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// Copyright 2009 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.
// Fixed-size object allocator. Returned memory is not zeroed.
// See malloc.go for overview.
package runtime
import "unsafe"
// FixAlloc is a simple free-list allocator for fixed size objects.
// Malloc uses a FixAlloc wrapped around sysAlloc to manages its
// MCache and MSpan objects.
// Memory returned by fixalloc.alloc is zeroed by default, but the
// caller may take responsibility for zeroing allocations by setting
// the zero flag to false. This is only safe if the memory never
// contains heap pointers.
// The caller is responsible for locking around FixAlloc calls.
// Callers can keep state in the object but the first word is
// smashed by freeing and reallocating.
// Consider marking fixalloc'd types go:notinheap.
type fixalloc struct {
size uintptr
first func(arg, p unsafe.Pointer) // called first time p is returned
arg unsafe.Pointer
list *mlink
chunk uintptr // use uintptr instead of unsafe.Pointer to avoid write barriers
nchunk uint32
inuse uintptr // in-use bytes now
stat *uint64
zero bool // zero allocations
// A generic linked list of blocks. (Typically the block is bigger than sizeof(MLink).)
// Since assignments to will result in a write barrier being performed
// this cannot be used by some of the internal GC structures. For example when
// the sweeper is placing an unmarked object on the free list it does not want the
// write barrier to be called since that could result in the object being reachable.
type mlink struct {
next *mlink
// Initialize f to allocate objects of the given size,
// using the allocator to obtain chunks of memory.
func (f *fixalloc) init(size uintptr, first func(arg, p unsafe.Pointer), arg unsafe.Pointer, stat *uint64) {
f.size = size
f.first = first
f.arg = arg
f.list = nil
f.chunk = 0
f.nchunk = 0
f.inuse = 0
f.stat = stat = true
func (f *fixalloc) alloc() unsafe.Pointer {
if f.size == 0 {
print("runtime: use of FixAlloc_Alloc before FixAlloc_Init\n")
throw("runtime: internal error")
if f.list != nil {
v := unsafe.Pointer(f.list)
f.list =
f.inuse += f.size
if {
memclrNoHeapPointers(v, f.size)
return v
if uintptr(f.nchunk) < f.size {
f.chunk = uintptr(persistentalloc(_FixAllocChunk, 0, f.stat))
f.nchunk = _FixAllocChunk
v := unsafe.Pointer(f.chunk)
if f.first != nil {
f.first(f.arg, v)
f.chunk = f.chunk + f.size
f.nchunk -= uint32(f.size)
f.inuse += f.size
return v
func (f *fixalloc) free(p unsafe.Pointer) {
f.inuse -= f.size
v := (*mlink)(p) = f.list
f.list = v