[all-commits] [llvm/llvm-project] 584cb6: [IRSymTab] Set FB_used on llvm.compiler.used symbols

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 584cb67d2df32163efe61cfa58b3c798ee5b6b5c
      https://github.com/llvm/llvm-project/commit/584cb67d2df32163efe61cfa58b3c798ee5b6b5c
  Author: Fangrui Song <i at maskray.me>
  Date:   2021-03-03 (Wed, 03 Mar 2021)

  Changed paths:
    A clang/test/CodeGen/thinlto-inline-asm2.c
    M llvm/lib/LTO/LTO.cpp
    M llvm/lib/Object/IRSymtab.cpp
    A llvm/test/ThinLTO/X86/asm.ll
    M llvm/test/tools/gold/X86/emit-llvm.ll

  Log Message:
  -----------
  [IRSymTab] Set FB_used on llvm.compiler.used symbols

IR symbol table does not parse inline asm. A symbol only referenced by inline
asm is not in the IR symbol table, so LTO does not know that the definition (in
another translation unit) is referenced and may internalize it, even if that
definition has `__attribute__((used))` (which lowers to `llvm.compiler.used` on
ELF targets since D97446).

```
// cabac.c
__attribute__((used)) const uint8_t ff_h264_cabac_tables[...] = {...};

// h264_cabac.c
  asm("lea ff_h264_cabac_tables(%rip), %0" : ...);
```

`__attribute__((used))` is the recommended way to tell the compiler there may
be inline asm references, so the usage is perfectly fine. This patch
conservatively sets the `FB_used` bit on `llvm.compiler.used` symbols to work
around the IR symbol table limitation. Note: before D97446, Clang never emitted
symbols in the `llvm.compiler.used` list, so this change does not punish any
Clang emitted global object.

Without the patch, `ff_h264_cabac_tables` may be assigned to a non-external
partition and get internalized. Then we will get a linker error because the
`cabac.c` definition is not exposed.

Differential Revision: https://reviews.llvm.org/D97755