Re: frame cache

[Daniel Jacobowitz <drow@false.org>]

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