[LLVMdev] [Debug Info PATCH] for support of ref_addr and removal of DIE duplication
David Blaikie
dblaikie at gmail.com
Tue Oct 15 11:34:26 PDT 2013
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.
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
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