Fangrui Song 25d19a9795 [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		2021-03-03 16:22:30 -08:00
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Caching.cpp
CMakeLists.txt
LTO.cpp
LTOBackend.cpp
LTOCodeGenerator.cpp
LTOModule.cpp
SummaryBasedOptimizations.cpp
ThinLTOCodeGenerator.cpp
UpdateCompilerUsed.cpp