Clang warns for unused Arm -march even when used by preprocessor

[DavidSpickett]

(derived from ClangBuiltLinux/linux#1315)

Summary: clang should not warn that compiler -march arguments are unused when the preprocessor is run on assembly files.

/tmp/test.s is just an empty file. Each time we're going to pass -march to the compiler and assembler with -Wa.

$ ./bin/clang --target=arm-linux-gnueabi -march=armv7-m -Wa,-march=armv7-m -Wunused-command-line-argument /tmp/test.s -c
clang-15: warning: argument unused during compilation: '-march=armv7-m' [-Wunused-command-line-argument]

If we assemble only then we are warned that the compiler -march is unused. So far so good.

$ ./bin/clang --target=arm-linux-gnueabi -march=armv7-m -Wa,-march=armv7-m -Wunused-command-line-argument -x assembler-with-cpp /tmp/test.s -c
clang-15: warning: argument unused during compilation: '-march=armv7-m' [-Wunused-command-line-argument]

If we assemble but apply the preprocessor first we are still warned that it is unused. Which isn't correct because this option actually sets some preprocessor defines (justification for that at the end).

$ ./bin/clang --target=arm-linux-gnueabi -march=armv7-m -Wa,-march=armv7-m -Wunused-command-line-argument /tmp/test.s -E
clang-15: warning: /tmp/test.s: 'assembler' input unused [-Wunused-command-line-argument]
clang-15: warning: argument unused during compilation: '-Wa,-march=armv7-m' [-Wunused-command-line-argument]
clang-15: warning: argument unused during compilation: '-Wunused-command-line-argument' [-Wunused-command-line-argument]

Now if we only preprocess an assembly file there's nothing to do so everything is unused. Fine.

$ ./bin/clang --target=arm-linux-gnueabi -march=armv7-m -Wa,-march=armv7-m -Wunused-command-line-argument -x assembler-with-cpp /tmp/test.s -E
# 1 "/tmp/test.s"
# 1 "<built-in>" 1
# 1 "/tmp/test.s" 2

If we run the preprocessor on the assembly file, everything is used. Part of me says that the -Wa,-march should be unused here. You are running the preprocessor on an assembly file but also we know that only the compiler -march is used to set defines. Not sure.

The key issue is that the compiler -march should be used if you're running the preprocessor. Clang should not be warning you that it is unused.

Since it governs defines like __thumb2__ so clearly it does something. Here we see that removing the compiler -march causes this file to fail to assemble, therefore it is "used". If you put it back clang then tells you it is unused despite clearly being used to add the define.

$ cat /tmp/test2.s
#ifndef __thumb2__
#error
#endif
$ ./bin/clang --target=arm-linux-gnueabi -Wa,-march=armv7-m -Wunused-command-line-argument -x assembler-with-cpp /tmp/test2.s -c
/tmp/test2.s:2:2: error:
#error
 ^
1 error generated.
$ ./bin/clang --target=arm-linux-gnueabi -march=armv7-m -Wa,-march=armv7-m -Wunused-command-line-argument -x assembler-with-cpp /tmp/test2.
s -c
clang-15: warning: argument unused during compilation: '-march=armv7-m' [-Wunused-command-line-argument]

Somehow -march fixes the issue but is also unused.

[llvmbot]

@llvm/issue-subscribers-clang-driver

[llvmbot]

@llvm/issue-subscribers-backend-arm

[DavidSpickett]

The warning is emitted in clang/lib/Driver/ToolChains/Arch/ARM.cpp::void arm::getARMTargetFeatures and this probably applies to -mcpu as well.

  if (WaArch) {
    if (ArchArg)
      D.Diag(clang::diag::warn_drv_unused_argument)
          << ArchArg->getAsString(Args);

Unfortunately it's not as simple as checking if ArchArg has been "claimed" since it's claimed 3/4 times even if it's going to be overriden by -Wa,-march. This can happen when you search for an argument so it doesn't (at least in practice) mean it is used but that someone looked at it.

The spirit of the warning is "hey you gave us two architecture values but we're going to prefer one hopefully it's the right one". So "unused" makes sense as the type of warning.

We do get told if this is for the assembler but it's true either way. We can't tell if this is for the preprocessor and the assembler. I could add that context.

$ ./bin/clang --target=arm-linux-gnueabi -march=armv7-m -Wa,-march=armv7-m -Wunused-command-line-argument /tmp/test.s -c
Warning unused, claimed: 1 ForAS: 1
clang-15: warning: argument unused during compilation: '-march=armv7-m' [-Wunused-command-line-argument]

$ ./bin/clang --target=arm-linux-gnueabi -march=armv7-m -Wa,-march=armv7-m -Wunused-command-line-argument -x assembler-with-cpp /tmp/test.s -c
Warning unused, claimed: 1 ForAS: 1
clang-15: warning: argument unused during compilation: '-march=armv7-m' [-Wunused-command-line-argument]

[nickdesaulniers]

I think that

  if (WaArch) {
    if (ArchArg)
      D.Diag(clang::diag::warn_drv_unused_argument)
          << ArchArg->getAsString(Args);

just needs an additional check for !getLangOpts().AsmPreprocessor.

The spirit of the warning is "hey you gave us two architecture values but we're going to prefer one hopefully it's the right one". So "unused" makes sense as the type of warning.

I guess you could do something odd like -march=armv8 -Wa,-march=armv7, which would be weird for .s, .S, or .c (with inline asm) files...

But I think for the kernel, the versions match. Maybe we could not warn if the versions match?

[nico]

https://reviews.llvm.org/D65233 (and abandoned https://reviews.llvm.org/D65108) were in this area, maybe they can provide inspiration. (Or maybe they can't, I didn't look into this issue very closely. It just looked "vaguely related" to me, and maybe that assessment is just wrong.)

[DavidSpickett]

Similar, thanks for the pointers. I think that issue is assembler vs not assembler where this is assembler vs preprocessor+assembler.

just needs an additional check for !getLangOpts().AsmPreprocessor.

Agreed, if I can find a way to get to that or something like it. Perhaps there is an equivalent hidden within the job object but I haven't found it yet.

But I think for the kernel, the versions match. Maybe we could not warn if the versions match?

As a new warning? Where the compiler flag is not unused, just different to the assembler one. Sounds like a good idea.
(once the current warning is correctly emitted we should only get it when the compiler march is being ignored)

[DavidSpickett]

I'm not going to have time for this for the next few weeks. Anyone else feel free to pick it up.

[simonbcn]

Same problem on Arch when I try to compile mpv with meson using clang: mpv-player/mpv#11242

❯ clang --version
clang version 15.0.7
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin