[LLVMdev] [Debug Info PATCH] for support of ref_addr and removal of DIE duplication

[Manman Ren]

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;
  }


> 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".


>     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".

  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.


>   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.


>   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.

  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.

Manman

>
>
>
> On Wed, Oct 9, 2013 at 10:45 AM, Manman Ren <manman.ren at gmail.com> wrote:
>
>>
>> Ping
>>
>> -Manman
>>
>>
>> On Fri, Oct 4, 2013 at 7:00 PM, Manman Ren <manman.ren at gmail.com> wrote:
>>
>>>
>>> Hi All,
>>>
>>> The first patch adds support for ref_addr.
>>> Most of it is from r176882, but instead of always using an integer for
>>> ref_addr, we use label + offset for relocation on non-darwin platforms.
>>>
>>> The second patch is a modified version of r191792.
>>> The main change is to use a single map instead of 3 maps in DwarfDebug
>>> and instead of calling DwarfDebug::getDIE|insertDIE directly, we delegate
>>> the function calls to DwarfDebug from CompileUnit.
>>>
>>> No testing case is added in the 1st patch, since the compiler does not
>>> use ref_addr yet.
>>>
>>> For the 2nd patch, testing cases are updated to make sure we remove
>>> duplicated DIEs and use ref_addr to refer to the type DIE. A testing case
>>> is also added to make sure we generate a relocation entry for ref_addr on
>>> linux platform.
>>>
>>> Thanks,
>>> Manman
>>>
>>>
>>>
>>
>
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