llvm dev - Nov 2013 - [LLVMdev] bit code file incompatibility due to debug info changes

I don't have a strong opinion on how we fix it, but I think it's really
important to do something and get it done for the 3.4 release.

Here's what I would do:

- implement Quentin's previous proposal for back-end diagnostics (and add a
diagnostic handler to Apple's linker), so that we can warn about out-of-date
debug info

- add a version number to the debug info metadata

- have the bit code reader strip out any debug info that is missing the version
number or has a different version than the current value

On Nov 19, 2013, at 10:40 AM, Eric Christopher <echristo at gmail.com>
wrote:
> On Tue, Nov 19, 2013 at 9:19 AM, Bob Wilson <bob.wilson at apple.com>
wrote:
>> 
>> On Nov 18, 2013, at 11:36 PM, Eric Christopher <echristo at
gmail.com> wrote:
>> 
>>> On Mon, Nov 18, 2013 at 11:02 PM, Bob Wilson <bob.wilson at
apple.com> wrote:
>>>> In the bug report, Rafael commented that the compiler should
never crash, even if the debug info metadata is old.  I certainly agree, and we
should fix that.  I don’t think that really solves this problem, at least not by
itself.  For one thing, if the metadata format is really changing so rapidly,
how do we ensure that we have fixed all the problems and that we don’t regress? 
Is there a plan for testing the behavior with a wide variety of old debug info?
>>>> 
>>> 
>>> Not a bit. The idea behind the verifier is that if it sees
something
>>> unexpected it can do something about it whether it's emitting a
>>> diagnostic (though we don't emit diagnostics from the back
end), or
>>> just removing the metadata in memory ala strip (not on disk - that
>>> really would be unfriendly).
>> 
>> I understand the idea behind the verifier.  I'm asking about
testing.  If we're relying on the verifier for correctness, then we need to
test that on a regular basis.  I know we have a good number of tests with valid
metadata.  How do you propose that we test to make sure the verifier catches all
invalid metadata?  This seems like a weak point of the proposal to rely on the
verifier rather than a version number.
>> 
> 
> Every time we change it check it in as an additional "don't
die" test?
> Right now there are a lot of asserts, moving those to being failures
> would probably work. How would you like to version it? Other than at
> bitcode load time I don't want to do anything with the version number
> in the debug info. The previous versioning scheme became quickly
> unmaintainable and I don't want to go back to that either.
> 
>>> 
>>>> What is the user experience in the case where the debug info is
out of date?  Not crashing is a pretty low bar.  Might we end up emitting DWARF
that is incorrect?  If we’re just going to drop the old debug info, I would
expect a warning diagnostic to report that.
>>>> 
>>> 
>>> A warning is about the best you can expect. The metadata is
basically
>>> locked with assumptions on fields throughout the code. There's
really
>>> no way to continue reliably.
>>> 
>>> -eric
>> 
>> This really is a pretty horrible user experience, even with a warning.
>> 
>> I'd just like to comment that we're actually running into this
problem at Apple.  It is not an idle concern.  We had a static library built
with LTO using an older version of clang.  When linking that with a recent
version of clang, the compiler crashed.  Obviously we need to fix the crash. 
Even if we don't crash, emitting incorrect debug info or silently removing
the debug info is only somewhat better.  Stripping the debug info and providing
a warning would be a reasonable short-term workaround.  I don't think we can
get away with doing anything less than that.  For the longer term, perhaps we
ought to revisit the decision to not auto-upgrade debug info metadata, at least
once the format stabilizes.  I had not realized the implications of that for LTO
at the time it was originally discussed.
>> 
> 
> I don't think we're really disagreeing with each other here.
It's not
> a good experience and what we've said previously is that you really
> can't rely upon the metadata being compatible between versions -
> including and especially LTO. In the future when we're not trying to
> change the various formats and needing to expand things so that we can
> cover more debug information then I think finally locking down the
> format will be a good thing - that time just isn't now. If you have
> some specific ideas in mind for this I'm very glad to discuss them
> with you.
> 
> -eric
> 
> 
> 
>>> 
>>>> On Nov 18, 2013, at 12:47 PM, Bob Wilson <bob.wilson at
apple.com> wrote:
>>>> 
>>>>> OK.  That is fine with me.  Saying "WONTFIX"
seems to be a bit different than "let's carry on the discussion on the
mailing list until we figure out what to do", though.  I had intended to
use that PR to track the fact that we have an issue here that I think needs to
be addressed before the 3.4 release.  I don't care where the discussion
happens.
>>>>> 
>>>>> On Nov 18, 2013, at 11:53 AM, Eric Christopher <echristo
at gmail.com> wrote:
>>>>> 
>>>>>> For the same reason that you asked me in two threads:
because it's
>>>>>> splitting the discussion up and making it more
complicated. Let's keep
>>>>>> it to one thread and then open a bug at the end when/if
we have
>>>>>> something that we can take action on.
>>>>>> 
>>>>>> -eric
>>>>>> 
>>>>>> On Mon, Nov 18, 2013 at 11:05 AM, Bob Wilson
<bob.wilson at apple.com> wrote:
>>>>>>> Eric, why did you move the PR back to
resolved/won't-fix again after I
>>>>>>> reopened it?  We really need to do *something*
here.  I'm  open to
>>>>>>> discussing any of the possible solutions that have
been offered but just
>>>>>>> letting the compiler crash does not seem
acceptable.
>>>>>>> 
>>>>>>> On Nov 18, 2013, at 11:00 AM, Chandler Carruth
<chandlerc at google.com> wrote:
>>>>>>> 
>>>>>>> 
>>>>>>> On Mon, Nov 18, 2013 at 10:55 AM, David Blaikie
<dblaikie at gmail.com> wrote:
>>>>>>>> 
>>>>>>>> It depends a bit, also, on what kind of
guarantees we need to offer. If
>>>>>>>> the guarantee when reading IR from disk is
"will not crash" then there's
>>>>>>>> nothing for it but to run full debug info
verification.
>>>>>>>> 
>>>>>>>> On the other hand, if we can assume that some
specific metadata implies
>>>>>>>> the correctness of some other metadata, then
all we need to do is check a
>>>>>>>> known debug info version number. If it's
the current number, do nothing
>>>>>>>> (assume the rest of the debug info is up to
date and well formed), otherwise
>>>>>>>> do all the work to find the debug info and dump
it (no need to verify it -
>>>>>>>> we just have to do the work to find it, so we
don't go following bad links
>>>>>>>> later on - that should be as easy as dropping
the llvm.dbg.cu named node and
>>>>>>>> removing all debug intrinsics and the
instruction metadata line references).
>>>>>>>> But this latter scheme isn't robust against
arbitrary metadata (that could,
>>>>>>>> coincidentally, have the right version number
and arbitrary metadata that
>>>>>>>> breaks all our debug info metadata assumptions)
>>>>>>>> 
>>>>>>>> If the latter is sufficient for everyone's
needs/principles, great.
>>>>>>> 
>>>>>>> 
>>>>>>> This makes sense to me, but I see Eric's
fundamental concern with upgrading
>>>>>>> test cases.
>>>>>>> 
>>>>>>> 
>>>>>>> One other possible idea: we could artificially
force coarseness on metadata
>>>>>>> while things are still iterating rapidly by just
having a version number
>>>>>>> that rotates every "release". (Even
non-open-source-releases could be
>>>>>>> handled by letting anyone, any time they need to
rev it, update what the
>>>>>>> current version is and update every test case to
reference that version.) If
>>>>>>> the version is nicely factored, it should at least
be an obvious diff if
>>>>>>> still huge.
>>>>>>> _______________________________________________
>>>>>>> LLVM Developers mailing list
>>>>>>> LLVMdev at cs.uiuc.edu        
http://llvm.cs.uiuc.edu
>>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>>>> 
>>>>>>> 
>>>>> 
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>> 
>>

On Tue, Nov 19, 2013 at 10:48 AM, Bob Wilson <bob.wilson at apple.com>
wrote:
> I don't have a strong opinion on how we fix it, but I think it's
really
> important to do something and get it done for the 3.4 release.
>
I agree with everything here being a reasonable step for the 3.4 release
except:

> - implement Quentin's previous proposal for back-end diagnostics (and
add
> a diagnostic handler to Apple's linker), so that we can warn about
> out-of-date debug info
>
I don't think this is important for the release. I don't think open
source
people have any expectation of stability here, and while it is a
nice-to-have, it doesn't seem necessary, and seems clearly like a
significant extension that shouldn't be cherry-picked late into the release.
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On Nov 19, 2013, at 10:58 AM, Chandler Carruth <chandlerc at google.com>
wrote:
> 
> On Tue, Nov 19, 2013 at 10:48 AM, Bob Wilson <bob.wilson at
apple.com> wrote:
> I don't have a strong opinion on how we fix it, but I think it's
really important to do something and get it done for the 3.4 release.
> 
> I agree with everything here being a reasonable step for the 3.4 release
except:
>  
> - implement Quentin's previous proposal for back-end diagnostics (and
add a diagnostic handler to Apple's linker), so that we can warn about
out-of-date debug info
> 
> I don't think this is important for the release. I don't think open
source people have any expectation of stability here, and while it is a
nice-to-have, it doesn't seem necessary, and seems clearly like a
significant extension that shouldn't be cherry-picked late into the release.
OK, that's fair.  As long as we can get it on trunk soon, we can cherry-pick
it to an internal release branch.

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On Nov 19, 2013, at 11:01 AM, Bob Wilson <bob.wilson at apple.com> wrote:
> 
> On Nov 19, 2013, at 10:58 AM, Chandler Carruth <chandlerc at
google.com> wrote:
> 
>> 
>> On Tue, Nov 19, 2013 at 10:48 AM, Bob Wilson <bob.wilson at
apple.com> wrote:
>> I don't have a strong opinion on how we fix it, but I think
it's really important to do something and get it done for the 3.4 release.
>> 
>> I agree with everything here being a reasonable step for the 3.4
release except:
>>  
>> - implement Quentin's previous proposal for back-end diagnostics
(and add a diagnostic handler to Apple's linker), so that we can warn about
out-of-date debug info
>> 
>> I don't think this is important for the release. I don't think
open source people have any expectation of stability here, and while it is a
nice-to-have, it doesn't seem necessary, and seems clearly like a
significant extension that shouldn't be cherry-picked late into the release.
> 
> OK, that's fair.  As long as we can get it on trunk soon, we can
cherry-pick it to an internal release branch.
> 
Two questions:

1) Can we reopen the bugzilla PR that I filed and use that to cover the
additional of a version number and code to ignore the debug info when the
version number does not match?  Eric has already closed it twice, so I'm
asking this time instead of just reopening it.

2) Who is going to do this work?
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> Two questions:
>
> 1) Can we reopen the bugzilla PR that I filed and use that to cover the
> additional of a version number and code to ignore the debug info when the
> version number does not match?  Eric has already closed it twice, so
I'm
> asking this time instead of just reopening it.
I'm not really sure the bug is needed, but if you want to open a bug
then that's fine. I think there are some holes in what's going to
specified where, but that can be discussed when someone starts working
on it.
>
> 2) Who is going to do this work?
No idea.

-eric

On Nov 19, 2013, at 1:31 PM, Eric Christopher <echristo at gmail.com>
wrote:
>> Two questions:
>> 
>> 1) Can we reopen the bugzilla PR that I filed and use that to cover the
>> additional of a version number and code to ignore the debug info when
the
>> version number does not match?  Eric has already closed it twice, so
I'm
>> asking this time instead of just reopening it.
> 
> I'm not really sure the bug is needed, but if you want to open a bug
> then that's fine. I think there are some holes in what's going to
> specified where, but that can be discussed when someone starts working
> on it.
OK.  I have reopened pr17982 to cover this issue.  I don't intend to specify
much in bugzilla, but we should have something to track the overall problem.
> 
>> 
>> 2) Who is going to do this work?
> 
> No idea.
> 
> -eric