src/runtime/mgc1.go

// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Garbage collector (GC)

package runtime

const (
	// Four bits per word (see #defines below).
	gcBits             = 4
	wordsPerBitmapByte = 8 / gcBits
)

const (
	// GC type info programs.
	// The programs allow to store type info required for GC in a compact form.
	// Most importantly arrays take O(1) space instead of O(n).
	// The program grammar is:
	//
	// Program = {Block} "insEnd"
	// Block = Data | Array
	// Data = "insData" DataSize DataBlock
	// DataSize = int // size of the DataBlock in bit pairs, 1 byte
	// DataBlock = binary // dense GC mask (2 bits per word) of size ]DataSize/4[ bytes
	// Array = "insArray" ArrayLen Block "insArrayEnd"
	// ArrayLen = int // length of the array, 8 bytes (4 bytes for 32-bit arch)
	//
	// Each instruction (insData, insArray, etc) is 1 byte.
	// For example, for type struct { x []byte; y [20]struct{ z int; w *byte }; }
	// the program looks as:
	//
	// insData 3 (BitsPointer BitsScalar BitsScalar)
	//	insArray 20 insData 2 (BitsScalar BitsPointer) insArrayEnd insEnd
	//
	// Total size of the program is 17 bytes (13 bytes on 32-bits).
	// The corresponding GC mask would take 43 bytes (it would be repeated
	// because the type has odd number of words).
	insData = 1 + iota
	insArray
	insArrayEnd
	insEnd
)

const (
	// Pointer map
	_BitsPerPointer  = 2
	_BitsMask        = (1 << _BitsPerPointer) - 1
	_PointersPerByte = 8 / _BitsPerPointer

	// If you change these, also change scanblock.
	// scanblock does "if(bits == BitsScalar || bits == BitsDead)" as "if(bits <= BitsScalar)".
	_BitsDead          = 0
	_BitsScalar        = 1                                // 01
	_BitsPointer       = 2                                // 10
	_BitsCheckMarkXor  = 1                                // 10
	_BitsScalarMarked  = _BitsScalar ^ _BitsCheckMarkXor  // 00
	_BitsPointerMarked = _BitsPointer ^ _BitsCheckMarkXor // 11

	// 64 bytes cover objects of size 1024/512 on 64/32 bits, respectively.
	_MaxGCMask = 65536 // TODO(rsc): change back to 64
)

// Bits in per-word bitmap.
// #defines because we shift the values beyond 32 bits.
//
// Each word in the bitmap describes wordsPerBitmapWord words
// of heap memory.  There are 4 bitmap bits dedicated to each heap word,
// so on a 64-bit system there is one bitmap word per 16 heap words.
//
// The bitmap starts at mheap.arena_start and extends *backward* from
// there.  On a 64-bit system the off'th word in the arena is tracked by
// the off/16+1'th word before mheap.arena_start.  (On a 32-bit system,
// the only difference is that the divisor is 8.)
const (
	bitBoundary = 1 // boundary of an object
	bitMarked   = 2 // marked object
	bitMask     = bitBoundary | bitMarked
	bitPtrMask  = _BitsMask << 2
)