Clang should never emit zero-length functions if they may be address-taken

[pcc]

Bugzilla Link	33033
Version	unspecified
OS	Linux
CC	@zmodem,@zygoloid

Extended Description

If the linker places a zero-length function at the same address as another function, function pointer equality may be broken.

$ cat addr-taken.c

#include <stdio.h>
void f1() { __builtin_unreachable(); }
void f2() { __builtin_unreachable(); }
int main() {
  printf("%p %p\n", &f1, &f2);
}

$ clang -Os addr-taken.c
$ ./a.out
0x400510 0x400510

[zmodem]

We ran into a related problem with duplicate function entries in the CFG tables on Windows, see r301040. Perhaps we should enable that code always?

[pcc]

Sounds reasonable to me. We could avoid emitting the noop in functions with the unnamed_addr attribute, but it doesn't seem worth it to me.

[Endilll]

Still reproduces as of post-17 trunk: https://godbolt.org/z/Gj8x7oj1M

[shafik]

This feels related to: #48943

Worth noting gcc does the same opt: https://godbolt.org/z/MazzcYb1T