netdev - Re: [PATCH] be2net: fix adapter->big_page

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Date:   Thu, 18 Jul 2019 14:21:29 -0700
From:   Bill Wendling <morbo@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Qian Cai <cai@....pw>, James Y Knight <jyknight@...gle.com>,
        David Miller <davem@...emloft.net>, sathya.perla@...adcom.com,
        ajit.khaparde@...adcom.com, sriharsha.basavapatna@...adcom.com,
        somnath.kotur@...adcom.com, Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, netdev@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: Re: [PATCH] be2net: fix adapter->big_page_size miscaculation

[My previous response was marked as spam...]

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.


On Thu, Jul 18, 2019 at 2:10 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Thu, Jul 18, 2019 at 2:01 PM Qian Cai <cai@....pw> wrote:
> >
> >
> >
> > > On Jul 12, 2019, at 8:50 PM, David Miller <davem@...emloft.net> wrote:
> > >
> > > From: Qian Cai <cai@....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