Clang Faulty Optimization when Allocating Memory?

Question

The following code

int foobar() {
    int* i = new int;
    return *i;
}

compiles in Clang 5.0.0 on x86-64 with -03 to

foobar(): # @foobar()
ret

(https://godbolt.org/g/uCHVo8)

I understand why it can just return an undefined value (since new'ing it returns an undefined value). But why is clang allowed to optimize away the new operator altogether; does that not change the behaviour of the code?

For example in this case:

static int foobar() {
    int* i = new int;
    return *i;
}

int main(int argc, char** argv) {
    int i = 0;
    try {
        for (;;) i += foobar();
   }
    catch (...) {}
    return i;
}

Clang generates:

main: # @main
.LBB0_1: # =>This Inner Loop Header: Depth=1
  jmp .LBB0_1

GCC on the other hand does more work, but at least it does some alloc (?) in the operator new:

main:
  push rbx
  xor ebx, ebx
.L2:
  mov edi, 4
  call operator new(unsigned long)
  add ebx, DWORD PTR [rax]
  jmp .L2
  mov rdi, rax
  call __cxa_begin_catch
  call __cxa_end_catch
  mov eax, ebx
  pop rbx
  ret

a) Which compiler is right? b) Bug in Clang?

Clang Faulty Optimization when Allocating Memory?

0 Answers0