summaryrefslogtreecommitdiffstats
path: root/Python/pyarena.c
blob: ead03370d153c35fa464959a36d7961fc64b18a6 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
#include "Python.h"
#include "pycore_pyarena.h"       // PyArena

/* A simple arena block structure.

   Measurements with standard library modules suggest the average
   allocation is about 20 bytes and that most compiles use a single
   block.

   TODO(jhylton): Think about a realloc API, maybe just for the last
   allocation?
*/

#define DEFAULT_BLOCK_SIZE 8192
#define ALIGNMENT               8

typedef struct _block {
    /* Total number of bytes owned by this block available to pass out.
     * Read-only after initialization.  The first such byte starts at
     * ab_mem.
     */
    size_t ab_size;

    /* Total number of bytes already passed out.  The next byte available
     * to pass out starts at ab_mem + ab_offset.
     */
    size_t ab_offset;

    /* An arena maintains a singly-linked, NULL-terminated list of
     * all blocks owned by the arena.  These are linked via the
     * ab_next member.
     */
    struct _block *ab_next;

    /* Pointer to the first allocatable byte owned by this block.  Read-
     * only after initialization.
     */
    void *ab_mem;
} block;

/* The arena manages two kinds of memory, blocks of raw memory
   and a list of PyObject* pointers.  PyObjects are decrefed
   when the arena is freed.
*/

struct _arena {
    /* Pointer to the first block allocated for the arena, never NULL.
       It is used only to find the first block when the arena is
       being freed.
     */
    block *a_head;

    /* Pointer to the block currently used for allocation.  Its
       ab_next field should be NULL.  If it is not-null after a
       call to block_alloc(), it means a new block has been allocated
       and a_cur should be reset to point it.
     */
    block *a_cur;

    /* A Python list object containing references to all the PyObject
       pointers associated with this arena.  They will be DECREFed
       when the arena is freed.
    */
    PyObject *a_objects;

#if defined(Py_DEBUG)
    /* Debug output */
    size_t total_allocs;
    size_t total_size;
    size_t total_blocks;
    size_t total_block_size;
    size_t total_big_blocks;
#endif
};

static block *
block_new(size_t size)
{
    /* Allocate header and block as one unit.
       ab_mem points just past header. */
    block *b = (block *)PyMem_Malloc(sizeof(block) + size);
    if (!b)
        return NULL;
    b->ab_size = size;
    b->ab_mem = (void *)(b + 1);
    b->ab_next = NULL;
    b->ab_offset = (char *)_Py_ALIGN_UP(b->ab_mem, ALIGNMENT) -
            (char *)(b->ab_mem);
    return b;
}

static void
block_free(block *b) {
    while (b) {
        block *next = b->ab_next;
        PyMem_Free(b);
        b = next;
    }
}

static void *
block_alloc(block *b, size_t size)
{
    void *p;
    assert(b);
    size = _Py_SIZE_ROUND_UP(size, ALIGNMENT);
    if (b->ab_offset + size > b->ab_size) {
        /* If we need to allocate more memory than will fit in
           the default block, allocate a one-off block that is
           exactly the right size. */
        /* TODO(jhylton): Think about space waste at end of block */
        block *newbl = block_new(
                        size < DEFAULT_BLOCK_SIZE ?
                        DEFAULT_BLOCK_SIZE : size);
        if (!newbl)
            return NULL;
        assert(!b->ab_next);
        b->ab_next = newbl;
        b = newbl;
    }

    assert(b->ab_offset + size <= b->ab_size);
    p = (void *)(((char *)b->ab_mem) + b->ab_offset);
    b->ab_offset += size;
    return p;
}

PyArena *
_PyArena_New(void)
{
    PyArena* arena = (PyArena *)PyMem_Malloc(sizeof(PyArena));
    if (!arena)
        return (PyArena*)PyErr_NoMemory();

    arena->a_head = block_new(DEFAULT_BLOCK_SIZE);
    arena->a_cur = arena->a_head;
    if (!arena->a_head) {
        PyMem_Free((void *)arena);
        return (PyArena*)PyErr_NoMemory();
    }
    arena->a_objects = PyList_New(0);
    if (!arena->a_objects) {
        block_free(arena->a_head);
        PyMem_Free((void *)arena);
        return (PyArena*)PyErr_NoMemory();
    }
#if defined(Py_DEBUG)
    arena->total_allocs = 0;
    arena->total_size = 0;
    arena->total_blocks = 1;
    arena->total_block_size = DEFAULT_BLOCK_SIZE;
    arena->total_big_blocks = 0;
#endif
    return arena;
}

void
_PyArena_Free(PyArena *arena)
{
    assert(arena);
#if defined(Py_DEBUG)
    /*
    fprintf(stderr,
        "alloc=%zu size=%zu blocks=%zu block_size=%zu big=%zu objects=%zu\n",
        arena->total_allocs, arena->total_size, arena->total_blocks,
        arena->total_block_size, arena->total_big_blocks,
        PyList_Size(arena->a_objects));
    */
#endif
    block_free(arena->a_head);
    /* This property normally holds, except when the code being compiled
       is sys.getobjects(0), in which case there will be two references.
    assert(arena->a_objects->ob_refcnt == 1);
    */

    Py_DECREF(arena->a_objects);
    PyMem_Free(arena);
}

void *
_PyArena_Malloc(PyArena *arena, size_t size)
{
    void *p = block_alloc(arena->a_cur, size);
    if (!p)
        return PyErr_NoMemory();
#if defined(Py_DEBUG)
    arena->total_allocs++;
    arena->total_size += size;
#endif
    /* Reset cur if we allocated a new block. */
    if (arena->a_cur->ab_next) {
        arena->a_cur = arena->a_cur->ab_next;
#if defined(Py_DEBUG)
        arena->total_blocks++;
        arena->total_block_size += arena->a_cur->ab_size;
        if (arena->a_cur->ab_size > DEFAULT_BLOCK_SIZE)
            ++arena->total_big_blocks;
#endif
    }
    return p;
}

int
_PyArena_AddPyObject(PyArena *arena, PyObject *obj)
{
    int r = PyList_Append(arena->a_objects, obj);
    if (r >= 0) {
        Py_DECREF(obj);
    }
    return r;
}