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From:   Bill Wendling <morbo@google.com>
Date:   Thu, 18 Jul 2019 14:28:39 -0700
Message-ID: <CAGG=3QXGqOkfTXC6LRqALJkWaX1L_nnYs3+1xXeojqKF3kftbw@mail.gmail.com>
Subject: Re: [PATCH] be2net: fix adapter->big_page_size miscaculation
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Qian Cai <cai@lca.pw>, James Y Knight <jyknight@google.com>,
        David Miller <davem@davemloft.net>, sathya.perla@broadcom.com,
        ajit.khaparde@broadcom.com, sriharsha.basavapatna@broadcom.com,
        somnath.kotur@broadcom.com, Arnd Bergmann <arnd@arndb.de>,
        David Howells <dhowells@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, netdev@vger.kernel.org,
        linux-arch <linux-arch@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Nathan Chancellor <natechancellor@gmail.com>
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Possibly. I'd need to ask him. :-)

On Thu, Jul 18, 2019 at 2:22 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Thu, Jul 18, 2019 at 2:18 PM Bill Wendling <morbo@google.com> wrote:
> >
> > Top-of-tree clang says that it's const:
> >
> > $ gcc a.c -O2 && ./a.out
> > a is a const.
> >
> > $ clang a.c -O2 && ./a.out
> > a is a const.
>
> Right, so I know you (Bill) did a lot of work to refactor
> __builtin_constant_p handling in Clang and LLVM in the
> pre-llvm-9-release timeframe.  I suspect Qian might not be using
> clang-9 built from source (as clang-8 is the current release) and thus
> observing differences.
>
> >
> > On Thu, Jul 18, 2019 at 2:10 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
> >>
> >> On Thu, Jul 18, 2019 at 2:01 PM Qian Cai <cai@lca.pw> wrote:
> >> >
> >> >
> >> >
> >> > > On Jul 12, 2019, at 8:50 PM, David Miller <davem@davemloft.net> wrote:
> >> > >
> >> > > From: Qian Cai <cai@lca.pw>
> >> > > Date: Fri, 12 Jul 2019 20:27:09 -0400
> >> > >
> >> > >> Actually, GCC would consider it a const with -O2 optimized level because it found that it was never modified and it does not understand it is a module parameter. Considering the following code.
> >> > >>
> >> > >> # cat const.c
> >> > >> #include <stdio.h>
> >> > >>
> >> > >> static int a = 1;
> >> > >>
> >> > >> int main(void)
> >> > >> {
> >> > >>      if (__builtin_constant_p(a))
> >> > >>              printf("a is a const.\n");
> >> > >>
> >> > >>      return 0;
> >> > >> }
> >> > >>
> >> > >> # gcc -O2 const.c -o const
> >> > >
> >> > > That's not a complete test case, and with a proper test case that
> >> > > shows the externalization of the address of &a done by the module
> >> > > parameter macros, gcc should not make this optimization or we should
> >> > > define the module parameter macros in a way that makes this properly
> >> > > clear to the compiler.
> >> > >
> >> > > It makes no sense to hack around this locally in drivers and other
> >> > > modules.
> >> >
> >> > If you see the warning in the original patch,
> >> >
> >> > https://lore.kernel.org/netdev/1562959401-19815-1-git-send-email-cai@lca.pw/
> >> >
> >> > GCC definitely optimize rx_frag_size  to be a constant while I just confirmed clang
> >> > -O2 does not. The problem is that I have no clue about how to let GCC not to
> >> > optimize a module parameter.
> >> >
> >> > Though, I have added a few people who might know more of compilers than myself.
> >>
> >> + Bill and James, who probably knows more than they'd like to about
> >> __builtin_constant_p and more than other LLVM folks at this point.
> >>
> >> --
> >> Thanks,
> >> ~Nick Desaulniers
>
>
>
> --
> Thanks,
> ~Nick Desaulniers