GCC - asm goto

Starting from GCC 4.5 the asm statement has new form: asm goto. Programmer can use any labels from C/C++ program, however output from this block is not allowed.

Using asm block in old form required more instructions and additional storage:

 uint32_t bit_set;
 asm (
  "bt %2, %%eax  \n"
  "setc %%al  \n"
  "movzx %%al, %%eax \n"
  : "=a" (bit_set)
  : "a" (number), "r" (bit)
  : "cc"
 );

 if (bit_set)
  goto has_bit;

Above code is compiled to:

 80483f6:       0f a3 d8                bt     %ebx,%eax
 80483f9:       0f 92 c0                setb   %al
 80483fc:       0f b6 c0                movzbl %al,%eax
 80483ff:       85 c0                   test   %eax,%eax
 8048401:       74 16                   je     8048419 

With asm goto the same task could be writting shorter and easier:

 asm goto (
  "bt %1, %0 \n"
  "jc %l[has_bit] \n"

  : /* no output */
  : "r" (number), "r" (bit)
  : "cc"
  : has_bit // name of label
 );

Complete demo is available. See also GCC documentation about Extended Asm.