You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: gcwb.go
    path of file: ./repos/gotosocial/vendor/github.com/bytedance/sonic/internal/rt
    
    the code of the file until where you have to start completion: /*
 * Copyright 2021 ByteDance Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package rt

import (
    `os`
    `sync/atomic`
    `unsafe`

    `golang.org/x/arch/x86/x86asm`
)

const (
    _MaxInstr = 15
)

func isvar(arg x86asm.Arg) bool {
    v, ok := arg.(x86asm.Mem)
    return ok && v.Base == x86asm.RIP
}

func iszero(arg x86asm.Arg) bool {
    v, ok :=
    
func GcwbAddr() uintptr {
    var err error
    var off uintptr
    var ins x86asm.Inst

    /* get the function address */
    pc := uintptr(0)
    fp := FuncAddr(atomic.StorePointer)

    /* search within the first 16 instructions */
    for i := 0; i < 16; i++ {
        mem := unsafe.Pointer(uintptr(fp) + pc)
        buf := BytesFrom(mem, _MaxInstr, _MaxInstr)

        /* disassemble the instruction */
        if ins, err = x86asm.Decode(buf, 64); err != nil {
            panic("gcwbaddr: " + err.Error())
        }

        /* check for a byte comparison with zero */
        if ins.Op == x86asm.CMP && ins.MemBytes == 1 && isvar(ins.Args[0]) && iszero(ins.Args[1]) {
            off = pc + uintptr(ins.Len) + uintptr(ins.Args[0].(x86asm.Mem).Disp)
            break
        }

        /* move to next instruction */
        nb := ins.Len
        pc += uintptr(nb)
    }

    /* check for address */
    if off == 0 {
        panic("gcwbaddr: could not locate the variable `writeBarrier`")
    } else {
        return uintptr(fp) + off
    }
}