| Author: | Wojciech Muła |
|---|---|
| Added on: | 2014-03-09 |
Starting from GCC 4.5 the asm statement has new form: asm goto. A programmer can use any label from a C/C++ program, however a output from this block is not allowed.
Using an asm block in an old form requires more instructions and an 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.