[cfe-dev] [LLVMdev] Issues with clang-llvm debug info validity

David Blaikie dblaikie at gmail.com
Fri Jun 27 14:19:07 PDT 2014 

On Fri, Jun 27, 2014 at 2:15 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
>> On Jun 27, 2014, at 1:58 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> On Fri, Jun 27, 2014 at 1:49 PM, Adrian Prantl <aprantl at apple.com> wrote:
>>> I looked into this a little.
>>
>> Thanks for taking a look!
>>
>>> My proposal is this:
>>>
>>> - extend DINamespace with an extra UniqueID filed (analogous to DICompositeType)
>>> - add an extra case to DIScope::getRef()
>>> - Let CGDebugInfo create a mangled name for each namespace to use as UID
>>>  -> the UID of the anonymous namespace is null.
>>>
>>> For example:
>>>
>>> namespace a { namespace b {} } -> “_ZN1a", “_ZN1a1b"
>>>
>>> !0 = [compile unit]
>>> !1 = metadata !{i32 786489, metadata !0, metadata !”_ZN1a", metadata !”b", i32 1, metadata !”_ZN1a1b"} ; [ DW_TAG_namespace ] [a]
>>> !2 = metadata !{i32 786489, metadata !0, null, metadata !”a", i32 1, metadata !”_ZN1a"} ; [ DW_TAG_namespace ] [a] [line 1]
>>>
>>> namespace a1b {} -> “_ZN3a1b"
>>> namespace {} -> null (!= “”) // Anonymous namespaces will remain old-fashioned MDNode references to prevent uniquing.
>>
>> Will this do the right thing if there's a named namespace inside or
>> outside the anonymous namespace?
>
> This is actually really tricky. Normally the ValueSymbolTable takes care of adding a unique suffix to all conflicting symbols. i think the most reasonable thing to do here is to add a rule:
>   -> the UID of any namespace that has an anonymous namespace in its parent chain is null.

Yep - I assume that's the right thing to do, just not sure if it
would/will require a little extra work & wanted to make sure it was on
the radar.

>>> With this change we will get the full type uniquing benefits. As a side effect, definitions that are in the same namespace will be children of the same DW_TAG_namespace in the debug info, which I’d argue is the expected behavior for LTO anyway.
>>
>> Sort of - what happens to functions defined in that namespace? Will
>> they all end up in one CU with one instance of the namespace under
>> LTO? Could do weird things to address ranges? (would we end up with
>> the address range in the CU the function was originally in - but the
>> subprogram DIE ends up in some other CU that doesn't include that
>> address range)
>
> I think you just convinced me to save this for later and clone a namespace for every compile unit instead. The decl_file/decl_line will have to identical for all CUs though (and come from the module linked in first).

OK - so we'll put one namespace in the metadata and just use that for
each CU? Should be easy enough.

One issue: what keeps the namespace metadata nodes live? Where do we
find them? Are we going to add another list to the CUs?

>
> thanks!
> adrian
>
>>
>> - David
>>
>>> -- adrian
>>>
>>>> On Jun 24, 2014, at 4:39 PM, Adrian Prantl <aprantl at apple.com> wrote:
>>>>
>>>>
>>>>> On Jun 24, 2014, at 4:34 PM, Eric Christopher <echristo at gmail.com> wrote:
>>>>>
>>>>>>> Without needing to change how namespaces are handled.
>>>>>>>
>>>>>>> Yes, using DIRefs for namespaces would cause the actual type nodes to
>>>>>>> be deduplicated when they aren't even with the patch above, and that
>>>>>>> would help reduce debug info metadata further if that's
>>>>>>> useful/necessary.
>>>>>>
>>>>>> Exactly. While the above patch will cover up the problem, we will still have many duplicate type entries during LTO that will take up memory, even though only the last one to be entered into DIRefMap will actually be used for emitting DWARF. Since DIRefs are also more expensive in terms of lookup speed, I think I’d prefer to only use them only where necessary (=as far down the MDNode DAG as possible).
>>>>>>
>>>>>
>>>>> Right, though, to be fair, some numbers here might be nice. The
>>>>> tradeoff of "extra lookup for ref" versus "extra memory allocation for
>>>>> larger debug info”.
>>>>
>>>> Then again, it’s not like the extra lookups introduced by David’s proposed patch would get rid of those extra memory allocations. Granted, they do get rid of the assertion, which obviously provides some value ;-)
>>>>
>>>>>
>>>>> I also don't think it'll matter for speed :)
>>>>>
>>>> You’re probably right about the lookup, but depending on the project you’re building the memory usage could matter.
>>>>
>>>> -- adrian
>>>>
>>>>> -eric
>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> Question to the C++ experts: Is the name (“test” in this case) a sufficient appropriate unique ID for a C++ namespace?
>>>>>>>
>>>>>>> Potentially - assuming you don't do this for anything in an anonymous
>>>>>>> namespace. I'd probably see about using the standard mangling
>>>>>> Good point! We should avoid unifying the anonymous namespace :-)
>>>>>>> machinery as we do for other things, though, if possible. Might keep
>>>>>>> it more consistent.
>>>>>>
>>>>>> I will do some experiments with nested namespaces, but it seems as if name mangling does not buy us much: this will merely turn into N4test.
>>>>>>
>>>>>> -- adrian
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> -- adrian
>>>>>>>>
>>>>>>>>
>>>>>>>>> On Jun 24, 2014, at 1:49 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>>>>>>>
>>>>>>>>> +Adrian & Manman,
>>>>>>>>>
>>>>>>>>> Looks like this is a case of non-DIRef references ending up in the IR,
>>>>>>>>> and thus the references not being deduplicated. This could lead to
>>>>>>>>> some of the IR bloat that you guys implemented the DIRef stuff to
>>>>>>>>> reduce/avoid.
>>>>>>>>>
>>>>>>>>> The specific issue is that the type is slightly different in the two
>>>>>>>>> TUs (since the namespace scope it's contained within is different in
>>>>>>>>> the two TUs - note the namespace preceeding the header include in
>>>>>>>>> test2.cpp) and the actual DIRef-usage failure is when building
>>>>>>>>> subroutine types. The subroutine types are made with direct references
>>>>>>>>> to types, not with DIRef references to types. This means they won't be
>>>>>>>>> deduplicated/resolved to a single type during linking correctly.
>>>>>>>>>
>>>>>>>>> It also leads to an assertion as per the original email. Figured you
>>>>>>>>> guys might want to look into/fix this (I assume the right fix is to
>>>>>>>>> use DIRef more pervasively in the frontend - I'm doing a little
>>>>>>>>> experimenting with clang now to see how that looks).
>>>>>>>>>
>>>>>>>>> - David
>>>>>>>>>
>>>>>>>>> On Fri, Jun 13, 2014 at 11:08 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>>>>>>>> Yep, looks like a legit bug to me. I'll try to reproduce it and reduce
>>>>>>>>>> a test case, etc.
>>>>>>>>>>
>>>>>>>>>> (the ODR doesn't require that the two copies of the Test class
>>>>>>>>>> template appear on the same line in the same file in two distinct
>>>>>>>>>> translation units, it only requires that the two copies have the same
>>>>>>>>>> sequence of tokens - which they do in your example)
>>>>>>>>>>
>>>>>>>>>> On Fri, Jun 13, 2014 at 5:33 PM, Kevin Modzelewski <kmod at dropbox.com> wrote:
>>>>>>>>>>> Hi all, sorry to post to both lists, but I'm running into an issue where
>>>>>>>>>>> clang-generated debug info is deemed to be invalid by LLVM tools (throws an
>>>>>>>>>>> assertion error in both llc and mcjit), and I'm not sure what the proper
>>>>>>>>>>> resolution is.
>>>>>>>>>>>
>>>>>>>>>>> Here's a test case; I last tested it on revision r210953:
>>>>>>>>>>>
>>>>>>>>>>> $ cat test1.cpp
>>>>>>>>>>> #include "test.h"
>>>>>>>>>>> test::Test<int> foo1() {
>>>>>>>>>>> return test::Test<int>();
>>>>>>>>>>> }
>>>>>>>>>>>
>>>>>>>>>>> $ cat test2.cpp
>>>>>>>>>>> namespace test {
>>>>>>>>>>> }
>>>>>>>>>>> #include "test.h"
>>>>>>>>>>> test::Test<int> foo2() {
>>>>>>>>>>> return test::Test<int>();
>>>>>>>>>>> }
>>>>>>>>>>>
>>>>>>>>>>> $ cat test.h
>>>>>>>>>>> namespace test {
>>>>>>>>>>> template <class T>
>>>>>>>>>>> class Test {
>>>>>>>>>>> };
>>>>>>>>>>> }
>>>>>>>>>>>
>>>>>>>>>>> $ clang++ -g -emit-llvm test1.cpp -S -o test1.ll
>>>>>>>>>>> $ clang++ -g -emit-llvm test2.cpp -S -o test2.ll
>>>>>>>>>>> $ llvm-link test1.ll test2.ll -S -o linked.ll
>>>>>>>>>>> $ llc linked.ll
>>>>>>>>>>>
>>>>>>>>>>> The last step raises an assertion error, assuming you have assertions
>>>>>>>>>>> enabled:
>>>>>>>>>>> llc: lib/CodeGen/AsmPrinter/DwarfUnit.cpp:953: llvm::DIE*
>>>>>>>>>>> llvm::DwarfUnit::getOrCreateTypeDIE(const llvm::MDNode*): Assertion `Ty ==
>>>>>>>>>>> resolve(Ty.getRef()) && "type was not uniqued, possible ODR violation."'
>>>>>>>>>>> failed
>>>>>>>>>>>
>>>>>>>>>>> The initial introduction of the "test" namespace in test2.cpp seems to
>>>>>>>>>>> confuse clang -- test1.cpp and test2.cpp have non-compatible debug
>>>>>>>>>>> information for the test::Test type.  test1.cpp says that test::Test's
>>>>>>>>>>> namespace is defined in test.h, and test2.cpp says that test::Test's
>>>>>>>>>>> namespace is defined in test2.cpp (they both say that the class itself is
>>>>>>>>>>> defined in test.h).  Regardless of whether LLVM tools should allow this,
>>>>>>>>>>> this seems like the wrong output for these files?
>>>>>>>>>>>
>>>>>>>>>>> Another related example, is if we simply defined the test::Test template in
>>>>>>>>>>> both files (ie, manually executed the #include "test.h") and got rid of the
>>>>>>>>>>> test2.cpp "namespace test {}" definition, we'd run into the same assertion,
>>>>>>>>>>> without hitting the weird clang behavior with the empty namespace
>>>>>>>>>>> pre-declaration.
>>>>>>>>>>>
>>>>>>>>>>> It's probably also worth noting that compiling directly with clang, ie
>>>>>>>>>>> adding a "int main() {}" function and then doing
>>>>>>>>>>> $ clang++ -g test1.cpp test2.cpp -o test
>>>>>>>>>>> works fine.
>>>>>>>>>>>
>>>>>>>>>>> I'm not a C++ expert and I don't know if these template issues really are
>>>>>>>>>>> violations of the ODR, but it feels like this should be allowed, and
>>>>>>>>>>> somewhere in the llvm toolchain (llvm-link? DwarfUnit.cpp?) this should be
>>>>>>>>>>> handled.  What do people think?
>>>>>>>>>>>
>>>>>>>>>>> kmod
>>>>>>>>
>>>>>>
>>>>>>
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>>>>
>>>>
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>

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