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Date:   Tue, 8 Dec 2020 17:04:29 +0000
From:   Brendan Jackman <jackmanb@google.com>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Yonghong Song <yhs@fb.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        KP Singh <kpsingh@chromium.org>,
        Florent Revest <revest@chromium.org>,
        open list <linux-kernel@vger.kernel.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH bpf-next v3 12/14] bpf: Pull tools/build/feature biz into
 selftests Makefile
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On Mon, Dec 07, 2020 at 06:19:12PM -0800, Andrii Nakryiko wrote:
> On Mon, Dec 7, 2020 at 3:00 AM Brendan Jackman <jackmanb@google.com> wrote:
> >
> > On Fri, Dec 04, 2020 at 11:00:24AM -0800, Andrii Nakryiko wrote:
> > > On Fri, Dec 4, 2020 at 1:41 AM Brendan Jackman <jackmanb@google.com> wrote:
> > > >
> > > > On Thu, Dec 03, 2020 at 01:01:27PM -0800, Andrii Nakryiko wrote:
> > > > > On Thu, Dec 3, 2020 at 8:07 AM Brendan Jackman <jackmanb@google.com> wrote:
> > > > > >
[...]
> >
> > Ah right gotcha. Then yeah I think we can do this:
> >
> >  BPF_ATOMICS_SUPPORTED = $(shell \
> >         echo "int x = 0; int foo(void) { return __sync_val_compare_and_swap(&x, 1, 2); }" \
> >         | $(CLANG) -x cpp-output -S -target bpf -mcpu=v3 - -o /dev/null && echo 1 || echo 0)
> 
> Looks like it would work, yes.
/
> Curious what "-x cpp-output" does?

That's just to tell Clang what language to expect, since it can't infer
it from a file extension:

  $ echo foo | clang -S -
  clang-10: error: -E or -x required when input is from standard input

Yonghong pointed out that we can actually just use `-x c`.