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

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Date:   Thu, 18 Jul 2019 17:01:27 -0400
From:   Qian Cai <cai@....pw>
To:     David Miller <davem@...emloft.net>
Cc:     sathya.perla@...adcom.com, ajit.khaparde@...adcom.com,
        sriharsha.basavapatna@...adcom.com, somnath.kotur@...adcom.com,
        arnd@...db.de, dhowells@...hat.com, hpa@...or.com,
        netdev@...r.kernel.org, linux-arch@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        natechancellor@...il.com
Subject: Re: [PATCH] be2net: fix adapter->big_page_size miscaculation



> 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.