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 (
    12  	"runtime/internal/sys"
    13  	"unsafe"
    14  )
    15  
    16  // fixalloc is a simple free-list allocator for fixed size objects.
    17  // Malloc uses a FixAlloc wrapped around sysAlloc to manage its
    18  // mcache and mspan objects.
    19  //
    20  // Memory returned by fixalloc.alloc is zeroed by default, but the
    21  // caller may take responsibility for zeroing allocations by setting
    22  // the zero flag to false. This is only safe if the memory never
    23  // contains heap pointers.
    24  //
    25  // The caller is responsible for locking around FixAlloc calls.
    26  // Callers can keep state in the object but the first word is
    27  // smashed by freeing and reallocating.
    28  //
    29  // Consider marking fixalloc'd types not in heap by embedding
    30  // runtime/internal/sys.NotInHeap.
    31  type fixalloc struct {
    32  	size   uintptr
    33  	first  func(arg, p unsafe.Pointer) // called first time p is returned
    34  	arg    unsafe.Pointer
    35  	list   *mlink
    36  	chunk  uintptr // use uintptr instead of unsafe.Pointer to avoid write barriers
    37  	nchunk uint32  // bytes remaining in current chunk
    38  	nalloc uint32  // size of new chunks in bytes
    39  	inuse  uintptr // in-use bytes now
    40  	stat   *sysMemStat
    41  	zero   bool // zero allocations
    42  }
    43  
    44  // A generic linked list of blocks.  (Typically the block is bigger than sizeof(MLink).)
    45  // Since assignments to mlink.next will result in a write barrier being performed
    46  // this cannot be used by some of the internal GC structures. For example when
    47  // the sweeper is placing an unmarked object on the free list it does not want the
    48  // write barrier to be called since that could result in the object being reachable.
    49  type mlink struct {
    50  	_    sys.NotInHeap
    51  	next *mlink
    52  }
    53  
    54  // Initialize f to allocate objects of the given size,
    55  // using the allocator to obtain chunks of memory.
    56  func (f *fixalloc) init(size uintptr, first func(arg, p unsafe.Pointer), arg unsafe.Pointer, stat *sysMemStat) {
    57  	if size > _FixAllocChunk {
    58  		throw("runtime: fixalloc size too large")
    59  	}
    60  	if min := unsafe.Sizeof(mlink{}); size < min {
    61  		size = min
    62  	}
    63  
    64  	f.size = size
    65  	f.first = first
    66  	f.arg = arg
    67  	f.list = nil
    68  	f.chunk = 0
    69  	f.nchunk = 0
    70  	f.nalloc = uint32(_FixAllocChunk / size * size) // Round _FixAllocChunk down to an exact multiple of size to eliminate tail waste
    71  	f.inuse = 0
    72  	f.stat = stat
    73  	f.zero = true
    74  }
    75  
    76  func (f *fixalloc) alloc() unsafe.Pointer {
    77  	if f.size == 0 {
    78  		print("runtime: use of FixAlloc_Alloc before FixAlloc_Init\n")
    79  		throw("runtime: internal error")
    80  	}
    81  
    82  	if f.list != nil {
    83  		v := unsafe.Pointer(f.list)
    84  		f.list = f.list.next
    85  		f.inuse += f.size
    86  		if f.zero {
    87  			memclrNoHeapPointers(v, f.size)
    88  		}
    89  		return v
    90  	}
    91  	if uintptr(f.nchunk) < f.size {
    92  		f.chunk = uintptr(persistentalloc(uintptr(f.nalloc), 0, f.stat))
    93  		f.nchunk = f.nalloc
    94  	}
    95  
    96  	v := unsafe.Pointer(f.chunk)
    97  	if f.first != nil {
    98  		f.first(f.arg, v)
    99  	}
   100  	f.chunk = f.chunk + f.size
   101  	f.nchunk -= uint32(f.size)
   102  	f.inuse += f.size
   103  	return v
   104  }
   105  
   106  func (f *fixalloc) free(p unsafe.Pointer) {
   107  	f.inuse -= f.size
   108  	v := (*mlink)(p)
   109  	v.next = f.list
   110  	f.list = v
   111  }
   112
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