On Thu, Jul 18, 2019 at 2:18 PM Bill Wendling <morbo@xxxxxxxxxx> 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@xxxxxxxxxx> wrote:
>>
>> On Thu, Jul 18, 2019 at 2:01 PM Qian Cai <cai@xxxxxx> wrote:
>> >
>> >
>> >
>> > > On Jul 12, 2019, at 8:50 PM, David Miller <davem@xxxxxxxxxxxxx> wrote:
>> > >
>> > > From: Qian Cai <cai@xxxxxx>
>> > > 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@xxxxxx/
>> >
>> > 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
