[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 <[email protected]> wrote: > > On Thu, Jul 18, 2019 at 2:01 PM Qian Cai <[email protected]> wrote: > > > > > > > > > On Jul 12, 2019, at 8:50 PM, David Miller <[email protected]> wrote: > > > > > > From: Qian Cai <[email protected]> > > > 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/[email protected]/ > > > > 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
