--rtlib does not trigger -Wunused-command-line-argument on AArch64

[glandium]

On some platforms, --rtlib=libgcc/--rtlib=compiler-rt only affect the libraries linked. On some others, it affects the generated code. As such, it needs to be passed as a CFLAGS and a LDFLAGS. The problem is that on platforms where the generated code, having it in CFLAGS triggers a unused-command-line-argument warning. Whether it affects the generated code is a clang implementation detail that build systems shouldn't be concerned with.

STR:

• touch foo.c
• clang -c foo.c --rtlib=libgcc --target=aarch64-linux-android -> no warning
• clang -c foo.c --rtlib=libgcc --target=i686-linux-android -> warning: argument unused during compilation: '--rtlib=libgcc' [-Wunused-command-line-argument]

[EugeneZelenko]

Could you please clarify platforms?

[llvmbot]

@llvm/issue-subscribers-clang-driver

[glandium]

Could you please clarify platforms?

See the --targets in the STR.

[MaskRay]

  } else if (Triple.isAArch64() &&
             getToolChain().IsAArch64OutlineAtomicsDefault(Args)) {
    CmdArgs.push_back("-target-feature");
    CmdArgs.push_back("+outline-atomics");
  }

added by [AArch64][Clang][Linux] Enable out-of-line atomics by default. sets the Claimed bit of OPT_rtlib_EQ, so [-Wunused-command-line-argument] is never triggered on AArch64. @ilinpv

I do think -rtlib= affecting code generation is unfortunate. There should be a diagnostic like the one issued for -c --unwindlib= Hope we can remove this when we stop supporting Clang using GCC<9.3.1 runtime. Alternatively, we can ask such users to specify -mno-outline-atomics.