asm習作

感想

• 寝癖およびAT&T formatとintel formatの違いで頭がぐしゃぐしゃ。


• divで割られるreigsterがedx:eax固定。恐怖のRISC脳はsrc/dst合計４つを想像する


• test x xとかxor x xとか。恐怖のry

unsigned int
asm_gcd(unsigned int x, unsigned int y)
{
  /* asm memo
   * mnemonic: div
   * to be diveded edx:eax
   * diveded by source: r/m32
   * result
   *   div -> eax
   *   mod -> edx
   */
  /*
    skelton
    div, mod = divmod(x, y)
    divmod(y, mod)
  */
  /*
    implementaion idea (intel format)
    eax, edx = div(edx:eax, ecx)
    mov eax, ecx; # next x(1)
    mov ecx, edx; # next y
    mov edx, 0; # next x(2)
    loop if ecx:
    return eax
  */
  assert(x >= y);
  __asm__ __volatile__(
    "init:               \n\t"
    "  xor %%edx,%%edx   \n\t"
    "while:              \n\t"
    "  div %%ecx         \n\t"
    "  mov %%ecx,%%eax   \n\t"
    "  mov %%edx,%%ecx   \n\t"
    "  xor %%edx,%%edx   \n\t"
    "  cmp %%edx,%%ecx   \n\t"
    "  jne while         \n\t"
    : "=a"(x):"a"(x),"c"(y));
  return x;
}

unsigned int
w_gcd(unsigned int x, unsigned int y)
{
  unsigned int z;
  while(y){
    z = y;
    y = x%y;
    x = z;
  }
  return x;
}

unsigned int
c_gcd(unsigned int x, unsigned int y)
{
  if(y){
    return c_gcd(y, x%y);
  }
  return x;
}

int
main(int argc, char**argv)
{
  assert(c_gcd(1, 1) == 1);
  assert(c_gcd(4, 2) == 2);
  assert(c_gcd(4, 3) == 1);
  assert(c_gcd(12, 3) == 3);
  assert(w_gcd(1, 1) == 1);
  assert(w_gcd(4, 2) == 2);
  assert(w_gcd(4, 3) == 1);
  assert(w_gcd(12, 3) == 3);
  assert(asm_gcd(1, 1) == 1);
  assert(asm_gcd(4, 2) == 2);
  assert(asm_gcd(4, 3) == 1);
  assert(asm_gcd(12, 3) == 3);
  assert(asm_gcd(15, 3) == 3);
  assert(asm_gcd(144, 64) == 16);
  return 0;
}

ちなみに-O2でコンパイルするとw_gcdもc_gcdは同一のasmに落ちて、手で組んだコードとほぼ同じになる。末尾最適化とレジスタ割付の最適化はちゃんと行われているようだ

.globl w_gcd
  .type w_gcd, @function
w_gcd:
.LFB28:
  test  %esi, %esi
  mov %eax, %edi
  jne .L7
  jmp .L2
  .p2align 4,,7
.L9:
  mov %esi, %edx
.L7:
  xor %edx, %edx
  div %esi
  mov %eax, %esi
  test  %edx, %edx
  jne .L9
  mov %eax, %esi
.L2:
  rep  ret

.globl c_gcd
  .type c_gcd, @function
c_gcd:
.LFB29:
  test  %esi, %esi
  mov %eax, %edi
  jne .L17
  jmp .L12
  .p2align 4,,7
.L18:
  mov %esi, %edx
.L17:
  xor %edx, %edx
  div %esi
  mov %eax, %esi
  test  %edx, %edx
  jne .L18
  mov %eax, %esi
.L12:
  rep  ret