[flang-commits] [flang] [flang][debuginfo] Emit DW_AT_type for DW_TAG_string_type DIEs [3/3] (PR #231960)

[via flang-commits]

jiel-nv wrote:

Thank you @timsmith78. This is a great enhancement! I only have two questions.

1. Should this be guarded on DWARF 5+? As the original issue #95440 notes, `DW_AT_type` on `DW_TAG_string_type` is a DWARF 5 feature.

2. The `stringLengthExpression` of descriptor-based strings is now visibly inconsistent with the character type. Slightly out of scope, since the issue is pre-existing, but this PR is what makes the character kind visible and so exposes it. For example, with a deferred-length string:

```fortran
character(kind=4, len=:), allocatable :: deferred
allocate(character(kind=4, len=7) :: deferred)    ! 7 characters = 28 bytes
deferred = 4_"abcdefg"
```

`DW_AT_string_length` points at the descriptor's `elem_len` field, which Flang stores in *bytes*, so it yields 28 rather than 7. This PR now additionally declares the character to be 4 bytes wide:

```
0x00000092:   DW_TAG_string_type
                DW_AT_string_length	(DW_OP_push_object_address, DW_OP_plus_uconst 0x8)   <=== yields 28, i.e. bytes (pre-existing)
                DW_AT_data_location	(DW_OP_push_object_address, DW_OP_deref)
                DW_AT_encoding	(DW_ATE_UCS)
                DW_AT_type	(0x0000008e "character(kind=4)")
0x0000008e:   DW_TAG_base_type
                DW_AT_name	("character(kind=4)")
                DW_AT_encoding	(DW_ATE_UCS)
                DW_AT_byte_size	(0x04)                                                   <=== inconsistent with the 28 above
```

It would be nice to fix it via something like,

metadata side:

```
stringLengthExpression: !DIExpression(DW_OP_push_object_address, DW_OP_plus_uconst, 8,
                                      DW_OP_deref, DW_OP_constu, <kind>, DW_OP_div,
                                      DW_OP_stack_value)
```

then the DWARF side should be as expected:

```
DW_AT_string_length	(DW_OP_push_object_address, DW_OP_plus_uconst 0x8, DW_OP_deref,
                     DW_OP_constu 0x4, DW_OP_div, DW_OP_stack_value)
```


https://github.com/llvm/llvm-project/pull/231960