Re: [PATCH] Handle OP_THIS in tracepoints

[Pedro Alves <pedro@codesourcery.com>]

On Thursday 24 December 2009 01:33:33, Stan Shebs wrote:
> This one comes up pretty quickly when doing tracepoints in C++; members 
> in expressions use OP_THIS, which amounts to shorthand for a reference 
> to the implicit argument "this".  Handling is uncomplicated, mimics 
> expression evaluation.

> 
> +     case OP_THIS:
> +       {
> +     char *name;
> +     struct frame_info *frame;
> +     struct symbol *func, *sym;
> +     struct block *b;
> +
> +     name = current_language->la_name_of_this;
> +     if (!name)
> +       error (_("no `this' in current language"));
> +
> +     frame = get_selected_frame (_("no frame selected"));
> +
> +     func = get_frame_function (frame);
> +     if (!func)
> +       error (_("no `%s' in nameless context"), name);
> +
> +     b = SYMBOL_BLOCK_VALUE (func);
> +     if (dict_empty (BLOCK_DICT (b)))
> +       error (_("no args, no `%s' in block"), name);
> +
> +     /* Calling lookup_block_symbol is necessary to get the LOC_REGISTER
> +        symbol instead of the LOC_ARG one (if both exist).  */
> +     sym = lookup_block_symbol (b, name, NULL, VAR_DOMAIN);
> +     if (!sym)
> +       error (_("no `%s' found"), name);
> +
> +     gen_var_ref (exp->gdbarch, ax, value, sym);
> +     (*pc) += 2;
> +       }
> +       break;
> +
>       case OP_TYPE:
>         error (_("Attempt to use a type name as an expression."));
>  

This is busted, and was later fixed in our internal
tree.  :-)  The selected frame, and the current language
don't have anything to do with the context in which
the tracepoint runs.  E.g.:

(gdb) trace 'Foo::Bar()'
(gdb) actions 
(gdb) collect foo_field

Here, the `this' for foo_field needs to be
looked up in the context of Foo::Bar, not of whatever
context/frame the user had selected when she issued
the 'actions' command.

(Note that `maint agent(-eval)' always works in the context
of the selected frame, so testing with that alone can
mask out these issues.)

-- 
Pedro Alves