clangDriver is the library implementing the compiler driver for Clang. It utilitizes LLVMOption to process command line options. As options are processed when required, as opposed to use a large switch, Clang gets the ability to detect unused options straightforwardly.

When an option is checked with APIs such as hasArg and getLastArg, 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 each unclaimed option. There are multiple possible messages, but argument 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 rather subjective. For instance, options that are relevant only during compilation do not result in -Wunused-command-line-argument diagnostics when linking is performed. This is necessary to support linking actions that utilitize CFLAGS or CXXFLAGS.

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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's consider a scenario where we specify --rtlib=compiler-rt --unwindlib=libunwind in CFLAGS and CXXFLAGS to utilize compiler-rt and LLVM 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)) {
    
  }
  if (!Args.hasArg(options::OPT_nostartfiles)) {
    
  }
}

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 be claimed, 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 a diagnostic, this seems more like a happenstance rather than a deliberate design choice.

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

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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 to suppress -Wunused-command-line-argument diagnostics regardless 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 are automatically claimed.

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cat >a.cfg <<e
-stdlib=libstdc++
--rtlib=libgcc
--unwindlib=libgcc
e
clang --config=./a.cfg a.c            
mkdir bin lib
ln -s /usr/bin/clang bin/
cp a.cfg bin/clang.cfg
ln -s /usr/lib/clang lib/
bin/clang -no-canonical-prefixes a.c  

In the last command, I specify -no-canonical-prefixes so that clang will find dirname(clang)/clang.cfg, otherwise Clang would try to find dirname(realname(clang))/clang.cfg.