1 #include "Python.h"
2 #include "pyarena.h"
3
4 /* A simple arena block structure.
5
6 Measurements with standard library modules suggest the average
7 allocation is about 20 bytes and that most compiles use a single
8 block.
9
10 TODO(jhylton): Think about a realloc API, maybe just for the last
11 allocation?
12 */
13
14 #define DEFAULT_BLOCK_SIZE 8192
15 #define ALIGNMENT 8
16 #define ALIGNMENT_MASK (ALIGNMENT - 1)
17 #define ROUNDUP(x) (((x) + ALIGNMENT_MASK) & ~ALIGNMENT_MASK)
18
19 typedef struct _block {
20 /* Total number of bytes owned by this block available to pass out.
21 * Read-only after initialization. The first such byte starts at
22 * ab_mem.
23 */
24 size_t ab_size;
25
26 /* Total number of bytes already passed out. The next byte available
27 * to pass out starts at ab_mem + ab_offset.
28 */
29 size_t ab_offset;
30
31 /* An arena maintains a singly-linked, NULL-terminated list of
32 * all blocks owned by the arena. These are linked via the
33 * ab_next member.
34 */
35 struct _block *ab_next;
36
37 /* Pointer to the first allocatable byte owned by this block. Read-
38 * only after initialization.
39 */
40 void *ab_mem;
41 } block;
42
43 /* The arena manages two kinds of memory, blocks of raw memory
44 and a list of PyObject* pointers. PyObjects are decrefed
45 when the arena is freed.
46 */
47
48 struct _arena {
49 /* Pointer to the first block allocated for the arena, never NULL.
50 It is used only to find the first block when the arena is
51 being freed.
52 */
53 block *a_head;
54
55 /* Pointer to the block currently used for allocation. It's
56 ab_next field should be NULL. If it is not-null after a
57 call to block_alloc(), it means a new block has been allocated
58 and a_cur should be reset to point it.
59 */
60 block *a_cur;
61
62 /* A Python list object containing references to all the PyObject
63 pointers associated with this area. They will be DECREFed
64 when the arena is freed.
65 */
66 PyObject *a_objects;
67
68 #if defined(Py_DEBUG)
69 /* Debug output */
70 size_t total_allocs;
71 size_t total_size;
72 size_t total_blocks;
73 size_t total_block_size;
74 size_t total_big_blocks;
75 #endif
76 };
77
78 static block *
block_new(size_t size)79 block_new(size_t size)
80 {
81 /* Allocate header and block as one unit.
82 ab_mem points just past header. */
83 block *b = (block *)malloc(sizeof(block) + size);
84 if (!b)
85 return NULL;
86 b->ab_size = size;
87 b->ab_mem = (void *)(b + 1);
88 b->ab_next = NULL;
89 b->ab_offset = ROUNDUP((Py_uintptr_t)(b->ab_mem)) -
90 (Py_uintptr_t)(b->ab_mem);
91 return b;
92 }
93
94 static void
block_free(block * b)95 block_free(block *b) {
96 while (b) {
97 block *next = b->ab_next;
98 free(b);
99 b = next;
100 }
101 }
102
103 static void *
block_alloc(block * b,size_t size)104 block_alloc(block *b, size_t size)
105 {
106 void *p;
107 assert(b);
108 size = ROUNDUP(size);
109 if (b->ab_offset + size > b->ab_size) {
110 /* If we need to allocate more memory than will fit in
111 the default block, allocate a one-off block that is
112 exactly the right size. */
113 /* TODO(jhylton): Think about space waste at end of block */
114 block *newbl = block_new(
115 size < DEFAULT_BLOCK_SIZE ?
116 DEFAULT_BLOCK_SIZE : size);
117 if (!newbl)
118 return NULL;
119 assert(!b->ab_next);
120 b->ab_next = newbl;
121 b = newbl;
122 }
123
124 assert(b->ab_offset + size <= b->ab_size);
125 p = (void *)(((char *)b->ab_mem) + b->ab_offset);
126 b->ab_offset += size;
127 return p;
128 }
129
130 PyArena *
PyArena_New()131 PyArena_New()
132 {
133 PyArena* arena = (PyArena *)malloc(sizeof(PyArena));
134 if (!arena)
135 return (PyArena*)PyErr_NoMemory();
136
137 arena->a_head = block_new(DEFAULT_BLOCK_SIZE);
138 arena->a_cur = arena->a_head;
139 if (!arena->a_head) {
140 free((void *)arena);
141 return (PyArena*)PyErr_NoMemory();
142 }
143 arena->a_objects = PyList_New(0);
144 if (!arena->a_objects) {
145 block_free(arena->a_head);
146 free((void *)arena);
147 return (PyArena*)PyErr_NoMemory();
148 }
149 #if defined(Py_DEBUG)
150 arena->total_allocs = 0;
151 arena->total_size = 0;
152 arena->total_blocks = 1;
153 arena->total_block_size = DEFAULT_BLOCK_SIZE;
154 arena->total_big_blocks = 0;
155 #endif
156 return arena;
157 }
158
159 void
PyArena_Free(PyArena * arena)160 PyArena_Free(PyArena *arena)
161 {
162 assert(arena);
163 #if defined(Py_DEBUG)
164 /*
165 fprintf(stderr,
166 "alloc=%d size=%d blocks=%d block_size=%d big=%d objects=%d\n",
167 arena->total_allocs, arena->total_size, arena->total_blocks,
168 arena->total_block_size, arena->total_big_blocks,
169 PyList_Size(arena->a_objects));
170 */
171 #endif
172 block_free(arena->a_head);
173 /* This property normally holds, except when the code being compiled
174 is sys.getobjects(0), in which case there will be two references.
175 assert(arena->a_objects->ob_refcnt == 1);
176 */
177
178 Py_DECREF(arena->a_objects);
179 free(arena);
180 }
181
182 void *
PyArena_Malloc(PyArena * arena,size_t size)183 PyArena_Malloc(PyArena *arena, size_t size)
184 {
185 void *p = block_alloc(arena->a_cur, size);
186 if (!p)
187 return PyErr_NoMemory();
188 #if defined(Py_DEBUG)
189 arena->total_allocs++;
190 arena->total_size += size;
191 #endif
192 /* Reset cur if we allocated a new block. */
193 if (arena->a_cur->ab_next) {
194 arena->a_cur = arena->a_cur->ab_next;
195 #if defined(Py_DEBUG)
196 arena->total_blocks++;
197 arena->total_block_size += arena->a_cur->ab_size;
198 if (arena->a_cur->ab_size > DEFAULT_BLOCK_SIZE)
199 ++arena->total_big_blocks;
200 #endif
201 }
202 return p;
203 }
204
205 int
PyArena_AddPyObject(PyArena * arena,PyObject * obj)206 PyArena_AddPyObject(PyArena *arena, PyObject *obj)
207 {
208 int r = PyList_Append(arena->a_objects, obj);
209 if (r >= 0) {
210 Py_DECREF(obj);
211 }
212 return r;
213 }
214