Is -march/-mtune=native actually supported on FreeBSD arm* or aarch64?

[Dimitry Andric]

On 5 Jan 2018, at 06:52, Jan Beich <jbeich at FreeBSD.org> wrote:
> 
> Some ports pass -march=native and/or -mtune=native. Both are extensively
> documented by GCC for x86. For other architectures some excerpts say
> "native" is only supported on Linux (via /proc/cpuinfo). For example,
> 
>  $ uname -p
>  armv6
>  $ echo 'int main() {}' >a.c
>  $ clang -march=native a.c
>  clang: error: the clang compiler does not support '-march=native'
>  $ clang -mtune=native a.c
>  $ pkg install -qy gcc7
>  $ gcc7 -march=native a.c
>  $ gcc7 -mtune=native a.c
> 
>  $ uname -p
>  aarch64
>  $ echo 'int main() {}' >a.c
>  $ clang -march=native a.c
>  clang: error: the clang compiler does not support '-march=native'
>  $ clang -mtune=native a.c
>  clang: error: the clang compiler does not support '-mtune=native'
>  $ pkg install -qy gcc7
>  $ gcc7 -march=native a.c
>  $ gcc7 -mtune=native a.c
> 
> What's the rationale for Clang vs. GCC difference? Is it safe to run
> FreeBSD arm* -mtune=native binaries on CPUs older than build machine?

On non-x86, llvm has not implemented CPU detection, as far as I know.
And since we don't export any CPU identification sysctl or even a
/proc/cpuinfo, getting that information from userland is tricky.  (I've
seen programs that attempt to parse /var/run/dmesg.boot, but it is
very error-prone, obviously.)

It is safer to just specify the target CPU explicitly, IMO.

-Dimitry

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