The operand constraint should be "r" since bswap works only on registers. You can also simplify it a little using an input/output operand: asm ("bswap %0"

The operand constraint should be "r" since bswap works only on registers. You can also simplify it a little using an input/output operand: asm ("bswap %0" : "+r"(x)); That said, since version 4.5, gcc recognises the typical byte-swapping pattern and uses the appropriate CPU instruction.