search indirect tag by binary search

searchtag_amd64.s

#include "textflag.h"

// func searchTag(m *Message, tag int) int
TEXT ·searchTag(SB), NOSPLIT, $0-24
    MOVQ m+0(FP), BX
    MOVQ tag+8(FP), R9
    MOVQ 16(BX), DI
    MOVQ 24(BX), CX
    MOVQ 40(BX), SI
    XORQ DX, DX
    LEAQ -1(CX), CX
loop:
    CMPQ DX, CX
    JG notfound
    LEAQ (DX)(CX*1), BX
    SHRQ $1, AX
    MOVQ (SI)(BX*8), AX
    MOVQ (DI)(AX*8), R8
    CMPQ R9, (R8)
    JZ return
    JL right
    LEAQ 1(BX), DX
    JMP loop
right:
    LEAQ -1(BX), CX
    JMP loop
notfound:
    XORQ AX, AX
    NOTQ AX
    JMP return
return:
    MOVQ AX, ret+16(FP)
    RET