netdev - Re: [PATCH bpf] selftests/bpf: adjust strobemeta loop to satisfy latest clang

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Message-ID: <CAEf4BzZREUDrjKKcsWjjTEjwV9PSXtHvb-+DB5Gs4uTk-05Pgg@mail.gmail.com>
Date:   Wed, 25 Sep 2019 13:35:14 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf] selftests/bpf: adjust strobemeta loop to satisfy
 latest clang

On Wed, Sep 25, 2019 at 1:33 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On Wed, Sep 25, 2019 at 11:52:05AM -0700, Andrii Nakryiko wrote:
> > Some recent changes in latest Clang started causing the following
> > warning when unrolling strobemeta test case main loop:
> >
> >   progs/strobemeta.h:416:2: warning: loop not unrolled: the optimizer was
> >   unable to perform the requested transformation; the transformation might
> >   be disabled or specified as part of an unsupported transformation
> >   ordering [-Wpass-failed=transform-warning]
> >
> > This patch simplifies loop's exit condition to depend only on constant
> > max iteration number (STROBE_MAX_MAP_ENTRIES), while moving early
> > termination logic inside the loop body. The changes are equivalent from
> > program logic standpoint, but fixes the warning. It also appears to
> > improve generated BPF code, as it fixes previously failing non-unrolled
> > strobemeta test cases.
> >
> > Cc: Alexei Starovoitov <ast@...com>
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
>
> Sounds like a clang regression? Was that from an official release?

It does, but I didn't dig deep enough to figure out what exactly
caused this. The version I used was latest Clang 10 built from
sources. Might be worth-while to investigate this further to prevent
some other unexpected breakages for user programs.

>
> Applied.

Thanks!