bpf: Replace deprecated "-target bpf" with "--target=bpf" for clang

[qmonnet]

Passing the target to clang with -target <name> has been considered deprecated since clang 3.4, released in 2013, in favour of the more recent syntax --target=<name>. Let's update our code and documentation to use the recommended syntax.

This replicates a similar patch submitted to the Linux kernel.

[qmonnet]

/test

[lmb]

FWIW, clang command line args seem like a mess. They mix double dashes, single dashes, space separated, equals separated.

Another point to think about: what kind of flags does gcc accept? Might make it easier to experiment with bpf-gcc in the future if both compilers agree.

[qmonnet]

FWIW, clang command line args seem like a mess. They mix double dashes, single dashes, space separated, equals separated.

@lmb Yep, I noticed :). I don't mind much between the two syntaxes to be honest, I simply thought it would be best to remain in sync with whatever the kernel goes with.

Another point to think about: what kind of flags does gcc accept? Might make it easier to experiment with bpf-gcc in the future if both compilers agree.

That's a good question. So apparently there's no direct equivalent to --target=bpf for GCC, instead you compile your BPF-dedicated version of GCC and call that. See for example the flags used on godbolt.org:

To compare with those used with clang:

(There is a --target=bpf flag mentioned on the reference page for the BPF backend, but it's used for compiling the BPF version of GCC, not to pick BPF with a generic GCC).

GCC does have a number of eBPF-related options though, none being identical to clang's, so the command line invocations won't be interchangeable anyway.

Thanks for the reviews!

[learnitall]

Looks good!