ref: 15b65b11b213ec857a8e39a814c90bcbc3ce85c9 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