[LLVMdev] [Debug Info PATCH] for support of ref_addr and removal of DIE duplication
David Blaikie
dblaikie at gmail.com
Tue Oct 15 13:37:36 PDT 2013
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. 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.
> 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
>>
>
>
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