clangDriver is the library implementing the compiler driver forClang. It utilitized LLVMOption to process command line options. Asoptions are processed when required, as opposed to use a largeswitch, Clang gets the ability to detect unused options.

When an option is checked with APIs such as hasArg andgetLastArg, its "claimed" bit is set.

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template<typename ...OptSpecifiers>
Arg *getLastArg(OptSpecifiers ...Ids) const {
  Arg *Res = nullptr;
  for (Arg *A : filtered(Ids...)) {
    Res = A;
    Res->claim();
  }
  return Res;
}

After all options are processed, Clang reports a-Wunused-command-line-argument diagnostic for eachunclaimed option. There are multiple possible messages, butargument unused during compilation is the most common one.

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% clang -c -ftime-trace a.s
clang: warning: argument unused during compilation: '-ftime-trace' [-Wunused-command-line-argument]
% clang -c a.c -la
clang: warning: -la: 'linker' input unused [-Wunused-command-line-argument]
% clang -c -ftime-trace -Werror a.s
clang: error: argument unused during compilation: '-ftime-trace' [-Werror,-Wunused-command-line-argument]
% clang -c -ftime-trace -Werror=unused-command-line-argument a.s
clang: error: argument unused during compilation: '-ftime-trace' [-Werror,-Wunused-command-line-argument]

There are many heuristics to enhance the desirability of-Wunused-command-line-argument, which can be rathersubjective. For instance, options that are relevant only duringcompilation do not result in -Wunused-command-line-argumentdiagnostics when linking is performed.

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% clang -faddrsig -fpic -march=generic a.o

Default options

There is a tension between-Wunused-command-line-argument and default options. Let'sconsider a scenario where we specify--rtlib=compiler-rt --unwindlib=libunwind inCFLAGS and CXXFLAGS to utilize compiler-rt andLLVM libunwind. ClangDriver claims --rtlib and--unwindlib in the following code snippet:

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if (!Args.hasArg(options::OPT_nostdlib, options::OPT_r)) {
  if (!Args.hasArg(options::OPT_nodefaultlibs)) {
    // Handle --rtlib and --unwindlib.
  }
  if (!Args.hasArg(options::OPT_nostartfiles)) {
    // Handle --rtlib. Append clang_rt.crtend.o or GCC style crtend{,S,T}.o
  }
}

However, if a build target employs -nostdlib or-nodefaultlibs, options such as --rtlib,--unwindlib, and many other linker options (e.g.-static-libstdc++ and -pthread) will not beclaimed, resulting in unused argument diagnostics:

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% clang --rtlib=compiler-rt --unwindlib=libunwind -stdlib=libstdc++ -static-libstdc++ -pthread -nostdlib a.o
clang: warning: argument unused during compilation: '--rtlib=compiler-rt' [-Wunused-command-line-argument]
clang: warning: argument unused during compilation: '--unwindlib=libunwind' [-Wunused-command-line-argument]
clang: warning: argument unused during compilation: '-static-libstdc++' [-Wunused-command-line-argument]
clang: warning: argument unused during compilation: '-pthread' [-Wunused-command-line-argument]

While some options like -stdlib= do not trigger adiagnostic, this seems more like a happenstance rather than a deliberatedesign choice.

To suppress the diagnostics, we can utilitize --start-no-unused-argumentsand --end-no-unused-arguments (Clang 14) like thefollowing:

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% clang --start-no-unused-arguments --rtlib=compiler-rt --unwindlib=libunwind -stdlib=libstdc++ -static-libstdc++ -pthread --end-no-unused-arguments -nostdlib a.o

There is also a heavy hammer -Qunused-arguments tosuppress -Wunused-command-line-argument diagnosticsregardless of options' positions:

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% clang -Qunused-arguments --rtlib=compiler-rt --unwindlib=libunwind -stdlib=libstdc++ -static-libstdc++ -pthread -nostdlib a.o

Conveniently, options specified in a Clang configuration file areautomatically claimed.