ref: c3c49fcd05049a0f89f84ed1a8ce3c7267ef8a8f
dir: /src/plan9_builtins_amd64.s/
TEXT __builtin_ctz(SB),$0 BYTE $0x0F; BYTE $0xBC; BYTE $0xC5 /* BSFL RARG, AX */ RET TEXT __builtin_clz(SB),$0 BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRL RARG, AX */ XORL $31, AX RET TEXT __builtin_clzll(SB),$0 BYTE $0x48; BYTE $0x0F; BYTE $0xBD; BYTE $0xC5 /* BSRQ RARG, AX */ XORL $63, AX RET