class addresses correctly

Starting with DWARF version 4, the description of the DW_AT_high_pc
attribute was amended to say:

   if it is of class constant, the value is an unsigned integer offset
   which when added to the low PC gives the address of the first
   location past the last instruction associated with the entity.

A change was made in Apr 27th, 2012 to reflect that change:

  | commit 91da14142c0171e58a91ad58a32fd010b700e761
  | Author: Mark Wielaard <mjw@redhat.com>
  | Date:   Fri Apr 27 18:55:19 2012 +0000
  |
  |     * dwarf2read.c (dwarf2_get_pc_bounds): Check DW_AT_high_pc form to
  |     see whether it is an address or a constant offset from DW_AT_low_pc.
  |     (dwarf2_record_block_ranges): Likewise.
  |     (read_partial_die): Likewise.

Unfortunately, this new interpretation is now used regardless of
the CU's DWARF version. It turns out that one of WindRiver's compilers
(FTR: Diabdata 4.4) is generating DWARF version 2 info with
DW_AT_high_pc attributes improperly using the data4 form. Because of
that, we miscompute all high PCs incorrectly. This leads to a lot of
symtabs having overlapping ranges, which in turn causes havoc in
pc-to-symtab-and-line translations.

One visible effect is when inserting a breakpoint on a given function:

    (gdb) b world
    Breakpoint 1 at 0x4005c4

The source location of the breakpoint is missing. The output should be:

    (gdb) b world
    Breakpoint 1 at 0x4005c8: file dw2-rel-hi-pc-world.c, line 24.

What happens in this case is that the pc-to-SAL translation first
starts be trying to find the symtab associated to our PC using
each symtab's ranges. Because of the high_pc miscomputation,
many symtabs end up matching, and the heuristic trying to select
the most probable one unfortunately returns one that is unrelated
(it really had no change in this case to do any better). Once we
have the wrong symtab, the start searching the associated linetable,
where the addresses are correct, thus finding no match, and therefore
no SAL.

This patch is an attempt at handling the situation as gracefully
as we can, without guarantees.  It introduces a new function
"attr_value_as_address" which uses the correct accessor for getting
the value of a given attribute.  It then adjust the code throughout
this unit to use this function instead of assuming that addresses always
have the DW_FORM_addr format.

It also fixes the original issue of miscomputing the high_pc
by limiting the new interpretation of constant form DW_AT_high_pc
attributes to units using DWARF version 4 or later.

gdb/ChangeLog:

        * dwarf2read.c (attr_value_as_address): New function.
        (dwarf2_find_base_address, read_call_site_scope): Use
        attr_value_as_address in place of DW_ADDR.
        (dwarf2_get_pc_bounds): Use attr_value_as_address to get
        the low and high addresses.  Slight rework of the handling
        of the high pc being a constant form, and limit it to
        DWARF verson 4 or higher.
        (dwarf2_record_block_ranges): Likewise.
        (read_partial_die): Likewise.
        (new_symbol_full): Use attr_value_as_address in place of DW_ADDR.

gdb/testsuite/ChangeLog:

        * gdb.dwarf2/dw2-abs-hi-pc-hello-dbg.S: New file.
        * gdb.dwarf2/dw2-abs-hi-pc-hello.c: New file.
        * gdb.dwarf2/dw2-abs-hi-pc-world-dbg.S: New file.
        * gdb.dwarf2/dw2-abs-hi-pc-world.c: New file.
        * gdb.dwarf2/dw2-abs-hi-pc.c: New file.
        * gdb.dwarf2/dw2-abs-hi-pc.exp: New file.

Tested on x86_64-linux.

1 files changed, 35 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.dwarf2/dw2-abs-hi-pc.exp b/gdb/testsuite/gdb.dwarf2/dw2-abs-hi-pc.exp
new file mode 100644
index 0000000..3353fda
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-abs-hi-pc.exp
@@ -0,0 +1,35 @@
+# Copyright 2014 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/>.
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile
+set executable ${testfile}
+
+if {[build_executable ${testfile}.exp ${executable} "${testfile}.c ${testfile}-hello-dbg.S ${testfile}-hello.c ${testfile}-world-dbg.S ${testfile}-world.c"] == -1} {
+    return -1
+}
+
+clean_restart $executable
+
+gdb_test "break hello" \
+    "Breakpoint $decimal at $hex: file .*dw2-abs-hi-pc-hello\\.c, line 24\\."
+
+gdb_test "break world" \
+    "Breakpoint $decimal at $hex: file .*dw2-abs-hi-pc-world\\.c, line 24\\."