libgo/go/runtime/mfixalloc.go

   1 // Copyright 2009 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 // Fixed-size object allocator. Returned memory is not zeroed.
   6 //
   7 // See malloc.go for overview.
   8
   9 package runtime
  10
  11 import "unsafe"
  12
  13 // FixAlloc is a simple free-list allocator for fixed size objects.
  14 // Malloc uses a FixAlloc wrapped around sysAlloc to manages its
  15 // MCache and MSpan objects.
  16 //
  17 // Memory returned by fixalloc.alloc is zeroed by default, but the
  18 // caller may take responsibility for zeroing allocations by setting
  19 // the zero flag to false. This is only safe if the memory never
  20 // contains heap pointers.
  21 //
  22 // The caller is responsible for locking around FixAlloc calls.
  23 // Callers can keep state in the object but the first word is
  24 // smashed by freeing and reallocating.
  25 //
  26 // Consider marking fixalloc'd types go:notinheap.
  27 type fixalloc struct {
  28         size   uintptr
  29         first  func(arg, p unsafe.Pointer) // called first time p is returned
  30         arg    unsafe.Pointer
  31         list   *mlink
  32         chunk  uintptr // use uintptr instead of unsafe.Pointer to avoid write barriers
  33         nchunk uint32
  34         inuse  uintptr // in-use bytes now
  35         stat   *uint64
  36         zero   bool // zero allocations
  37 }
  38
  39 // A generic linked list of blocks.  (Typically the block is bigger than sizeof(MLink).)
  40 // Since assignments to mlink.next will result in a write barrier being performed
  41 // this cannot be used by some of the internal GC structures. For example when
  42 // the sweeper is placing an unmarked object on the free list it does not want the
  43 // write barrier to be called since that could result in the object being reachable.
  44 //
  45 //go:notinheap
  46 type mlink struct {
  47         next *mlink
  48 }
  49
  50 // Initialize f to allocate objects of the given size,
  51 // using the allocator to obtain chunks of memory.
  52 func (f *fixalloc) init(size uintptr, first func(arg, p unsafe.Pointer), arg unsafe.Pointer, stat *uint64) {
  53         f.size = size
  54         f.first = first
  55         f.arg = arg
  56         f.list = nil
  57         f.chunk = 0
  58         f.nchunk = 0
  59         f.inuse = 0
  60         f.stat = stat
  61         f.zero = true
  62 }
  63
  64 func (f *fixalloc) alloc() unsafe.Pointer {
  65         if f.size == 0 {
  66                 print("runtime: use of FixAlloc_Alloc before FixAlloc_Init\n")
  67                 throw("runtime: internal error")
  68         }
  69
  70         if f.list != nil {
  71                 v := unsafe.Pointer(f.list)
  72                 f.list = f.list.next
  73                 f.inuse += f.size
  74                 if f.zero {
  75                         memclrNoHeapPointers(v, f.size)
  76                 }
  77                 return v
  78         }
  79         if uintptr(f.nchunk) < f.size {
  80                 f.chunk = uintptr(persistentalloc(_FixAllocChunk, 0, f.stat))
  81                 f.nchunk = _FixAllocChunk
  82         }
  83
  84         v := unsafe.Pointer(f.chunk)
  85         if f.first != nil {
  86                 f.first(f.arg, v)
  87         }
  88         f.chunk = f.chunk + f.size
  89         f.nchunk -= uint32(f.size)
  90         f.inuse += f.size
  91         return v
  92 }
  93
  94 func (f *fixalloc) free(p unsafe.Pointer) {
  95         f.inuse -= f.size
  96         v := (*mlink)(p)
  97         v.next = f.list
  98         f.list = v
  99 }