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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 manage 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 // bytes remaining in current chunk
nalloc uint32 // size of new chunks in bytes
inuse uintptr // in-use bytes now
stat *sysMemStat
zero bool // zero allocations
}
// A generic linked list of blocks. (Typically the block is bigger than sizeof(MLink).)
// Since assignments to mlink.next 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.
//
//go:notinheap
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 *sysMemStat) {
if size > _FixAllocChunk {
throw("runtime: fixalloc size too large")
}
if min := unsafe.Sizeof(mlink{}); size < min {
size = min
}
f.size = size
f.first = first
f.arg = arg
f.list = nil
f.chunk = 0
f.nchunk = 0
f.nalloc = uint32(_FixAllocChunk / size * size) // Round _FixAllocChunk down to an exact multiple of size to eliminate tail waste
f.inuse = 0
f.stat = stat
f.zero = 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.list.next
f.inuse += f.size
if f.zero {
memclrNoHeapPointers(v, f.size)
}
return v
}
if uintptr(f.nchunk) < f.size {
f.chunk = uintptr(persistentalloc(uintptr(f.nalloc), 0, f.stat))
f.nchunk = f.nalloc
}
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)
v.next = f.list
f.list = v
}
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