But in fact compilers do regularly prove such things as, "this function call did...

jart · on May 10, 2021

You don't need to worry about null check removal optimizations unless you do this:

    int main() {
      char *p;
      p = mmap(0, 65536, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
      // ...
      return __builtin_popcountl((uintptr_t)p);
    }

Or you do this:

    void ContinueOnError(int sig, siginfo_t *si, ucontext_t *ctx) {
      xed_decoded_inst_zero_set_mode(&xedd, XED_MACHINE_MODE_LONG_64);
      xed_instruction_length_decode(&xedd, (void *)ctx->uc_mcontext.rip, 15);
      ctx->uc_mcontext.rip += xedd.length;
    }

    int main() {
      signal(SIGSEGV, ContinueOnError);
      volatile long *x = NULL;
      printf("*NULL = %ld\n", *x);
    }

wruza · on May 9, 2021

warning that it removed code based on an assumption that was inferred by the compiler

That would dump a ton of warnings from various macro/meta routines, which real-world C is usually peppered with. Not that it’s particularly hard to do (at the very least compilers know which lines are missing from debug info alone).