commit ad12af4a6f94b5e341871283c1841cdce2deba7d
author Fangrui Song <i@maskray.me> Thu Dec 21 18:59:11 2023 -0800
committer Copybara-Service <copybara-worker@google.com> Thu Dec 21 19:00:41 2023 -0800
tree 69df73d60c304da04d90cc55fa2848a3018c9a63
parent a6966af0efdd221cc58a065537ab330fe1ecccff

[ELF] Change .debug_names tombstone value to UINT32_MAX/UINT64_MAX (#74686)

`clang -g -gpubnames -fdebug-types-section` now emits .debug_names
section with references to local type unit entries defined in COMDAT
.debug_info sections.

```
.section        .debug_info,"G",@progbits,5657452045627120676,comdat
.Ltu_begin0:
...

.section        .debug_names,"",@progbits
...
// DWARF32
.long   .Ltu_begin0                     # Type unit 0
// DWARF64
// .long   .Ltu_begin0                     # Type unit 0
```

When `.Ltu_begin0` is relative to a non-prevailing .debug_info section,
the relocation resolves to 0, which is a valid offset within the
.debug_info section.

```
cat > a.cc <<e
struct A { int x; };
inline A foo() { return {1}; }
int main() { foo(); }
e
cat > b.cc <<e
struct A { int x; };
inline A foo() { return {1}; }
void use() { foo(); }
e
clang++ -g -gpubnames -fdebug-types-section -fuse-ld=lld a.cc b.cc -o old
```
```
% llvm-dwarfdump old
...
  Local Type Unit offsets [
    LocalTU[0]: 0x00000000
  ]
...
  Local Type Unit offsets [
    LocalTU[0]: 0x00000000  // indistinguishable from a valid offset within .debug_info
  ]
```

https://dwarfstd.org/issues/231013.1.html proposes that we use a
tombstone value instead to inform consumers. This patch implements the
idea. The second LocalTU entry will now use 0xffffffff.

https://reviews.llvm.org/D84825 has a TODO that we should switch the
tombstone value for most `.debug_*` sections to UINT64_MAX. We have
postponed the change for more than three years for consumers to migrate.
At some point we shall make the change, so that .debug_names is no long
different from other debug section that is not .debug_loc/.debug_ranges.

Co-authored-by: Alexander Yermolovich <ayermolo@meta.com>
GitOrigin-RevId: 26ddf4eee2a009147faa3000c55d7822c2087dce