[Bug gdb/26905] New: Inconsistent DW_AT_count evaluation

["gbenson at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26905

            Bug ID: 26905
           Summary: Inconsistent DW_AT_count evaluation
           Product: gdb
           Version: unknown
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: gbenson at redhat dot com
  Target Milestone: ---

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 a
DW_AT_location.  In some cases GDB correctly handles these, but other times GDB
attempts to dereference the result of evaluating the location expression and
fails.

An example where GDB does not dereference the value:

 $ gdb /path/to/gdb/testsuite/outputs/gdb.mi/mi-vla-c99/mi-vla-c99
 (gdb) b vla.c:28
 (gdb) r
  ...
 Breakpoint 1, func (n=5) at /gdbtest/src/gdb/testsuite/gdb.mi/vla.c:28
 28       return n;                 /* vla-filled */
 (gdb) p vla
 $1 = {0, 1, 2, 3, 4}
 (gdb) p __vla_expr0
 $2 = 5

An example where GDB does dereference the value:

 $ gdb
/path/to/gdb/testsuite/outputs/gdb.base/vla-optimized-out/vla-optimized-out-o1 
 (gdb) b f1
 Breakpoint 1 at 0x401110: file
/gdbtest/src/gdb/testsuite/gdb.base/vla-optimized-out.c, line 34.
 (gdb) r
  ...
 Breakpoint 1, f1 (i=5) at
/gdbtest/src/gdb/testsuite/gdb.base/vla-optimized-out.c:34
 34     }
 (gdb) p a
 Cannot access memory at address 0x6
 (gdb) 
 (gdb) p __vla_expr0
 $1 = 6

In both these examples, __vla_expr0 is the name Clang assigned the synthetic
variable containing the value of DW_AT_count.

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