[LLVMdev] [Debug Info PATCH] for support of ref_addr and removal of DIE duplication
Manman Ren
manman.ren at gmail.com
Tue Oct 15 13:29:08 PDT 2013
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 place.
>
>
>>
>> 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.
>
Eric,
Do you want to take a look at the 1st patch?
>
> There's one or two cases of {} on single-statement blocks you could fix up.
>
I couldn't find that.
Are you referring to:
+ } else {
+ Asm->EmitInt32(Addr);
+ }
It has a multi-statement if, I thought the rule is to have a parenthesis
for the else as well.
>
> "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?)?
>
Eric, any opinion here?
Thanks,
Manman
>
>
> - David
>
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