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Re: [patch] Convert frame_stash to a hash table
- From: Phil Muldoon <pmuldoon at redhat dot com>
- To: Pedro Alves <palves at redhat dot com>
- Cc: "gdb-patches at sourceware dot org" <gdb-patches at sourceware dot org>
- Date: Thu, 16 May 2013 14:50:28 +0100
- Subject: Re: [patch] Convert frame_stash to a hash table
- References: <5194DA88 dot 6020705 at redhat dot com> <5194E257 dot 4010807 at redhat dot com>
On 16/05/13 14:42, Pedro Alves wrote:
> On 05/16/2013 02:09 PM, Phil Muldoon wrote:
>
> When doing a backtrace, you'll end up linearly walking the frame
> chain, and normally you don't go back to newer frames -- unwind a
> frame (frame.prev()), print info about it, unwind the next, print it,
> on and on. As such, a single frame stashed in the frame stash should be
> sufficient. But it's not. frapy_older does:
When using frame filters, in the case of eliding frames this may not
be the case. In fact we cannot predict how frame filters will
navigate the stack.
> TRY_CATCH (except, RETURN_MASK_ALL)
> {
>
> /* Try to get the previous frame, to determine if this is the last frame
> in a corrupt stack. If so, we need to store the frame_id of the next
> frame and not of this one (which is possibly invalid). */
> if (get_prev_frame (frame) == NULL
> && get_frame_unwind_stop_reason (frame) != UNWIND_NO_REASON
> && get_next_frame (frame) != NULL)
> {
> frame_obj->frame_id = get_frame_id (get_next_frame (frame));
> frame_obj->frame_id_is_next = 1;
> }
Yes, this is bogus. But even if you remove this, the performance hits
still register as significant.
> and given the present frame stash can only hold one frame,
> these get_prev_frame/get_next_frame calls constantly invalidate it.
> Now, I don't get this "detect corrupt stack" code at all.
Me either, it should be removed. Hiding the corrupt stack from a
Python consumer seems all kinds of wrong. I am going to fix this
next. I decided not to include it in this patch, as I wanted the
focus to be on frame_stash issues where Python scripts can randomly
access frame from all over the stack.
Take this example
f = gdb.newest_frame()
do some other inferior operations happen, stop.
g = gdb.newest_frame()
Now is I access f, say f.type(), that will not be in the frame_stash,
it was from awhile ago. These kinds of patterns do crop up in frame
filters, because we are filtering, eliding frames.
> To be clear, I'm not against the hash stash idea at all. It's likely to
> speed up use cases, even if frame_info_to_frame_object was changed
> to not do that dance.
It will be changed, very soon.
Thanks for the comments,
Cheers
Phil