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Re: [RFC 00/12] Merge value optimized_out and unavailable
- From: Pedro Alves <palves at redhat dot com>
- To: Andrew Burgess <aburgess at broadcom dot com>
- Cc: "gdb-patches at sourceware dot org" <gdb-patches at sourceware dot org>
- Date: Fri, 29 Nov 2013 20:20:40 +0000
- Subject: Re: [RFC 00/12] Merge value optimized_out and unavailable
- Authentication-results: sourceware.org; auth=none
- References: <5208D1DF dot 1090201 at broadcom dot com>
On 08/12/2013 01:15 PM, Andrew Burgess wrote:
> This patch set merges together how gdb handles values that are
> optimized out and values that are unavailable.
> I think that in most cases gdb should not care why the contents of
> a value are not fetch-able, it is only when we need to display
> something to the user that we should have to figure out was this
> optimized-out or unavailable?
Kind of, but not quite. Going through the series, and thinking about
it a lot, I'm not convinced the parts that handle unavailable and
optimized out errors the same way are right. The frame machinery
handles unavailable registers especially, with the _if_available
wrappers, because it's possible to have a traceframe with no
collected PC, or trimmed core file with no registers info,
and consequently, a frame #0 with an <unavailable> PC / function.
I'm not seeing how it's possible to end up with a frame_info that
has an optimized out PC. If unwinding the PC results in a not
saved PC, we use that as indication that we've reached the
outermost frame, so we stop the backtrace before that could happen.
So all the _if_available functions that _don't_ try to unwind the
prev frame (which are most, except frame_unwind_caller_pc_if_available),
shouldn't ever throw an optimized error for PC/function.
Since frame_unwind_caller_pc_if_available is the only
_if_available wrapper that unwinds, that's the case where the
caller would need to likewise handle optimized out / not saved
PCs. But then, after the series, "info frame", the only caller
of frame_unwind_caller_pc_if_available, ends printing <unavailable>
instead of <not saved>, because these different errors passed
through the same sieve hole:
if (frame_unwind_caller_pc_if_available (fi, &caller_pc))
fputs_filtered (paddress (gdbarch, caller_pc), gdb_stdout);
fputs_filtered ("<unavailable>", gdb_stdout);
(gdb) info frame
Stack level 2, frame at 0x0:
rip = 0x323d4f168d in clone (../sysdeps/unix/sysv/linux/x86_64/clone.S:115); saved rip <unavailable>
Outermost frame: outermost
caller of frame at 0x7ffff7fcafc0
source language asm.
Arglist at 0x7ffff7fcafb8, args:
Locals at 0x7ffff7fcafb8, Previous frame's sp is 0x7ffff7fcafc8
Another place showing the issue with "merging" the errors
frame_unwind_pc (struct frame_info *this_frame)
if (!frame_unwind_pc_if_available (this_frame, &pc))
throw_error (NOT_AVAILABLE_ERROR, _("PC not available"));
This throws the wrong error with the wrong string if
frame_unwind_pc_if_available returned false due to
So for that part of the series, I'd rather not go around and
sprinkle the is_unavailable_error wrapper function wherever
we use catch NOT_AVAILABLE_ERROR, but instead handle
OPTIMIZED_OUT_ERROR as needed.
I've posted a mini series that fixes the "info frame" case
(I had actually tried to fix that before by making
frame_unwind_caller_pc return struct value, and gave up as that
grew quite messy quick. Making use of your new OPTIMIZED_OUT_ERROR
ends up much simpler.)
> After this patch set there will be a single unified interface to ask
> if a value is available (either fully, partially, or for a range of
> bit/bytes), this will answer in terms of both optimized out and
> unavailable state.
On terminology: I'd much rather not overload the "available/unavailable"
words for this. It'll end up confusing, like "This value is
not available, because it was unavailable? No, because it
was optimized out.". Etc.