1//===----------------------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include "fallback_malloc.h"
10#include "abort_message.h"
11
12#include <__thread/support.h>
13#ifndef _LIBCXXABI_HAS_NO_THREADS
14#if defined(__ELF__) && defined(_LIBCXXABI_LINK_PTHREAD_LIB)
15#pragma comment(lib, "pthread")
16#endif
17#endif
18
19#include <__memory/aligned_alloc.h>
20#include <__assert>
21#include <stdlib.h> // for malloc, calloc, free
22#include <string.h> // for memset
23
24// A small, simple heap manager based (loosely) on
25// the startup heap manager from FreeBSD, optimized for space.
26//
27// Manages a fixed-size memory pool, supports malloc and free only.
28// No support for realloc.
29//
30// Allocates chunks in multiples of four bytes, with a four byte header
31// for each chunk. The overhead of each chunk is kept low by keeping pointers
32// as two byte offsets within the heap, rather than (4 or 8 byte) pointers.
33
34namespace {
35
36// When POSIX threads are not available, make the mutex operations a nop
37#ifndef _LIBCXXABI_HAS_NO_THREADS
38static _LIBCPP_CONSTINIT std::__libcpp_mutex_t heap_mutex = _LIBCPP_MUTEX_INITIALIZER;
39#else
40static _LIBCPP_CONSTINIT void* heap_mutex = 0;
41#endif
42
43class mutexor {
44public:
45#ifndef _LIBCXXABI_HAS_NO_THREADS
46 mutexor(std::__libcpp_mutex_t* m) : mtx_(m) {
47 std::__libcpp_mutex_lock(mtx_);
48 }
49 ~mutexor() { std::__libcpp_mutex_unlock(mtx_); }
50#else
51 mutexor(void*) {}
52 ~mutexor() {}
53#endif
54private:
55 mutexor(const mutexor& rhs);
56 mutexor& operator=(const mutexor& rhs);
57#ifndef _LIBCXXABI_HAS_NO_THREADS
58 std::__libcpp_mutex_t* mtx_;
59#endif
60};
61
62static const size_t HEAP_SIZE = 512;
63char heap[HEAP_SIZE] __attribute__((aligned));
64
65typedef unsigned short heap_offset;
66typedef unsigned short heap_size;
67
68// On both 64 and 32 bit targets heap_node should have the following properties
69// Size: 4
70// Alignment: 2
71struct heap_node {
72 heap_offset next_node; // offset into heap
73 heap_size len; // size in units of "sizeof(heap_node)"
74};
75
76// All pointers returned by fallback_malloc must be at least aligned
77// as RequiredAligned. Note that RequiredAlignment can be greater than
78// alignof(std::max_align_t) on 64 bit systems compiling 32 bit code.
79struct FallbackMaxAlignType {
80} __attribute__((aligned));
81const size_t RequiredAlignment = alignof(FallbackMaxAlignType);
82
83static_assert(alignof(FallbackMaxAlignType) % sizeof(heap_node) == 0,
84 "The required alignment must be evenly divisible by the sizeof(heap_node)");
85
86// The number of heap_node's that can fit in a chunk of memory with the size
87// of the RequiredAlignment. On 64 bit targets NodesPerAlignment should be 4.
88const size_t NodesPerAlignment = alignof(FallbackMaxAlignType) / sizeof(heap_node);
89
90static const heap_node* list_end =
91 (heap_node*)(&heap[HEAP_SIZE]); // one past the end of the heap
92static heap_node* freelist = NULL;
93
94heap_node* node_from_offset(const heap_offset offset) {
95 return (heap_node*)(heap + (offset * sizeof(heap_node)));
96}
97
98heap_offset offset_from_node(const heap_node* ptr) {
99 return static_cast<heap_offset>(
100 static_cast<size_t>(reinterpret_cast<const char*>(ptr) - heap) /
101 sizeof(heap_node));
102}
103
104// Return a pointer to the first address, 'A', in `heap` that can actually be
105// used to represent a heap_node. 'A' must be aligned so that
106// '(A + sizeof(heap_node)) % RequiredAlignment == 0'. On 64 bit systems this
107// address should be 12 bytes after the first 16 byte boundary.
108heap_node* getFirstAlignedNodeInHeap() {
109 heap_node* node = (heap_node*)heap;
110 const size_t alignNBytesAfterBoundary = RequiredAlignment - sizeof(heap_node);
111 size_t boundaryOffset = reinterpret_cast<size_t>(node) % RequiredAlignment;
112 size_t requiredOffset = alignNBytesAfterBoundary - boundaryOffset;
113 size_t NElemOffset = requiredOffset / sizeof(heap_node);
114 return node + NElemOffset;
115}
116
117void init_heap() {
118 freelist = getFirstAlignedNodeInHeap();
119 freelist->next_node = offset_from_node(ptr: list_end);
120 freelist->len = static_cast<heap_size>(list_end - freelist);
121}
122
123// How big a chunk we allocate
124size_t alloc_size(size_t len) {
125 return (len + sizeof(heap_node) - 1) / sizeof(heap_node) + 1;
126}
127
128bool is_fallback_ptr(void* ptr) {
129 return ptr >= heap && ptr < (heap + HEAP_SIZE);
130}
131
132void* fallback_malloc(size_t len) {
133 heap_node *p, *prev;
134 const size_t nelems = alloc_size(len);
135 mutexor mtx(&heap_mutex);
136
137 if (NULL == freelist)
138 init_heap();
139
140 // Walk the free list, looking for a "big enough" chunk
141 for (p = freelist, prev = 0; p && p != list_end;
142 prev = p, p = node_from_offset(offset: p->next_node)) {
143
144 // Check the invariant that all heap_nodes pointers 'p' are aligned
145 // so that 'p + 1' has an alignment of at least RequiredAlignment
146 _LIBCXXABI_ASSERT(reinterpret_cast<size_t>(p + 1) % RequiredAlignment == 0, "");
147
148 // Calculate the number of extra padding elements needed in order
149 // to split 'p' and create a properly aligned heap_node from the tail
150 // of 'p'. We calculate aligned_nelems such that 'p->len - aligned_nelems'
151 // will be a multiple of NodesPerAlignment.
152 size_t aligned_nelems = nelems;
153 if (p->len > nelems) {
154 heap_size remaining_len = static_cast<heap_size>(p->len - nelems);
155 aligned_nelems += remaining_len % NodesPerAlignment;
156 }
157
158 // chunk is larger and we can create a properly aligned heap_node
159 // from the tail. In this case we shorten 'p' and return the tail.
160 if (p->len > aligned_nelems) {
161 heap_node* q;
162 p->len = static_cast<heap_size>(p->len - aligned_nelems);
163 q = p + p->len;
164 q->next_node = 0;
165 q->len = static_cast<heap_size>(aligned_nelems);
166 void* ptr = q + 1;
167 _LIBCXXABI_ASSERT(reinterpret_cast<size_t>(ptr) % RequiredAlignment == 0, "");
168 return ptr;
169 }
170
171 // The chunk is the exact size or the chunk is larger but not large
172 // enough to split due to alignment constraints.
173 if (p->len >= nelems) {
174 if (prev == 0)
175 freelist = node_from_offset(offset: p->next_node);
176 else
177 prev->next_node = p->next_node;
178 p->next_node = 0;
179 void* ptr = p + 1;
180 _LIBCXXABI_ASSERT(reinterpret_cast<size_t>(ptr) % RequiredAlignment == 0, "");
181 return ptr;
182 }
183 }
184 return NULL; // couldn't find a spot big enough
185}
186
187// Return the start of the next block
188heap_node* after(struct heap_node* p) { return p + p->len; }
189
190void fallback_free(void* ptr) {
191 struct heap_node* cp = ((struct heap_node*)ptr) - 1; // retrieve the chunk
192 struct heap_node *p, *prev;
193
194 mutexor mtx(&heap_mutex);
195
196#ifdef DEBUG_FALLBACK_MALLOC
197 std::printf("Freeing item at %d of size %d\n", offset_from_node(cp), cp->len);
198#endif
199
200 for (p = freelist, prev = 0; p && p != list_end;
201 prev = p, p = node_from_offset(offset: p->next_node)) {
202#ifdef DEBUG_FALLBACK_MALLOC
203 std::printf(" p=%d, cp=%d, after(p)=%d, after(cp)=%d\n",
204 offset_from_node(p), offset_from_node(cp),
205 offset_from_node(after(p)), offset_from_node(after(cp)));
206#endif
207 if (after(p) == cp) {
208#ifdef DEBUG_FALLBACK_MALLOC
209 std::printf(" Appending onto chunk at %d\n", offset_from_node(p));
210#endif
211 p->len = static_cast<heap_size>(
212 p->len + cp->len); // make the free heap_node larger
213 return;
214 } else if (after(p: cp) == p) { // there's a free heap_node right after
215#ifdef DEBUG_FALLBACK_MALLOC
216 std::printf(" Appending free chunk at %d\n", offset_from_node(p));
217#endif
218 cp->len = static_cast<heap_size>(cp->len + p->len);
219 if (prev == 0) {
220 freelist = cp;
221 cp->next_node = p->next_node;
222 } else
223 prev->next_node = offset_from_node(ptr: cp);
224 return;
225 }
226 }
227// Nothing to merge with, add it to the start of the free list
228#ifdef DEBUG_FALLBACK_MALLOC
229 std::printf(" Making new free list entry %d\n", offset_from_node(cp));
230#endif
231 cp->next_node = offset_from_node(ptr: freelist);
232 freelist = cp;
233}
234
235#ifdef INSTRUMENT_FALLBACK_MALLOC
236size_t print_free_list() {
237 struct heap_node *p, *prev;
238 heap_size total_free = 0;
239 if (NULL == freelist)
240 init_heap();
241
242 for (p = freelist, prev = 0; p && p != list_end;
243 prev = p, p = node_from_offset(p->next_node)) {
244 std::printf("%sOffset: %d\tsize: %d Next: %d\n",
245 (prev == 0 ? "" : " "), offset_from_node(p), p->len, p->next_node);
246 total_free += p->len;
247 }
248 std::printf("Total Free space: %d\n", total_free);
249 return total_free;
250}
251#endif
252} // end unnamed namespace
253
254namespace __cxxabiv1 {
255
256struct __attribute__((aligned)) __aligned_type {};
257
258void* __aligned_malloc_with_fallback(size_t size) {
259#if defined(_WIN32)
260 if (void* dest = std::__libcpp_aligned_alloc(alignof(__aligned_type), size))
261 return dest;
262#elif defined(_LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION)
263 if (void* dest = ::malloc(size))
264 return dest;
265#else
266 if (size == 0)
267 size = 1;
268 if (void* dest = std::__libcpp_aligned_alloc(__alignof(__aligned_type), size))
269 return dest;
270#endif
271 return fallback_malloc(len: size);
272}
273
274void* __calloc_with_fallback(size_t count, size_t size) {
275 void* ptr = ::calloc(nmemb: count, size: size);
276 if (NULL != ptr)
277 return ptr;
278 // if calloc fails, fall back to emergency stash
279 ptr = fallback_malloc(len: size * count);
280 if (NULL != ptr)
281 ::memset(ptr, 0, size * count);
282 return ptr;
283}
284
285void __aligned_free_with_fallback(void* ptr) {
286 if (is_fallback_ptr(ptr))
287 fallback_free(ptr);
288 else {
289#if defined(_LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION)
290 ::free(ptr);
291#else
292 std::__libcpp_aligned_free(ptr);
293#endif
294 }
295}
296
297void __free_with_fallback(void* ptr) {
298 if (is_fallback_ptr(ptr))
299 fallback_free(ptr);
300 else
301 ::free(ptr: ptr);
302}
303
304} // namespace __cxxabiv1
305

source code of libcxxabi/src/fallback_malloc.cpp