> On Jul 18, 2019, at 5:21 PM, Bill Wendling <mo...@google.com> wrote:
>
> [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.
I used clang-7.0.1. So, this is getting worse where both GCC and clang will
start to suffer the
same problem.
>
>
> On Thu, Jul 18, 2019 at 2:10 PM Nick Desaulniers
> <ndesaulni...@google.com> wrote:
>>
>> On Thu, Jul 18, 2019 at 2:01 PM Qian Cai <c...@lca.pw> wrote:
>>>
>>>
>>>
>>>> On Jul 12, 2019, at 8:50 PM, David Miller <da...@davemloft.net> wrote:
>>>>
>>>> From: Qian Cai <c...@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-...@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
