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[MC] Add support for -mcpu=native. #159414

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Merged
merged 4 commits into from
Sep 19, 2025
Merged

[MC] Add support for -mcpu=native. #159414

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[MC] Add support for -mcpu=native.
Sep 17, 2025
c0fa4ef
Handle -mcpu=native default Host features.
Sep 18, 2025
54aa0fd
Run clang-format.
Sep 18, 2025
2ef30f7
-mattr processing does not need to be guarded now.
Sep 19, 2025
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3 changes: 3 additions & 0 deletions llvm/test/MC/AsmParser/native.s
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
# RUN: llvm-mc -filetype=obj -mcpu=native %s 2>&1 | FileCheck %s

# CHECK-NOT: 'native' is not a recognized processor for this target (ignoring processor)
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This is too specific for a not check. Better to verify the output is empty

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Looks like FileCheck doesn't like an empty input file (really stderr piped to stdout):

FileCheck error: '<stdin>' is empty.

I could make the CHECK-NOT more narrow. Maybe something like:

# CHECK-NOT: native

Does that seem acceptable, @arsenm?

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I know there's some trick to do this. I ran into this recently but forget what I did

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I guess checking native works though

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--allow-empty

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Updated in latest patch.

4 changes: 4 additions & 0 deletions llvm/tools/llvm-mc/llvm-mc.cpp
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Expand Up @@ -467,6 +467,10 @@ int main(int argc, char **argv) {
FeaturesStr = Features.getString();
}

// Replace -mcpu=native with Host CPU.
if (MCPU == "native")
MCPU = std::string(llvm::sys::getHostCPUName());

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Do we need to pass call getHostCPUFeatures too and pass that information to MAttr?

I know on X86, getHostCPUName can return "haswell" on Pentium CPUs that don't support AVX2, but use the haswell microarchitecture. "haswell" implies AVX2. getHostCPUFeatures will return -avx2 for these CPUs.

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Hm, that's a good point.

It doesn't look like X86 supports -mcpu= in Clang.

I do see that -march= is supported, but Clang just maps to llvm::sys::getHostCPUName(), like this patch, but with the obvious FIXME.

clang/lib/Driver/ToolChains/Arch/X86.cpp:

std::string x86::getX86TargetCPU(const Driver &D, const ArgList &Args,
                                 const llvm::Triple &Triple) {
  if (const Arg *A = Args.getLastArg(clang::driver::options::OPT_march_EQ)) {
    StringRef CPU = A->getValue();
    if (CPU != "native")
      return std::string(CPU);

    // FIXME: Reject attempts to use -march=native unless the target matches
    // the host.
    CPU = llvm::sys::getHostCPUName();
    if (!CPU.empty() && CPU != "generic")
      return std::string(CPU);

I don't have a strong opinion here, but do have a strong opinion that MC needs to support -mcpu=native in some way. So whatever you suggest is fine with me.

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Clang does use getHostCPUFeatures with -mcpu=native. I wrote the code originally. It's here.

void x86::getX86TargetFeatures(const Driver &D, const llvm::Triple &Triple,      
                               const ArgList &Args,                              
                               std::vector<StringRef> &Features) {               
  // Claim and report unsupported -mabi=. Note: we don't support "sysv_abi" or   
  // "ms_abi" as default function attributes.                                    
  if (const Arg *A = Args.getLastArg(clang::driver::options::OPT_mabi_EQ)) {     
    StringRef DefaultAbi =                                                       
        (Triple.isOSWindows() || Triple.isUEFI()) ? "ms" : "sysv";               
    if (A->getValue() != DefaultAbi)                                             
      D.Diag(diag::err_drv_unsupported_opt_for_target)                           
          << A->getSpelling() << Triple.getTriple();                             
  }                                                                              
                                                                                 
  // If -march=native, autodetect the feature list.                              
  if (const Arg *A = Args.getLastArg(clang::driver::options::OPT_march_EQ)) {    
    if (StringRef(A->getValue()) == "native") {                                  
      for (auto &F : llvm::sys::getHostCPUFeatures())                            
        Features.push_back(                                                      
            Args.MakeArgString((F.second ? "+" : "-") + F.first()));             
    }                                                                            
  }

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I do see that -march= is supported, but Clang just maps to llvm::sys::getHostCPUName(), like this patch, but with the obvious FIXME.

I think that fixme is maybe saying we shouldn't call getHostCPU if you explicitly pass --target=x86_64 on a non-x86 machine?

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Clang does use getHostCPUFeatures with -mcpu=native. I wrote the code originally. It's here.

That's -march now. Maybe changed when -mcpu was deprecated on X86.

$ clang test.c -mcpu=native
clang: error: unsupported option '-mcpu=' for target 'x86_64-unknown-linux-gnu'

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Clang does use getHostCPUFeatures with -mcpu=native. I wrote the code originally. It's here.

That's -march now. Maybe changed when -mcpu was deprecated on X86.

$ clang test.c -mcpu=native
clang: error: unsupported option '-mcpu=' for target 'x86_64-unknown-linux-gnu'

Sorry I meant -march=native. For all intents and purpose -march on X86 is -mcpu. I've been on RISC-V for too long now so I my brain thinks -mcpu.

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Latest patch addresses this. I think it's best to allow the command line -mattr's to override the default with native, but I don't feel strongly about it. Thoughts?

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Latest patch addresses this. I think it's best to allow the command line -mattr's to override the default with native, but I don't feel strongly about it. Thoughts?

I agree.

std::unique_ptr<MCSubtargetInfo> STI(
TheTarget->createMCSubtargetInfo(TheTriple, MCPU, FeaturesStr));
assert(STI && "Unable to create subtarget info!");
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