Optimizing C to know ranges

README.md

Can I hint the optimizer by giving the range of an integer?

Unexpected output from x86-x64 GCC 7 -O3 only fully optimizes func3, proof that compilers still require good programmers.

func(int):
        mov     edx, edi
        imul    edx, edi
        imul    edx, edi
        imul    edx, edx
        mov     eax, edx
        imul    eax, edi
        ret
func2(int):
        cmp     edi, 1
        mov     eax, 1
        je      .L2
        mov     eax, edi
        imul    eax, edi
        imul    eax, edi
        imul    eax, eax
        imul    eax, edi
.L2:
        rep ret
func3(int):
        mov     eax, edi
        ret
func4(unsigned int):
        xor     eax, eax
        test    edi, edi
        setne   al
        ret

test.c

#define assume(cond) do { if (!(cond)) __builtin_unreachable(); } while (0)

int func(int x){
  assume(x == 0 || x == 1);
  return x*x*x*x*x*x*x;
}

int func2(int x){
  if(x == 1)
    return x*x*x*x*x*x*x;
  else
    return x*x*x*x*x*x*x;
}

int func3(int x){
  assume(x == 0 || x == 1);
  if(x == 1)
    return x*x*x*x*x*x*x;
  else
    return x*x*x*x*x*x*x;
}

int func4(unsigned int x){
  if(x == 0)
    return x*x*x*x*x*x*x;
  else if (x == 1)
    return x*x*x*x*x*x*x;
  __builtin_unreachable();
}