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Re: RFC: tracepoints: Use constant-size trace opcodes where possible
- From: Michael Snyder <msnyder at specifix dot com>
- To: Jim Blandy <jimb at codesourcery dot com>
- Cc: gdb-patches at sourceware dot org
- Date: Sat, 27 Oct 2007 01:07:18 -0700
- Subject: Re: RFC: tracepoints: Use constant-size trace opcodes where possible
- References: <m3r6jhbhgx.fsf@codesourcery.com>
On Fri, 2007-10-26 at 16:10 -0700, Jim Blandy wrote:
> The comments should explain what's going on here. The Linux
> tracepoint agent was much simplified by restricting it to expressions
> whose data size could be estimated in advance. This should have no
> effect on other agents.
>
> Okay to commit?
Looks fine.
>
> gdb/ChangeLog:
> 2007-10-25 Jim Blandy <jimb@codesourcery.com>
>
> * ax-gdb.c (gen_traced_pop): Generate trace_quick or trace16
> bytecodes whenever possible.
>
> diff -r cdc626576c72 -r 84b7570f26f4 gdb/ax-gdb.c
> --- a/gdb/ax-gdb.c Thu Oct 25 12:43:14 2007 -0700
> +++ b/gdb/ax-gdb.c Thu Oct 25 12:47:24 2007 -0700
> @@ -321,13 +321,22 @@ gen_traced_pop (struct agent_expr *ax, s
> {
> int length = TYPE_LENGTH (check_typedef (value->type));
>
> - /* There's no point in trying to use a trace_quick bytecode
> - here, since "trace_quick SIZE pop" is three bytes, whereas
> - "const8 SIZE trace" is also three bytes, does the same
> - thing, and the simplest code which generates that will also
> - work correctly for objects with large sizes. */
> - ax_const_l (ax, length);
> - ax_simple (ax, aop_trace);
> + /* Try to generate a trace_quick here if we can. The
> + bytecode stream generated isn't any shorter ('quick <len>
> + pop' and 'const8 <len> trace' are both three bytes long),
> + but this simplifies agent expression evaluators that want
> + to be able to easily put an upper bound on the memory
> + that will be collected by an expression. */
> + if (length < 1 << 16)
> + {
> + ax_trace_quick (ax, length);
> + ax_simple (ax, aop_pop);
> + }
> + else
> + {
> + ax_const_l (ax, length);
> + ax_simple (ax, aop_trace);
> + }
> }
> break;
>