[LLVMdev] [Debug Info PATCH] for support of ref_addr and removal of DIE duplication
David Blaikie
dblaikie at gmail.com
Tue Oct 15 14:24:39 PDT 2013
On Tue, Oct 15, 2013 at 1:56 PM, Manman Ren <manman.ren at gmail.com> wrote:
>
>
>
> On Tue, Oct 15, 2013 at 1:37 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>>
>> On Tue, Oct 15, 2013 at 1:22 PM, Manman Ren <manman.ren at gmail.com> wrote:
>>
>>>
>>>
>>>
>>> On Tue, Oct 15, 2013 at 11:34 AM, David Blaikie <dblaikie at gmail.com>wrote:
>>>
>>>>
>>>>
>>>>
>>>> On Tue, Oct 15, 2013 at 10:51 AM, Manman Ren <manman.ren at gmail.com>wrote:
>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Tue, Oct 15, 2013 at 10:10 AM, David Blaikie <dblaikie at gmail.com>wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Tue, Oct 15, 2013 at 10:05 AM, Manman Ren <manman.ren at gmail.com>wrote:
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Wed, Oct 9, 2013 at 5:22 PM, Manman Ren <manman.ren at gmail.com>wrote:
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Wed, Oct 9, 2013 at 1:32 PM, Manman Ren <manman.ren at gmail.com>wrote:
>>>>>>>>
>>>>>>>>>
>>>>>>>>> David,
>>>>>>>>>
>>>>>>>>> Thanks for reviewing!
>>>>>>>>>
>>>>>>>>> On Wed, Oct 9, 2013 at 11:36 AM, David Blaikie <dblaikie at gmail.com
>>>>>>>>> > wrote:
>>>>>>>>>
>>>>>>>>>> Might be easier if these were on Phabricator, but here are some
>>>>>>>>>> thoughts:
>>>>>>>>>>
>>>>>>>>>> 0001:
>>>>>>>>>> This patch generally, while separated for legibility, is
>>>>>>>>>> untested & difficult to discuss in isolation.
>>>>>>>>>>
>>>>>>>>> I agree, this patch adds the functionality but does not use it,
>>>>>>>>> the 2nd patch uses ref_addr.
>>>>>>>>> If you think I should merge the two and commit as a single patch,
>>>>>>>>> let me know.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> I may need to refer to the second patch in reviewing this first
>>>>>>>>>> one.
>>>>>>>>>> DwarfDebug.cpp:
>>>>>>>>>> computeSizeAndOffsets:
>>>>>>>>>> I believe this produces the wrong offset for the 3rd CU and
>>>>>>>>>> onwards. computeSizeAndOffset returns the EndOffset which is absolute, not
>>>>>>>>>> relative to the Offset passed in, so it should be assigned to SecOffset,
>>>>>>>>>> not added to it. (eg: if you have CUs at 0, 42, and 57 - the first pass
>>>>>>>>>> through SecOffset will be zero, then it'll be 0 + 42, then on the 3rd it'll
>>>>>>>>>> be (0 + 42) + 57 which isn't right, it should just be 57). Please add more
>>>>>>>>>> test coverage while fixing this issue.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> computeSizeAndOffset takes an offset that is relative to the start
>>>>>>>>> of the CU and returns the offset relative to the CU after laying out the
>>>>>>>>> DIE.
>>>>>>>>> The initial offset before laying out the CU DIE is the header
>>>>>>>>> size, EndOffset will be the offset relative to the CU after laying out the
>>>>>>>>> whole CU DIE.
>>>>>>>>> We can think of EndOffset as the size of the whole CU DIE.
>>>>>>>>> SecOffset is the offset from the Debug Info section, so we can update it by
>>>>>>>>> adding the CU size.
>>>>>>>>>
>>>>>>>>> // Offset from the beginning of debug info section.
>>>>>>>>> unsigned SecOffset = 0;
>>>>>>>>> for (SmallVectorImpl<CompileUnit *>::iterator I = CUs.begin(),
>>>>>>>>> E = CUs.end(); I != E; ++I) {
>>>>>>>>> (*I)->setDebugInfoOffset(SecOffset);
>>>>>>>>> unsigned Offset =
>>>>>>>>> sizeof(int32_t) + // Length of Compilation Unit Info
>>>>>>>>> sizeof(int16_t) + // DWARF version number
>>>>>>>>> sizeof(int32_t) + // Offset Into Abbrev. Section
>>>>>>>>> sizeof(int8_t); // Pointer Size (in bytes)
>>>>>>>>>
>>>>>>>>> unsigned EndOffset = computeSizeAndOffset((*I)->getCUDie(),
>>>>>>>>> Offset);
>>>>>>>>> SecOffset += EndOffset;
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>
>>>>>>>> Added more comments in attached patch.
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> Eric/Manman: rough design question: compute the absolute offset
>>>>>>>>>> of each CU within the debug_info section and describe them all relative to
>>>>>>>>>> a single symbol at the start of the debug_info section, or should we put a
>>>>>>>>>> label at the start of each CU?
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Either way should work. But since we already have the section
>>>>>>>>> offset for each CU, describing relative to the start of debug_info section
>>>>>>>>> saves us a few symbols :)
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> 0002:
>>>>>>>>>> ref_addr_relocation.ll:
>>>>>>>>>> seems a bit vague in terms of how you test for the
>>>>>>>>>> relocation. I think it'd make more sense to test the assembly, than the
>>>>>>>>>> reafobj output, that way you can test that the correct bytes have the
>>>>>>>>>> relocation rather than just that there's "some" .debug_info relocation in
>>>>>>>>>> the file. For an example, see test/DebugInfo/X86/tls.ll I wrote - it has
>>>>>>>>>> some "unannotated" bytes because we still don't have a nice way to annotate
>>>>>>>>>> location bytes that are DWARF expressions, but it's close - I guess those
>>>>>>>>>> should be CHECK-NEXTs, though. In any case, you should be able to check a
>>>>>>>>>> few lines of assembly with the # DW_AT/DW_TAG annotation comments.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I can check for ".quad .Lsection_info+38 #DW_AT_type".
>>>>>>>>>
>>>>>>>> Done in attached patch.
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> You'd need to add the tu3.cpp from my example if you want to
>>>>>>>>>> demonstrate that the relocation is actually working as intended and
>>>>>>>>>> avoiding the bogus result I showed.
>>>>>>>>>> type-unique-simple-a.ll
>>>>>>>>>> While I agree that having common test cases helps reduce the
>>>>>>>>>> number of separate invocations we have, this test case seems like it might
>>>>>>>>>> be becoming a little hard to follow what's under test. Just going from the
>>>>>>>>>> diff I'm not sure what's what. Could you give a brief description of the
>>>>>>>>>> state of type-unique-simple2 files? What's involved? What's it meant to
>>>>>>>>>> test? What's it actually testing?
>>>>>>>>>>
>>>>>>>>> I can add more comments. The source files are included in the
>>>>>>>>> testing case. I am checking that llvm-link only generates a single copy of
>>>>>>>>> the struct and the backend generates a single DIE and uses ref_addr.
>>>>>>>>> The changes are to check "the backend generates a single DIE and
>>>>>>>>> uses ref_addr".
>>>>>>>>>
>>>>>>>> Done in attached patch.
>>>>>>>>
>>>>>>>>>
>>>>>>>>> DIE.h:
>>>>>>>>>> checkCompileUnit could probably be called
>>>>>>>>>> "getCompileUnitOrNull", the name "check*" seems to imply that it does some
>>>>>>>>>> checking, which isn't true.
>>>>>>>>>>
>>>>>>>>> Will do.
>>>>>>>>>
>>>>>>>> Done in attached patch.
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> DwarfCompileUnit.cpp:
>>>>>>>>>> the "istype || issubprogram" check should probably be pulled
>>>>>>>>>> out into a separate function, something like "isShareableAcrossCUs" or
>>>>>>>>>> something like that (please, that's not the best name, let's discuss it
>>>>>>>>>> further before we settle on a name) so that getDIE and insertDIE are sure
>>>>>>>>>> to use the same test at all times.
>>>>>>>>>>
>>>>>>>>> Yes, the condition is shared between getDIE and insertDIE. I like
>>>>>>>>> isSharableAcrossCUs, because that is why the map is in DwarfDebug instead
>>>>>>>>> of CompileUnit.
>>>>>>>>>
>>>>>>>> Done in attached patch.
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> Why does addDIEEntry take a form? When does the caller get to
>>>>>>>>>> choose the form rather than the callee deciding between ref4 and ref_addr
>>>>>>>>>> based on context?
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> addDIEEntry took a form before my change and I didn't check why it
>>>>>>>>> did.
>>>>>>>>> I will check if all callers always use ref4, if it it true, I will
>>>>>>>>> submit a cleanup patch first.
>>>>>>>>>
>>>>>>>> Done in attached patch.
>>>>>>>>
>>>>>>>>>
>>>>>>>>> I'm still unsure about this worklist thing - do your current
>>>>>>>>>> tests cover the need for the worklist? ie: if we removed that logic, would
>>>>>>>>>> tests fail? Can you describe a specific sequence where the worklist is
>>>>>>>>>> necessary?
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> If we are sure that DIEs are always added to an owner before
>>>>>>>>> calling addDIEEntry, we don't need the worklist.
>>>>>>>>> But I saw cases where that was not true, I will get a reduced
>>>>>>>>> testing case.
>>>>>>>>>
>>>>>>>>
>>>>>>>> If we try to assert both DIEs have owner in addDIEEntry, the
>>>>>>>> following testing cases will fail:
>>>>>>>> LLVM :: DebugInfo/X86/multiple-aranges.ll
>>>>>>>> LLVM :: DebugInfo/X86/ref_addr_relocation.ll
>>>>>>>> LLVM :: DebugInfo/X86/stmt-list-multiple-compile-units.ll
>>>>>>>> LLVM :: DebugInfo/two-cus-from-same-file.ll
>>>>>>>> LLVM :: Linker/type-unique-simple-a.ll
>>>>>>>> LLVM :: Linker/type-unique-simple2.ll
>>>>>>>>
>>>>>>>> For ref_addr_relocation, it failed in
>>>>>>>> #5 0x0000000100b169ba in llvm::DwarfDebug::addDIEEntry
>>>>>>>> (this=0x103023600, Die=0x102e14090, Attribute=73, Entry=0x10302a9d0) at
>>>>>>>> /Users/manmanren/llvm_git/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp:3071
>>>>>>>> #6 0x0000000100b040e0 in llvm::CompileUnit::addType
>>>>>>>> (this=0x102e13ec0, Entity=0x102e14090, Ty={<llvm::DIScope> =
>>>>>>>> {<llvm::DIDescriptor> = {DbgNode = 0x102e05f30}, <No data fields>}, <No
>>>>>>>> data fields>}, Attribute=73) at
>>>>>>>> /Users/manmanren/llvm_git/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp:910
>>>>>>>> #7 0x0000000100b05bff in
>>>>>>>> llvm::CompileUnit::createGlobalVariableDIE (this=0x102e13ec0,
>>>>>>>> N=0x102e068c0) at
>>>>>>>> /Users/manmanren/llvm_git/llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp:1505
>>>>>>>>
>>>>>>>> If we look at DwarfCompileUnit.cpp:
>>>>>>>> VariableDIE = new DIE(GV.getTag());
>>>>>>>> // Add to map.
>>>>>>>> insertDIE(N, VariableDIE);
>>>>>>>>
>>>>>>>> // Add name and type.
>>>>>>>> addString(VariableDIE, dwarf::DW_AT_name, GV.getDisplayName());
>>>>>>>> addType(VariableDIE, GTy);
>>>>>>>>
>>>>>>>> The VariableDIE is not added to an owner yet when calling addType.
>>>>>>>>
>>>>>>>
>>>>>>> I believe I have addressed all review comments, the patches are
>>>>>>> re-attached here for convenience.
>>>>>>>
>>>>>>
>>>>>> So I'm still thinking about the work list work.
>>>>>>
>>>>>> If we don't know which CU a DIE is in - isn't it, necessarily, going
>>>>>> to be in the current CU (& thus referenced by ref_data4 (using a CU-local
>>>>>> offset), not ref_addr)?
>>>>>>
>>>>>
>>>>> That may be true. But can we prove that?
>>>>>
>>>>
>>>> We really shouldn't add extra complexity to the codebase just because
>>>> we don't know how the codebase works - that's what leads to the kind of
>>>> complexity we see in the debug info handling today.
>>>>
>>>
>>>>
>>>>> There are two ways of handling this:
>>>>> 1> use a worklist to be conservative
>>>>> 2> do not use a worklist, but add an assertion when emitting a DIE A,
>>>>> make sure the DIE referenced with ref4 is indeed in the same CU as DIE A.
>>>>>
>>>>
>>>> Please just add this assertion. If it fires we'll have a test case and
>>>> a concrete justification for this complexity such that should anyone remove
>>>> it later because it looked unnecessary, they'll at least have a failing
>>>> test to explain why it was there in the first plac .
>>>>
>>>
>>> The assertion fails even with a simple testing case when the referenced
>>> DIE has an owner and the DIE itself does not have an owner.
>>>
>>
>> OK, sorry - I should've read your description of the assertion more
>> carefully. I believe the assertion you added wasn't the right thing to test
>> for.
>>
>> I'm not sure there is a correct assertion to add here to detect the case
>> you're describing - perhaps a complex verification after-the-fact could be
>> done, but essentially if we have a DIE that's partially constructed/has no
>> parent we should assume it's in the current unit.
>>
>
> Why should we make the assumption that a DIE without a parent at some
> point should belong to the CU that is constructing the DIE?
>
Because the code we have today only constructs one CU at a time. I know of
no code that adds anything to prior CUs. Indeed this may become an
invariant one day when we do CU-at-a-time DWARF emission to reduce memory
overhead. There's nothing in the design today that I know of that would
make CU-at-a-time DWARF emission anything other than trivial. We build all
the DIEs for a CU in one go, then move on to the next CU - the CU_Nodes
loop in beginModule does this.
> The CU constructing a DIE will add the DIE to the context owner, which may
> not belong to the CU itself.
>
That's the question, isn't it? When is it ever /not/ the current CU under
construction?
> In the case that a DIE is added to an owner in DwarfDebug, I don't think
> we should try to enforce that DwarfDebug will add the DIE to the CU that
> constructed the DIE earlier.
>
My claim is that this is already an invariant. That the code you are adding
is never needed and thus is additional, unjustified/unused complexity that
comes at a cost to all future developers/development. This codebase needs
/much/ less of this than it already has, let alone adding more of it.
Though I wouldn't mind an assertion, I suspect it'll be more code than is
really worth it to keep track of this information/state.
I think if we more to CU-at-a-time DWARF emission it'll be more obvious
that this invariant is true and cannot be violated.
>
>
>> If we can demonstrate a case where this isn't true, then we should work
>> to address that problem - until we demonstrate that, we should not (though
>> we might want to search for such cases - but without type units I can't
>> imagine how they could occur - we build the DIE tree for one CU at a time,
>> at no point do we have DIEs under construction for multiple CUs).
>>
>> So if we want to build a reference to a DIE, if the DIE is not in another
>> CU we should use ref4. (then the only other case is that it's either in
>> this CU or it's in no known CU at all - in which case it's under
>> construction and, without evidence to the contrary, will be added to the
>> current CU when it's done).
>>
>> About the only assertion we could add would involve keeping a side-table
>> of "assumptions" ("we expect this DIE will be added to this CU") and check
>> that those assumptions are fulfilled at some point.
>>
> In my opinion, if we can't verify the assumption with a reasonable amount
> of effort, then we should not make the assumption.
>
This leads to unbounded, unjustified complexity that makes the codebase
impossible to maintain. It just cannot be an acceptable method of
development.
- David
>
> Manman
>
>
>>
>>> For that case, we can't assume ref4 should be used. I don't think we can
>>> enforce that all DIEs must be added to a parent before calling addDIEEntry.
>>>
>>> For the specific testing case, when constructing children of a scope
>>> DIE, we call addDIEEntry on the children, after that, we add the children
>>> to the scope DIE.
>>> cat foo.cpp
>>>
>>> #include "a.hpp"
>>> void f(int a) {
>>> Base t;
>>> }
>>> cat bar.cpp
>>>
>>> #include "a.hpp"
>>> void f(int);
>>> void g(int a) {
>>> Base t;
>>> }
>>> int main() {
>>> f(0);
>>> g(1);
>>> return 0;
>>> }
>>> cat a.hpp
>>> struct Base {
>>> int a;
>>> };
>>>
>>> Let me know if I should investigate further.
>>>
>>> Thanks,
>>> Manman
>>>
>>>
>>>>
>>>>
>>>>>
>>>>> Let me know which one you prefer.
>>>>>
>>>>> Do you have any comments on the ref_addr patch (the 1st patch of the
>>>>> two)?
>>>>>
>>>>
>>>> Nothing much - I wouldn't mind Eric taking a look (& would rather you
>>>> not commit this until you're committing the second patch, since it's
>>>> otherwise untested/unjustified code) on the label/offset related stuff
>>>> since I'm less familiar with that.
>>>>
>>>> There's one or two cases of {} on single-statement blocks you could fix
>>>> up.
>>>>
>>>> "DebugInfoOffset" (both the member and the functions to set/get it)
>>>> might be more meaningfully named "SectionOffset" - but I'm not sure. Eric?
>>>> Other names (DebugInfoSectionOffset?)?
>>>>
>>>> - David
>>>>
>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20131015/e06c6d34/attachment.html>
More information about the llvm-dev
mailing list