Re: RFA: revised implementation of permanent breakpoints

[Andrew Cagney <ac131313 AT cygnus dot com>]

Jim Blandy wrote:

> I added the SKIP_PERMANENT_BREAKPOINT macro in such a way that it
> should be easy to adapt it to the gdbarch framework.  It is always
> used as a typed value, never in an #ifdef.  So one should be able to
> add a line to gdbarch.sh, and change the #define to a proper
> gdbarch-style initialization, and everything will just work.

That is all I ask :-)


> + /* We can't step off a permanent breakpoint in the ordinary way, because we
> +    can't remove it.  Instead, we have to advance the PC to the next
> +    instruction.  This macro should expand to a pointer to a function that
> +    does that, or zero if we have no such function.  If we don't have a
> +    definition for it, we have to report an error.  */
> + #ifndef SKIP_PERMANENT_BREAKPOINT
> + #define SKIP_PERMANENT_BREAKPOINT ((void (*) (void)) 0)
> + #endif
> +
> +

Puzzled expression.  Why is it being cast in that strange way.

> +
> +   /* Normally, by the time we reach `resume', the breakpoints are either
> +      removed or inserted, as appropriate.  The exception is if we're sitting
> +      at a permanent breakpoint; we need to step over it, but permanent
> +      breakpoints can't be removed.  So we have to test for it here.  */
> +   if (breakpoint_here_p (read_pc ()) == permanent_breakpoint_here)
> +     {
> +       if (SKIP_PERMANENT_BREAKPOINT)
> +       SKIP_PERMANENT_BREAKPOINT ();
> +       else
> +       {
> +         error_begin ();
> +         fprintf_filtered (gdb_stderr, "\
> + The program is stopped at a permanent breakpoint, but GDB does not know\n\
> + how to step past a permanent breakpoint on this architecture.  Try using\n\
> + a command like `return' or `jump' to continue execution.\n");
> +         return_to_top_level (RETURN_ERROR);
> +       }
> +     }

Ah, no. You can't directly test a macro for NULL. The good old ``have
the macro name return a function pointer and then call that'' trick
won't work.  This is because GDB arch is internally parameterized by the
current architecture.

Instead, either a predicate needs to be added:

	if (SKIP_PERMENANT_BREAPOINT_P)
	  SKIP_PERMENANT_BREAPOINT (...);

or better, just turn the else clause from the above into a function and
default the macro to that:

	SKIP_PERMENANT_BREAKPOINT (...)

	Andrew