[llvm] [DebugInfo] Support to get TU for hash from .debug_types.dwo section in DWARF4. (PR #161067)

[David Blaikie via llvm-commits]

dwblaikie wrote:

Can you add a test case?

My rudimentary testing seems to show this working correctly already - but perhaps by accident or something?

```
struct t1 { };
void f1(t1) {
}
```
```
clang++-tot -fdebug-types-section -c test.cpp -gdwarf-4 -gsplit-dwarf  && llvm-dwarfdump-tot test.dwo -debug-info -debug-types
```
```
test.dwo:       file format elf64-x86-64

.debug_info.dwo contents:
0x00000000: Compile Unit: length = 0x00000036, format = DWARF32, version = 0x0004, abbr_offset = 0x0000, addr_size = 0x08 (next unit at 0x0000003a)

0x0000000b: DW_TAG_compile_unit
              DW_AT_producer    ("clang version 21.1.1 (git at github.com:llvm/llvm-project.git f4907049285ca0875cc91770e3ceb3f162ec7c48)")
              DW_AT_language    (DW_LANG_C_plus_plus_14)
              DW_AT_name        ("test.cpp")
              DW_AT_GNU_dwo_name        ("test.dwo")
              DW_AT_GNU_dwo_id  (0x9a627b6109b8affc)

0x00000019:   DW_TAG_subprogram
                DW_AT_low_pc    (indexed (00000000) address = <unresolved>)
                DW_AT_high_pc   (0x00000006)
                DW_AT_frame_base        (DW_OP_reg6 RBP)
                DW_AT_linkage_name      ("_Z2f12t1")
                DW_AT_name      ("f1")
                DW_AT_decl_file (0x01)
                DW_AT_decl_line (2)
                DW_AT_external  (true)

0x00000025:     DW_TAG_formal_parameter
                  DW_AT_location        (DW_OP_fbreg -1)
                  DW_AT_decl_file       (0x01)
                  DW_AT_decl_line       (2)
                  DW_AT_type    (0x00000030 "t1")

0x0000002f:     NULL

0x00000030:   DW_TAG_structure_type
                DW_AT_declaration       (true)
                DW_AT_signature (0xc6694e51369161f2)

0x00000039:   NULL

.debug_types.dwo contents:
0x00000000: Type Unit: length = 0x00000021, format = DWARF32, version = 0x0004, abbr_offset = 0x0000, addr_size = 0x08, name = 't1', type_signature = 0xc6694e51369161f2, type_offset = 0x001e (next unit at 0x00000025)

0x00000017: DW_TAG_type_unit
              DW_AT_language    (DW_LANG_C_plus_plus_14)
              DW_AT_stmt_list   (0x00000000)

0x0000001e:   DW_TAG_structure_type
                DW_AT_calling_convention        (DW_CC_pass_by_value)
                DW_AT_name      ("t1")
                DW_AT_byte_size (0x01)
                DW_AT_decl_file ("test.cpp")
                DW_AT_decl_line (1)

0x00000024:   NULL
```

Specifically, when dumping the CU it shows the name of the type at offset 0x30, which indicates that it's successfully looking through the `DW_TAG_structure_type` and using the `DW_AT_signature` to find the DWARFv4 type unit in `.debug_types.dwo` to retrieve the name... 

https://github.com/llvm/llvm-project/pull/161067