[llvm-bugs] [Bug 49293] New: Clang ignores -march when targeting armv7-apple-darwin

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49293

            Bug ID: 49293
           Summary: Clang ignores -march when targeting armv7-apple-darwin
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Driver
          Assignee: unassignedclangbugs at nondot.org
          Reporter: orivej at gmx.fr
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

This is a low priority bug since it affects only old hardware targets,
but since it exists in the codebase and I've stumbled upon it I'll
report it.

When running "clang --target=armv7-apple-darwin14 -march=armv8.2-a+dotprod"
MachO::TranslateArgs is called with BoundArch "armv7" and appends
"-march=armv7a" to the list of the command line args:
https://github.com/llvm/llvm-project/blob/llvmorg-13-init/clang/lib/Driver/ToolChains/Darwin.cpp#L2380
Then clang -cc1 uses it, silently ignoring the custom -march.

The same issue was encountered in
https://github.com/tensorflow/tensorflow/issues/40026
and worker around in https://github.com/google/XNNPACK/pull/692/files
by adding -mcpu to the command line. (In my case the workaround would
be "-mcpu=cyclone+dotprod".)

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