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Re: frame cache
Thanks for the quick reply.
Daniel Jacobowitz wrote:
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.
I don't see any links to the <target>_frame_cache in frame_info.
I don't see anything in frame.c which looks like it searches
for the correct <target>_frame_cache. Can you point me at the
right place?
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 :-)
I don't know about creating FDEs, but keeping persistent data
like frame pointer and size should be simple.
When I put debugging code in <target>_analyze_prologue(), I see
that it is called over and over while executing a "next" command.
All those bits going back and forth over the serial line to the
target.
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?
I'd appreciate anything, naturally.
--
Michael Eager eager@eagercon.com
1960 Park Blvd., Palo Alto, CA 94306 650-325-8077