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`-Cllvm-args` usability improvement fixes: #26338 fixes: #115564 Two problems were found during playing with `-Cllvm-args` 1. When `llvm.link-shared` is set to `false` in `config.toml`, output of `rustc -C llvm-args='--help-list-hidden'` doesn't contain `--emit-dwarf-unwind` and `--emulated-tls`. When it is set to `true`, `rustc -C llvm-args='--help-list-hidden'` emits `--emit-dwarf-unwind`, but `--emulated-tls` is still missing. 2. Setting `-Cllvm-args=--emit-dwarf-unwind=always` doesn't take any effect, but `-Cllvm-args=-machine-outliner-reruns=3` does work. ### 1 Adding `RegisterCodeGenFlags` to register codegen flags fixed the first problem. `rustc -C llvm-args='--help-list-hidden'` emits full codegen flags including `--emit-dwarf-unwind` and `--emulated-tls`. ### 2 Constructing `TargetOptions` from `InitTargetOptionsFromCodeGenFlags` in `LLVMRustCreateTargetMachine` fixed the second problem. The `LLVMRustSetLLVMOptions` calls `ParseCommandLineOptions` which parses given `llvm-args`. For options like `machine-outliner-reruns`, it just works, since the codegen logic directly consumes the parsing result: [machine-outliner-reruns register](0537f6354c/llvm/lib/CodeGen/MachineOutliner.cpp (L114)) [machine-outliner-reruns consumption](0537f6354c/llvm/lib/CodeGen/MachineOutliner.cpp (L1138)) But for flags defined in `TargetOptions` and `MCTargetOptions` to take effect, constructing them with `InitTargetOptionsFromCodeGenFlags` is essential, or the parsing result is just not consumed. Similar patterns can be observed in [lli](0537f6354c/llvm/tools/llc/llc.cpp (L494)), [llc](0537f6354c/llvm/tools/lli/lli.cpp (L517)), etc.