Re: [patch/mips, rfc] Don't define FP_REGNUM

[Daniel Jacobowitz <drow at mvista dot com>]

On Fri, Sep 13, 2002 at 04:16:02PM -0400, Andrew Cagney wrote:
> Hello,
> 
> Ok, so this takes things out of the theory stage :-)
> 
> The attached patch eliminates the totally bogus definition of FP_REGNUM 
> from the MIPS target.  It was being mapped to a pseudo register and that 
> pseudo register, through luck, not good management, sometimes contained 
> the contents of the SP.  It now always returns the SP.
> 
> To get this patch working, I needed to modify 
> ``legacy_virtual_frame_pointer'' (used by the tracepoint code) so that 
> it tries FP_REGNUM and then SP_REGNUM as a possible frame pointer 
> register.  As I write:
> 
> -  gdb_assert (FP_REGNUM >= 0);
> -  *frame_regnum = FP_REGNUM;
> +  /* FIXME: cagney/2002-09-13: This code is used when identifying the
> +     frame pointer of the current PC.  It is assuming that a single
> +     register and an offset can determine this.  Instead it should
> +     generate a byte code expression that computes the current FP.  */
> +  if (FP_REGNUM >= 0 && FP_REGNUM < NUM_REGS)
> +    *frame_regnum = FP_REGNUM;
> +  else if (SP_REGNUM >= 0 && SP_REGNUM < NUM_REGS)
> +    *frame_regnum = SP_REGNUM;
> +  else
> +    /* Should this be an internal error?  I guess so, it is reflecting
> +       an architectural limitation in the current design.  */
> +    internal_error (__FILE__, __LINE__, "No virtual frame pointer 
> available");
>    *frame_offset = 0;
> 
> For the MIPS to work, a custom virtual_frame_pointer() function would be 
> needed.  I don't see any point in implementing this, though, as there is 
> no tracepoint target to test it against.  Anyway, coments on this aspect 
> of the patch?    If not, I'll check it in.
> 
> [Hmm, need to mention this in the NEWS file.  The MIPS $fp will finally 
> behave as specified in the documentation.]

Looks good to me.  And there may someday be a tracepoint target to
test it against - I hope to add tracepoints to gdbserver, eventually.
When that happens I can revisit this.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer