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

[Manman Ren]

On Mon, 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.
>

Adding a debug info version number and ignoring the debug info MDNodes when
version does not match makes sense. And changing the version number with
every release sounds reasonable.

One implementation is
1> completely get rid of version number in the tag
  LLVMDebugVersion is only used in two places and we should be able to
remove it.
    include/llvm/DebugInfo.h:    return getUnsignedField(0) &
~LLVMDebugVersionMask;
    lib/IR/DIBuilder.cpp:  assert((Tag & LLVMDebugVersionMask) == 0 &&
    lib/IR/DIBuilder.cpp:  return
ConstantInt::get(Type::getInt32Ty(VMContext), Tag | LLVMDebugVersion);
  I remember there was something that blocks David from completely getting
rid of version number, but it seems the blocker is gone now.
  This can be done separately as well.

2> introduce a debug info version in module flags similar to "dwarf
version" module flag
     LTO can correctly handle the module flag during linking and we only
need to change one line in the debug info testing cases when we update
debug info version number.
     One issue is that we don't have a mode that emits a warning and
outputs the largest value (we have a mode that emits a warning and outputs
the value from the first module, but we can definitely add another mode if
necessary)

3> when to drop the debug info MDNodes?
     We could do this in the bit code loader but it seems a violation of
layers. One possibility is to run a module pass right after loading the bit
code, to remove debug intrinsics and debug tags on instructions.

4> how to report warning?
    Is errs() << "WARNING: " good enough? BTW this is how we emit warning
when linking module flags.

Any objection to the above? Any other suggestion?

Thanks,
Manman


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