[PATCH] D147270: [DebugInfo] Support more than 2 operands in DWARF operations

[Carlos Alberto Enciso via Phabricator via llvm-commits]

CarlosAlbertoEnciso added a comment.

In D147270#4346528 <https://reviews.llvm.org/D147270#4346528>, @scott.linder wrote:

> I did a little digging and it seems like there is a faulty assumption in `LVELFReader`:
>
>   // We are assuming that DW_AT_specification, DW_AT_abstract_origin,
>   // DW_AT_type and DW_AT_extension do not appear at the same time
>   // in the same DIE.

Thanks very much for your analysis and for creating a small reproducible test.

> This seems true for GCC, but not for Clang, at least for the simplest reproducer I could create:
>
>   $ cat a.cpp
>   struct S {
>       static const int Arr[];
>   };
>   const int S::Arr[] = {
>       0, 1, 2
>   };
>   $ gcc -g -c a.cpp -o a.o
>   $ build/bin/llvm-dwarfdump --debug-info a.o | grep -B1 -A2 DW_AT_specification
>   0x00000053:   DW_TAG_variable
>                   DW_AT_specification     (0x00000028 "Arr")
>                   DW_AT_decl_line (4)
>                   DW_AT_decl_column       (0x0b)
>   $ clang++ -g -c a.cpp -o a.o
>   $ build/bin/llvm-dwarfdump --debug-info a.o | grep -B1 -A2 DW_AT_specification
>   0x0000001e:   DW_TAG_variable
>                   DW_AT_specification     (0x0000003e "Arr")
>                   DW_AT_type      (0x00000068 "const int[3]")
>                   DW_AT_location  (DW_OP_addr 0x0)
>
> It seems like the code could be adapted to track these independently, rather than assume only one is present? I can prepare a patch if that sounds reasonable to you!

I am happy with your proposal and a patch.

> Edit: This may also be a Clang bug. I haven't read the relevant DWARF sections with enough detail to say whether the DWARF itself is valid or not, but either way it seems nice to support it in llvm-debuginfo-analyzer, especially considering how much flexibility the tool already affords in terms of input

I compiled your test case with the latest Clang:

  0x0000001e:   DW_TAG_variable
                       DW_AT_specification	(0x00000031 "Arr")
                       DW_AT_type	(0x00000052 "const int[3]")
                       DW_AT_location	(DW_OP_addrx 0x0)
                       DW_AT_linkage_name	("_ZN1S3ArrE")
  
  0x00000031:     DW_TAG_member
                         DW_AT_name	("Arr")
                         DW_AT_type	(0x0000003a "const int[]")
                         DW_AT_decl_file	("/data/projects/sandbox/pr-62716.cpp")
                         DW_AT_decl_line	(2)
                         DW_AT_external	(true)
                         DW_AT_declaration	(true)

>From the DWARF section for DW_AT_specification:

  A debugging information entry with a DW_AT_specification attribute does not need to duplicate information provided by the debugging information entry referenced by that specification attribute.

Clang:

- Added an extra entry for DW_AT_type which seems redundant in DIE (0x1e) as by following the DW_AT_specification we can get the type
- The referenced types are pointing to different DIEs (0x52 and 0x3a).


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