Here's what I committed.  I incorporated the feedback (thanks!), 
including adding BINOP_ASSIGN_MODIFY (messier than one would think, 
required refactoring binary operation compilation), and added a NEWS 
item.  Now on to the next new tracing feature! :-)

Stan

2009-12-28  Stan Shebs  <stan@codesourcery.com>

    Add trace state variables.
    * ax.h (enum agent_op): Add getv, setv, and tracev.
    (ax_tsv): Declare.
    * ax-gdb.c: Include tracepoint.h.
    (gen_expr): Handle BINOP_ASSIGN, BINOP_ASSIGN_MODIFY, and
    OP_INTERNALVAR.
    (gen_expr_binop_rest): New function, split from gen_expr.
    * ax-general.c (ax_tsv): New function.
    (aop_map): Add new bytecodes.
    * tracepoint.h (struct trace_state_variable): New struct.
    (tsv_s): New typedef.
    (find_trace_state_variable): Declare.
    * tracepoint.c (tvariables): New global.
    (next_tsv_number): New global.
    (create_trace_state_variable): New function.
    (find_trace_state_variable): New function.
    (delete_trace_state_variable): New function.
    (trace_variable_command): New function.
    (delete_trace_variable_command): New function.
    (tvariables_info): New function.
    (trace_start_command): Download tsvs with initial values.
    (_initialize_tracepoint): Add new commands.
    * NEWS: Mention the addition of trace state variables.

    * gdb.texinfo (Trace State Variables): New section.
    (Tracepoint Packets): Describe trace state variable packets.
    * agentexpr.texi (Bytecode Descriptions): Describe trace state
    variable bytecodes.

    * gdb.trace/tsv.exp: New file.
    * gdb.base/completion.exp: Update ambiguous info output.