mfixalloc.go

Documentation: runtime

     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 manage 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  // bytes remaining in current chunk
    34  	nalloc uint32  // size of new chunks in bytes
    35  	inuse  uintptr // in-use bytes now
    36  	stat   *sysMemStat
    37  	zero   bool // zero allocations
    38  }
    39  
    40  // A generic linked list of blocks.  (Typically the block is bigger than sizeof(MLink).)
    41  // Since assignments to mlink.next will result in a write barrier being performed
    42  // this cannot be used by some of the internal GC structures. For example when
    43  // the sweeper is placing an unmarked object on the free list it does not want the
    44  // write barrier to be called since that could result in the object being reachable.
    45  //
    46  //go:notinheap
    47  type mlink struct {
    48  	next *mlink
    49  }
    50  
    51  // Initialize f to allocate objects of the given size,
    52  // using the allocator to obtain chunks of memory.
    53  func (f *fixalloc) init(size uintptr, first func(arg, p unsafe.Pointer), arg unsafe.Pointer, stat *sysMemStat) {
    54  	if size > _FixAllocChunk {
    55  		throw("runtime: fixalloc size too large")
    56  	}
    57  	if min := unsafe.Sizeof(mlink{}); size < min {
    58  		size = min
    59  	}
    60  
    61  	f.size = size
    62  	f.first = first
    63  	f.arg = arg
    64  	f.list = nil
    65  	f.chunk = 0
    66  	f.nchunk = 0
    67  	f.nalloc = uint32(_FixAllocChunk / size * size) // Round _FixAllocChunk down to an exact multiple of size to eliminate tail waste
    68  	f.inuse = 0
    69  	f.stat = stat
    70  	f.zero = true
    71  }
    72  
    73  func (f *fixalloc) alloc() unsafe.Pointer {
    74  	if f.size == 0 {
    75  		print("runtime: use of FixAlloc_Alloc before FixAlloc_Init\n")
    76  		throw("runtime: internal error")
    77  	}
    78  
    79  	if f.list != nil {
    80  		v := unsafe.Pointer(f.list)
    81  		f.list = f.list.next
    82  		f.inuse += f.size
    83  		if f.zero {
    84  			memclrNoHeapPointers(v, f.size)
    85  		}
    86  		return v
    87  	}
    88  	if uintptr(f.nchunk) < f.size {
    89  		f.chunk = uintptr(persistentalloc(uintptr(f.nalloc), 0, f.stat))
    90  		f.nchunk = f.nalloc
    91  	}
    92  
    93  	v := unsafe.Pointer(f.chunk)
    94  	if f.first != nil {
    95  		f.first(f.arg, v)
    96  	}
    97  	f.chunk = f.chunk + f.size
    98  	f.nchunk -= uint32(f.size)
    99  	f.inuse += f.size
   100  	return v
   101  }
   102  
   103  func (f *fixalloc) free(p unsafe.Pointer) {
   104  	f.inuse -= f.size
   105  	v := (*mlink)(p)
   106  	v.next = f.list
   107  	f.list = v
   108  }
   109
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