ref: bd75d8270da313ccbfd06d25799d346803a623bd 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