On Fri, Dec 9, 2022 at 7:17 PM 刘畅 via Gcc <gcc@gcc.gnu.org> wrote: > > Hi all, > > I met a problem when I was testing the weak attribute and the weakref > attribute of GCC. I've read the documentation and in the 6.33.1 Common > Function Attributes - weakref part I found: > > Without a target given as an argument to weakref or to alias, > weakref is equivalent to weak (in that case the declaration may be > extern). > > To verify this statement, I wrote the following two C programs: > > a.c > #include <stdio.h> > > void func(void) __attribute__((weak)); > > int main() { > if (func) > printf("1\n"); > else > printf("0\n"); > > return 0; > } > > b.c > #include <stdio.h> > > extern void func(void) __attribute__((weakref)); > > int main() { > if (func) > printf("1\n"); > else > printf("0\n"); > > return 0; > } > > The only difference is a.c uses __attribute__((weak)) while b.c uses > __attribute__((weakref)). According to the statement I referred above, > I expect the two programs have the smae behavior. However, after I > compiled the two programs with: > > $ gcc a.c -o a.out; gcc b.c -o b.out > > I got a warning: > > b.c:3:13: warning: ‘weakref’ attribute should be accompanied with an > ‘alias’ attribute [-Wattributes] > 3 | extern void func(void) __attribute__((weakref)); > | > > then I found they have different output: > > $ ./a.out; ./b.out > 0 > 1 > > Then I disassembled the main function of a.out and b.out, and found > the func symbol didn't even appear in the assemble code of b.c (I > recompiled the 2 programs with -g option): > > assemble code of a.c: > 5 int main() { > 0x0000000000001149 <+0>: f3 0f 1e fa endbr64 > 0x000000000000114d <+4>: 55 push %rbp > 0x000000000000114e <+5>: 48 89 e5 mov %rsp,%rbp > > 6 if (func) > 0x0000000000001151 <+8>: 48 8b 05 90 2e 00 00 mov > 0x2e90(%rip),%rax # 0x3fe8 > 0x0000000000001158 <+15>: 48 85 c0 test %rax,%rax > 0x000000000000115b <+18>: 74 0e je 0x116b <main+34> > > 7 printf("1\n"); > 0x000000000000115d <+20>: 48 8d 3d a0 0e 00 00 lea > 0xea0(%rip),%rdi # 0x2004 > 0x0000000000001164 <+27>: e8 e7 fe ff ff callq 0x1050 <puts@plt> > 0x0000000000001169 <+32>: eb 0c jmp 0x1177 <main+46> > > 8 else > 9 printf("0\n"); > 0x000000000000116b <+34>: 48 8d 3d 94 0e 00 00 lea > 0xe94(%rip),%rdi # 0x2006 > 0x0000000000001172 <+41>: e8 d9 fe ff ff callq 0x1050 <puts@plt> > > 10 > 11 return 0; > 0x0000000000001177 <+46>: b8 00 00 00 00 mov $0x0,%eax > > 12 } > 0x000000000000117c <+51>: 5d pop %rbp > 0x000000000000117d <+52>: c3 retq > > assemble code of b.c: > 5 int main() { > 0x0000000000001149 <+0>: f3 0f 1e fa endbr64 > 0x000000000000114d <+4>: 55 push %rbp > 0x000000000000114e <+5>: 48 89 e5 mov %rsp,%rbp > > 6 if (func) > 7 printf("1\n"); > 0x0000000000001151 <+8>: 48 8d 3d ac 0e 00 00 lea > 0xeac(%rip),%rdi # 0x2004 > 0x0000000000001158 <+15>: e8 f3 fe ff ff callq 0x1050 <puts@plt> > > 8 else > 9 printf("0\n"); > 10 > 11 return 0; > 0x000000000000115d <+20>: b8 00 00 00 00 mov $0x0,%eax > > 12 } > 0x0000000000001162 <+25>: 5d pop %rbp > 0x0000000000001163 <+26>: c3 retq > > In my test, the weak attribute and the weakref attribute without a > target given as an argument to weakref or to alias have different > behavior, which is different from the documentation. I don't know if > it's because I misunderstood the documentation. I would be appreciate > if anyone can help me :)
No it is a bug, at least the documentation no longer matches the implementation. I filed https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108042 and even pointed out the (old) revision which changed the behavior to no longer match the documentation. Thanks, Andrew > > Best regards, > > Chang Liu