vm使用了stack.go裡面的物件Stack來作為虛擬機器的堆疊。memory代表了虛擬機器裡面使用的記憶體物件。

stack

比較簡單，就是用1024個big.Int的定長陣列來作為堆疊的儲存。

構造

// stack is an object for basic stack operations. Items popped to the stack are
// expected to be changed and modified. stack does not take care of adding newly
// initialised objects.
type Stack struct {
    data []*big.Int
}

func newstack() *Stack {
    return &Stack{data: make([]*big.Int, 0, 1024)}
}
 

push操作

func (st *Stack) push(d *big.Int) { //追加到最末尾
    // NOTE push limit (1024) is checked in baseCheck
    //stackItem := new(big.Int).Set(d)
    //st.data = append(st.data, stackItem)
    st.data = append(st.data, d)
}
func (st *Stack) pushN(ds ...*big.Int) {
    st.data = append(st.data, ds...)
}

pop操作

func (st *Stack) pop() (ret *big.Int) { //從最末尾取出。
    ret = st.data[len(st.data)-1]
    st.data = st.data[:len(st.data)-1]
    return
}

交換元素的值操作，還有這種操作？

func (st *Stack) swap(n int) { 交換堆疊頂的元素和離棧頂n距離的元素的值。
    st.data[st.len()-n], st.data[st.len()-1] = st.data[st.len()-1], st.data[st.len()-n]
}
 

dup操作 像複製指定位置的值到堆頂

func (st *Stack) dup(pool *intPool, n int) {
    st.push(pool.get().Set(st.data[st.len()-n]))
}

peek 操作. 偷看棧頂元素

func (st *Stack) peek() *big.Int {
    return st.data[st.len()-1]
}

Back 偷看指定位置的元素

// Back returns the n'th item in stack
func (st *Stack) Back(n int) *big.Int {
    return st.data[st.len()-n-1]
}

require 保證堆疊元素的數量要大於等於n.

func (st *Stack) require(n int) error {
    if st.len() < n {
        return fmt.Errorf("stack underflow (%d <=> %d)", len(st.data), n)
    }
    return nil
}

intpool

非常簡單. 就是256大小的 big.int的池,用來加速bit.Int的分配

var checkVal = big.NewInt(-42)

const poolLimit = 256

// intPool is a pool of big integers that
// can be reused for all big.Int operations.
type intPool struct {
    pool *Stack
}

func newIntPool() *intPool {
    return &intPool{pool: newstack()}
}

func (p *intPool) get() *big.Int {
    if p.pool.len() > 0 {
        return p.pool.pop()
    }
    return new(big.Int)
}
func (p *intPool) put(is ...*big.Int) {
    if len(p.pool.data) > poolLimit {
        return
    }

    for _, i := range is {
        // verifyPool is a build flag. Pool verification makes sure the integrity
        // of the integer pool by comparing values to a default value.
        if verifyPool {
            i.Set(checkVal)
        }

        p.pool.push(i)
    }
}

memory

構造, memory的儲存就是byte[]. 還有一個lastGasCost的記錄.

type Memory struct {
    store       []byte
    lastGasCost uint64
}

func NewMemory() *Memory {
    return &Memory{}
}

使用首先需要使用Resize分配空間

// Resize resizes the memory to size
func (m *Memory) Resize(size uint64) {
    if uint64(m.Len()) < size {
        m.store = append(m.store, make([]byte, size-uint64(m.Len()))...)
    }
}

然後使用Set來設定值

// Set sets offset + size to value
func (m *Memory) Set(offset, size uint64, value []byte) {
    // length of store may never be less than offset + size.
    // The store should be resized PRIOR to setting the memory
    if size > uint64(len(m.store)) {
        panic("INVALID memory: store empty")
    }

    // It's possible the offset is greater than 0 and size equals 0. This is because
    // the calcMemSize (common.go) could potentially return 0 when size is zero (NO-OP)
    if size > 0 {
        copy(m.store[offset:offset+size], value)
    }
}

Get來取值, 一個是獲取拷貝, 一個是獲取指標.

// Get returns offset + size as a new slice
func (self *Memory) Get(offset, size int64) (cpy []byte) {
    if size == 0 {
        return nil
    }

    if len(self.store) > int(offset) {
        cpy = make([]byte, size)
        copy(cpy, self.store[offset:offset+size])

        return
    }

    return
}

// GetPtr returns the offset + size
func (self *Memory) GetPtr(offset, size int64) []byte {
    if size == 0 {
        return nil
    }

    if len(self.store) > int(offset) {
        return self.store[offset : offset+size]
    }

    return nil
}

一些額外的幫助函式 在stack_table.go裡面

func makeStackFunc(pop, push int) stackValidationFunc {
    return func(stack *Stack) error {
        if err := stack.require(pop); err != nil {
            return err
        }

        if stack.len()+push-pop > int(params.StackLimit) {
            return fmt.Errorf("stack limit reached %d (%d)", stack.len(), params.StackLimit)
        }
        return nil
    }
}

func makeDupStackFunc(n int) stackValidationFunc {
    return makeStackFunc(n, n+1)
}

func makeSwapStackFunc(n int) stackValidationFunc {
    return makeStackFunc(n, n)
}