[PATCH] D100630: [Debug-Info][DBX] DW_TAG_rvalue_reference_type should not be generated when dwarf version is smaller than 4

[ChenZheng via Phabricator via cfe-commits]

shchenz added a comment.

In D100630#2749594 <https://reviews.llvm.org/D100630#2749594>, @dblaikie wrote:

> In D100630#2749550 <https://reviews.llvm.org/D100630#2749550>, @shchenz wrote:
>
>> In D100630#2748899 <https://reviews.llvm.org/D100630#2748899>, @dblaikie wrote:
>>
>>> Does this cause duplicate DW_TAG_reference_types in the output, if the input IR Has both DW_TAG_reference_type and DW_TAG_rvalue_reference_types?
>>
>> Yes, it will cause such issue in theory. I can not cook a case that has a DW_TAG_reference_type to a DW_TAG_rvalue_reference_types or has a DW_TAG_rvalue_reference_types to a DW_TAG_reference_type for now. Is there such usage that we both be rvalue-reference and lvalue-reference to a basic type?
>
> Ah, I was less thinking of a case of an rvalue reference and a normal reference in the same chain - I was thinking more about two unrelated reference types.
>
> Try some code like:
>
>   void f1(int &&x, int &y) {
>   }
>
> I'm guessing without this change - you get 'x's type pointing to the rvalue_reference_type, 'y's type pointing to the reference_type, and both of those pointing to the 'int' DW_TAG_basic_type.
>
> But with this change I'm worried you'll get two copies of the DW_TAG_reference_type - one that replaces the rvalue reference, and then the real one. Instead you should get one copy that both 'x' and 'y' refer to, I think.

Hmm, yeah, indeed, a duplicated `DW_TAG_reference_type` is generated.

  0x00000069:   DW_TAG_reference_type
                  DW_AT_type      (0x0000006e "int")
  
  0x0000006e:   DW_TAG_base_type
                  DW_AT_name      ("int")
                  DW_AT_encoding  (DW_ATE_signed)
                  DW_AT_byte_size (0x04)
  
  0x00000075:   DW_TAG_reference_type
                  DW_AT_type      (0x0000006e "int")

I think rather than fixing the duplicated `DW_TAG_reference_type` in the backend, maybe it is better to fix this strict DWARF issue in the FE like the fix in the previous patch https://reviews.llvm.org/D100630?id=338755

What do you think? @dblaikie


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