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// 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)
// GC instruction opcodes.
// The opcode of an instruction is followed by zero or more
// arguments to the instruction.
// Meaning of arguments:
// off Offset (in bytes) from the start of the current object
// objgc Pointer to GC info of an object
// objgcrel Offset to GC info of an object
// len Length of an array
// elemsize Size (in bytes) of an element
// size Size (in bytes)
// NOTE: There is a copy of these in ../reflect/type.go.
// They must be kept in sync.
enum {
GC_END, // End of object, loop or subroutine. Args: none
GC_PTR, // A typed pointer. Args: (off, objgc)
GC_APTR, // Pointer to an arbitrary object. Args: (off)
GC_ARRAY_START, // Start an array with a fixed length. Args: (off, len, elemsize)
GC_ARRAY_NEXT, // The next element of an array. Args: none
GC_CALL, // Call a subroutine. Args: (off, objgcrel)
GC_CHAN_PTR, // Go channel. Args: (off, ChanType*)
GC_STRING, // Go string. Args: (off)
GC_EFACE, // interface{}. Args: (off)
GC_IFACE, // interface{...}. Args: (off)
GC_SLICE, // Go slice. Args: (off, objgc)
GC_REGION, // A region/part of the current object. Args: (off, size, objgc)
GC_NUM_INSTR, // Number of instruction opcodes
enum {
// Size of GC's fixed stack.
// The current GC implementation permits:
// - at most 1 stack allocation because of GC_CALL
// - at most GC_STACK_CAPACITY allocations because of GC_ARRAY_START
enum {
ScanStackByFrames = 1,
IgnorePreciseGC = 0,
// Four bits per word (see #defines below).
wordsPerBitmapWord = sizeof(void*)*8/4,
bitShift = sizeof(void*)*8/4,
// Bits in per-word bitmap.
// #defines because enum might not be able to hold the values.
// 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 bits in the word are packed together by type first, then by
// heap location, so each 64-bit bitmap word consists of, from top to bottom,
// the 16 bitMarked bits for the corresponding heap words,
// then the 16 bitScan/bitBlockBoundary bits, then the 16 bitAllocated bits.
// This layout makes it easier to iterate over the bits of a given type.
// 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.)
// To pull out the bits corresponding to a given pointer p, we use:
// off = p - (uintptr*)mheap.arena_start; // word offset
// b = (uintptr*)mheap.arena_start - off/wordsPerBitmapWord - 1;
// shift = off % wordsPerBitmapWord
// bits = *b >> shift;
// /* then test bits & bitAllocated, bits & bitMarked, etc. */
#define bitAllocated ((uintptr)1<<(bitShift*0)) /* block start; eligible for garbage collection */
#define bitScan ((uintptr)1<<(bitShift*1)) /* when bitAllocated is set */
#define bitMarked ((uintptr)1<<(bitShift*2)) /* when bitAllocated is set */
#define bitBlockBoundary ((uintptr)1<<(bitShift*1)) /* when bitAllocated is NOT set - mark for FlagNoGC objects */
#define bitMask (bitAllocated | bitScan | bitMarked)