[LLVMdev] bit code file incompatibility due to debug info changes

[Eric Christopher]

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).

> What is the user experience in the case where the debug info is out of date?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

> 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.
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>>>>
>>
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