ref: a1a213fab617a3f238cb2984dd0c905d45904074 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