110 lines
2.6 KiB
Go
110 lines
2.6 KiB
Go
/*
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* Copyright 2017 Dgraph Labs, Inc. and Contributors
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* Modifications copyright (C) 2017 Andy Kimball and Contributors
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package arenaskl
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import (
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"math"
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"sync/atomic"
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"unsafe"
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"github.com/cockroachdb/errors"
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)
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// Arena is lock-free.
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type Arena struct {
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n uint64
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buf []byte
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}
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const (
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align4 = 3
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)
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var (
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// ErrArenaFull indicates that the arena is full and cannot perform any more
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// allocations.
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ErrArenaFull = errors.New("allocation failed because arena is full")
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)
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// NewArena allocates a new arena using the specified buffer as the backing
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// store.
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func NewArena(buf []byte) *Arena {
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// Don't store data at position 0 in order to reserve offset=0 as a kind
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// of nil pointer.
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return &Arena{
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n: 1,
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buf: buf,
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}
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}
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// Size returns the number of bytes allocated by the arena.
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func (a *Arena) Size() uint32 {
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s := atomic.LoadUint64(&a.n)
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if s > math.MaxUint32 {
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// Saturate at MaxUint32.
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return math.MaxUint32
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}
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return uint32(s)
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}
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// Capacity returns the capacity of the arena.
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func (a *Arena) Capacity() uint32 {
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return uint32(len(a.buf))
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}
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func (a *Arena) alloc(size, align, overflow uint32) (uint32, uint32, error) {
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// Verify that the arena isn't already full.
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origSize := atomic.LoadUint64(&a.n)
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if int(origSize) > len(a.buf) {
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return 0, 0, ErrArenaFull
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}
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// Pad the allocation with enough bytes to ensure the requested alignment.
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padded := uint32(size) + align
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newSize := atomic.AddUint64(&a.n, uint64(padded))
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if int(newSize)+int(overflow) > len(a.buf) {
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return 0, 0, ErrArenaFull
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}
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// Return the aligned offset.
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offset := (uint32(newSize) - padded + align) & ^align
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return offset, padded, nil
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}
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func (a *Arena) getBytes(offset uint32, size uint32) []byte {
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if offset == 0 {
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return nil
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}
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return a.buf[offset : offset+size : offset+size]
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}
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func (a *Arena) getPointer(offset uint32) unsafe.Pointer {
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if offset == 0 {
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return nil
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}
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return unsafe.Pointer(&a.buf[offset])
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}
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func (a *Arena) getPointerOffset(ptr unsafe.Pointer) uint32 {
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if ptr == nil {
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return 0
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}
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return uint32(uintptr(ptr) - uintptr(unsafe.Pointer(&a.buf[0])))
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}
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