Re: frame cache

Daniel Jacobowitz
2007-07-24T17:14:58Z

On Tue, Jul 24, 2007 at 10:01:05AM -0700, Michael Eager wrote:
> I have a couple questions about the <target>_frame_cache
> structure and functions.
>
> 1) This appears to be a single-entry cache. Why not keep
> multiple entries?

No, it isn't single-entry. The common code in frame.c is responsible
for passing a pointer to the correct place to store the cache for the
current frame.

> 2) The data in the frame cache seems to be of two different
> types:
> a) Fixed, based on analyzing the code: register offsets,
> stack alignment, framelessness, etc.
> b) Variable, based on the call: return pc, frame base
>
> It looks to me that the object code is analyzed repeatedly
> and this fixed information is discarded along with the
> variable information.
>
> Why not keep a persistent cache of function specific fixed
> data and only discard the call-specific data when the frame
> cache is cleared?

No good reason. I have thought about doing this before. It's not
fundamentally different from the way the DWARF unwinder works; the
persistent part of the cache would be approximately an FDE. It's a
little tricky to implement, since we still need to detect stopping
within the prologue, but not too tricky. I suppose bonus points would
be awarded for constructing an actual FDE :-)

> Is there any documentation about what target-specific data
> the frame cache is supposed to contain or how the functions are
> supposed to work?

Not really, because it's completely up to the target what goes in
them; it varies quite a bit between targets.

Vlad, didn't you say a week or two ago that you'd been working on some
frame docs?

--
Daniel Jacobowitz
CodeSourcery