ref: fa63d0c70294c1e4a50d81528c59e0f0bb95e95f 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