[Bug gdb/20448] New: target descriptions: bitfields vs flags backward/forward incompatible change

["palves at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=20448

            Bug ID: 20448
           Summary: target descriptions: bitfields vs flags
                    backward/forward incompatible change
           Product: gdb
           Version: 7.12
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: palves at redhat dot com
  Target Milestone: ---

The new target description bitfields support in (unreleased yet) 7.12
introduced a backward/forward incompatible change.

E.g., gdb 7.11.1 against master gdbserver:

 Remote debugging using :9999
 warning: while parsing target description (at line 24): Field "CF" has neither
type nor bit position
 warning: Could not load XML target description; ignoring

This was caused by removing the "end" attribute of flags described in target
descriptions.

However, simply reverting that change (commit 49b7ae7bb8f2) alone, as in:

 -    <field name="CF" start="0"/>
 +    <field name="CF" start="0" end="0"/>

etc.

makes current gdb consider some fields that were previously considered flags,
as bitfields now:

  rip            0x4005ea        0x4005ea <terminal_func+4>
 -eflags         0x202   [ IF ]
 +eflags         0x202   [ CF=0 PF=0 AF=0 ZF=0 SF=0 TF=0 IF=1 DF=0 OF=0 NT=0
RF=0 VM=0 AC=0 VIF=0 VIP=0 ID=0 ]
  cs             0x33    51

which is undesirable.

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