Re: frame->unwind->this_base()

Daniel Jacobowitz
2003-03-17T16:38:51Z

On Mon, Mar 17, 2003 at 11:22:35AM -0500, Andrew Cagney wrote:
>
> >>However, shouldn't the only thing needing the `virtual frame pointer' /
> >>get_frame_base() be the code that needs a virtual base pointer when
> >>computing the value of a local variable?
> >
> >
> >Yes, and that's the only time that we search for the frame base. But
> >what difference does it make?
>
> (gdb) info frame
>
> will display the correct value.

What does "correct" mean though?

> >At that point we have an offset that we
> >know is relative to DW_AT_frame_base, but we don't know if it's
> >relative to what the rest of GDB considers the frame base (since we
> >never use DW_AT_frame_base to compute the frame base in the first
> >place; and it's not clear to me that we should be).
>
> Where, apart from `info frame', and variable evaluation, is it correct
> for GDB to use the frame base?

I'm sorry, but I just don't understand what you're asking. We use the
frame base all over.

The current frame base (i.e. id.base) is produced by target specific
code - often via prologue analysis; on x86-64 via CFI; etc.

The prologue analyzer, CFI code, etc. use the frame base when finding
saved registers, the saved PC value, et cetera. For instance i386 uses
it to read the saved PC off of the stack. This defines what GDB
expects to be the frame base.

The compiler doesn't necessarily have the same idea of the frame base.
It can pick a convenient location, which may be biased some number of
words away from what GDB considers the frame base. There's any number
of reasons this might happen. Depending on, for instance, the
architecture's ABI and the available instructions for frame-relative
loads.

When using information computed by GDB we may need to use the current
frame base. When using information provided by the compiler we
probably need to use the compiler provided frame base.

Are you saying that you want GDB always to use the DW_AT_frame_base?
That's a bit of a leap from where we are today.