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 } 100
