ref: 439f4f0582f31b81cad4bb4c7a388a5f76080544 dir: /c_builtins_amd64.s/
TEXT __builtin_ctz(SB),1,$0 BSFL RARG, AX RET TEXT __builtin_clz(SB),1,$0 BSRL RARG, AX XORL $31, AX RET TEXT __builtin_clzll(SB),1,$0 BSRQ RARG, AX XORL $63, AX RET