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memory.go
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memory.go
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package wasm
import (
"encoding/binary"
"fmt"
"math"
"reflect"
"sync"
"unsafe"
"github.com/tetratelabs/wazero/api"
"github.com/tetratelabs/wazero/internal/internalapi"
)
const (
// MemoryPageSize is the unit of memory length in WebAssembly,
// and is defined as 2^16 = 65536.
// See https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#memory-instances%E2%91%A0
MemoryPageSize = uint32(65536)
// MemoryLimitPages is maximum number of pages defined (2^16).
// See https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#grow-mem
MemoryLimitPages = uint32(65536)
// MemoryPageSizeInBits satisfies the relation: "1 << MemoryPageSizeInBits == MemoryPageSize".
MemoryPageSizeInBits = 16
)
// compile-time check to ensure MemoryInstance implements api.Memory
var _ api.Memory = &MemoryInstance{}
// MemoryInstance represents a memory instance in a store, and implements api.Memory.
//
// Note: In WebAssembly 1.0 (20191205), there may be up to one Memory per store, which means the precise memory is always
// wasm.Store Memories index zero: `store.Memories[0]`
// See https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#memory-instances%E2%91%A0.
type MemoryInstance struct {
internalapi.WazeroOnlyType
Buffer []byte
Min, Cap, Max uint32
// mux is used to prevent overlapping calls to Grow.
mux sync.RWMutex
// definition is known at compile time.
definition api.MemoryDefinition
}
// NewMemoryInstance creates a new instance based on the parameters in the SectionIDMemory.
func NewMemoryInstance(memSec *Memory) *MemoryInstance {
min := MemoryPagesToBytesNum(memSec.Min)
capacity := MemoryPagesToBytesNum(memSec.Cap)
return &MemoryInstance{
Buffer: make([]byte, min, capacity),
Min: memSec.Min,
Cap: memSec.Cap,
Max: memSec.Max,
}
}
// Definition implements the same method as documented on api.Memory.
func (m *MemoryInstance) Definition() api.MemoryDefinition {
return m.definition
}
// Size implements the same method as documented on api.Memory.
func (m *MemoryInstance) Size() uint32 {
return m.size()
}
// ReadByte implements the same method as documented on api.Memory.
func (m *MemoryInstance) ReadByte(offset uint32) (byte, bool) {
if offset >= m.size() {
return 0, false
}
return m.Buffer[offset], true
}
// ReadUint16Le implements the same method as documented on api.Memory.
func (m *MemoryInstance) ReadUint16Le(offset uint32) (uint16, bool) {
if !m.hasSize(offset, 2) {
return 0, false
}
return binary.LittleEndian.Uint16(m.Buffer[offset : offset+2]), true
}
// ReadUint32Le implements the same method as documented on api.Memory.
func (m *MemoryInstance) ReadUint32Le(offset uint32) (uint32, bool) {
return m.readUint32Le(offset)
}
// ReadFloat32Le implements the same method as documented on api.Memory.
func (m *MemoryInstance) ReadFloat32Le(offset uint32) (float32, bool) {
v, ok := m.readUint32Le(offset)
if !ok {
return 0, false
}
return math.Float32frombits(v), true
}
// ReadUint64Le implements the same method as documented on api.Memory.
func (m *MemoryInstance) ReadUint64Le(offset uint32) (uint64, bool) {
return m.readUint64Le(offset)
}
// ReadFloat64Le implements the same method as documented on api.Memory.
func (m *MemoryInstance) ReadFloat64Le(offset uint32) (float64, bool) {
v, ok := m.readUint64Le(offset)
if !ok {
return 0, false
}
return math.Float64frombits(v), true
}
// Read implements the same method as documented on api.Memory.
func (m *MemoryInstance) Read(offset, byteCount uint32) ([]byte, bool) {
if !m.hasSize(offset, uint64(byteCount)) {
return nil, false
}
return m.Buffer[offset : offset+byteCount : offset+byteCount], true
}
// WriteByte implements the same method as documented on api.Memory.
func (m *MemoryInstance) WriteByte(offset uint32, v byte) bool {
if offset >= m.size() {
return false
}
m.Buffer[offset] = v
return true
}
// WriteUint16Le implements the same method as documented on api.Memory.
func (m *MemoryInstance) WriteUint16Le(offset uint32, v uint16) bool {
if !m.hasSize(offset, 2) {
return false
}
binary.LittleEndian.PutUint16(m.Buffer[offset:], v)
return true
}
// WriteUint32Le implements the same method as documented on api.Memory.
func (m *MemoryInstance) WriteUint32Le(offset, v uint32) bool {
return m.writeUint32Le(offset, v)
}
// WriteFloat32Le implements the same method as documented on api.Memory.
func (m *MemoryInstance) WriteFloat32Le(offset uint32, v float32) bool {
return m.writeUint32Le(offset, math.Float32bits(v))
}
// WriteUint64Le implements the same method as documented on api.Memory.
func (m *MemoryInstance) WriteUint64Le(offset uint32, v uint64) bool {
return m.writeUint64Le(offset, v)
}
// WriteFloat64Le implements the same method as documented on api.Memory.
func (m *MemoryInstance) WriteFloat64Le(offset uint32, v float64) bool {
return m.writeUint64Le(offset, math.Float64bits(v))
}
// Write implements the same method as documented on api.Memory.
func (m *MemoryInstance) Write(offset uint32, val []byte) bool {
if !m.hasSize(offset, uint64(len(val))) {
return false
}
copy(m.Buffer[offset:], val)
return true
}
// WriteString implements the same method as documented on api.Memory.
func (m *MemoryInstance) WriteString(offset uint32, val string) bool {
if !m.hasSize(offset, uint64(len(val))) {
return false
}
copy(m.Buffer[offset:], val)
return true
}
// MemoryPagesToBytesNum converts the given pages into the number of bytes contained in these pages.
func MemoryPagesToBytesNum(pages uint32) (bytesNum uint64) {
return uint64(pages) << MemoryPageSizeInBits
}
// Grow implements the same method as documented on api.Memory.
func (m *MemoryInstance) Grow(delta uint32) (result uint32, ok bool) {
// We take write-lock here as the following might result in a new slice
m.mux.Lock()
defer m.mux.Unlock()
currentPages := memoryBytesNumToPages(uint64(len(m.Buffer)))
if delta == 0 {
return currentPages, true
}
// If exceeds the max of memory size, we push -1 according to the spec.
newPages := currentPages + delta
if newPages > m.Max {
return 0, false
} else if newPages > m.Cap { // grow the memory.
m.Buffer = append(m.Buffer, make([]byte, MemoryPagesToBytesNum(delta))...)
m.Cap = newPages
return currentPages, true
} else { // We already have the capacity we need.
sp := (*reflect.SliceHeader)(unsafe.Pointer(&m.Buffer))
sp.Len = int(MemoryPagesToBytesNum(newPages))
return currentPages, true
}
}
// PageSize returns the current memory buffer size in pages.
func (m *MemoryInstance) PageSize() (result uint32) {
return memoryBytesNumToPages(uint64(len(m.Buffer)))
}
// PagesToUnitOfBytes converts the pages to a human-readable form similar to what's specified. e.g. 1 -> "64Ki"
//
// See https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#memory-instances%E2%91%A0
func PagesToUnitOfBytes(pages uint32) string {
k := pages * 64
if k < 1024 {
return fmt.Sprintf("%d Ki", k)
}
m := k / 1024
if m < 1024 {
return fmt.Sprintf("%d Mi", m)
}
g := m / 1024
if g < 1024 {
return fmt.Sprintf("%d Gi", g)
}
return fmt.Sprintf("%d Ti", g/1024)
}
// Below are raw functions used to implement the api.Memory API:
// memoryBytesNumToPages converts the given number of bytes into the number of pages.
func memoryBytesNumToPages(bytesNum uint64) (pages uint32) {
return uint32(bytesNum >> MemoryPageSizeInBits)
}
// size returns the size in bytes of the buffer.
func (m *MemoryInstance) size() uint32 {
return uint32(len(m.Buffer)) // We don't lock here because size can't become smaller.
}
// hasSize returns true if Len is sufficient for byteCount at the given offset.
//
// Note: This is always fine, because memory can grow, but never shrink.
func (m *MemoryInstance) hasSize(offset uint32, byteCount uint64) bool {
return uint64(offset)+byteCount <= uint64(len(m.Buffer)) // uint64 prevents overflow on add
}
// readUint32Le implements ReadUint32Le without using a context. This is extracted as both ints and floats are stored in
// memory as uint32le.
func (m *MemoryInstance) readUint32Le(offset uint32) (uint32, bool) {
if !m.hasSize(offset, 4) {
return 0, false
}
return binary.LittleEndian.Uint32(m.Buffer[offset : offset+4]), true
}
// readUint64Le implements ReadUint64Le without using a context. This is extracted as both ints and floats are stored in
// memory as uint64le.
func (m *MemoryInstance) readUint64Le(offset uint32) (uint64, bool) {
if !m.hasSize(offset, 8) {
return 0, false
}
return binary.LittleEndian.Uint64(m.Buffer[offset : offset+8]), true
}
// writeUint32Le implements WriteUint32Le without using a context. This is extracted as both ints and floats are stored
// in memory as uint32le.
func (m *MemoryInstance) writeUint32Le(offset uint32, v uint32) bool {
if !m.hasSize(offset, 4) {
return false
}
binary.LittleEndian.PutUint32(m.Buffer[offset:], v)
return true
}
// writeUint64Le implements WriteUint64Le without using a context. This is extracted as both ints and floats are stored
// in memory as uint64le.
func (m *MemoryInstance) writeUint64Le(offset uint32, v uint64) bool {
if !m.hasSize(offset, 8) {
return false
}
binary.LittleEndian.PutUint64(m.Buffer[offset:], v)
return true
}