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Date:   Mon, 6 May 2019 14:35:59 -0700
From:   Jakub Kicinski <jakub.kicinski@netronome.com>
To:     Oleksandr Natalenko <oleksandr@natalenko.name>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        oss-drivers@netronome.com, linux-kernel@vger.kernel.org,
        xdp-newbies@vger.kernel.org, valdis@vt.edu
Subject: Re: netronome/nfp/bpf/jit.c cannot be build with -O3
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On Mon, 06 May 2019 23:24:39 +0200, Oleksandr Natalenko wrote:
> Hi.
> 
> On 06.05.2019 23:00, Jakub Kicinski wrote:
> > Any chance you could try different compiler versions?  The code in
> > question does not look too unusual.  Could you try if removing
> > FIELD_FIT() on line 326 makes a difference?  
> 
> If building with gcc from CentOS 7:
> 
> gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-36)
> 
> the issue is not reproducible.

I just did a make CC=gcc-8 CFLAGS=-O3 with GCC 8.2 here, and doesn't
seem to trigger either.

> Also, commenting out the whole "if" block with FIELD_FIT() prevents the 
> issue from occurring too.

Hm, could it be that GCC 8.3 has some constant propagation bug which
breaks BUILD_BUG_ON()?  My uneducated guess it tries to pull the mask
validation out of FIELD_FIT() and FIELD_PREP() into one place but in
doing so loses some information.