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.
Is this other file needed for the test?