libcxxabi/src/fallback_malloc.cpp

   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 <__threading_support>
  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
  34 namespace {
  35
  36 // When POSIX threads are not available, make the mutex operations a nop
  37 #ifndef _LIBCXXABI_HAS_NO_THREADS
  38 static _LIBCPP_CONSTINIT std::__libcpp_mutex_t heap_mutex = _LIBCPP_MUTEX_INITIALIZER;
  39 #else
  40 static _LIBCPP_CONSTINIT void* heap_mutex = 0;
  41 #endif
  42
  43 class mutexor {
  44 public:
  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
  54 private:
  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
  62 static const size_t HEAP_SIZE = 512;
  63 char heap[HEAP_SIZE] __attribute__((aligned));
  64
  65 typedef unsigned short heap_offset;
  66 typedef 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
  71 struct 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.
  79 struct FallbackMaxAlignType {
  80 } __attribute__((aligned));
  81 const size_t RequiredAlignment = alignof(FallbackMaxAlignType);
  82
  83 static_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.
  88 const size_t NodesPerAlignment = alignof(FallbackMaxAlignType) / sizeof(heap_node);
  89
  90 static const heap_node* list_end =
  91     (heap_node*)(&heap[HEAP_SIZE]); // one past the end of the heap
  92 static heap_node* freelist = NULL;
  93
  94 heap_node* node_from_offset(const heap_offset offset) {
  95   return (heap_node*)(heap + (offset * sizeof(heap_node)));
  96 }
  97
  98 heap_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.
 108 heap_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
 117 void init_heap() {
 118   freelist = getFirstAlignedNodeInHeap();
 119   freelist->next_node = offset_from_node(list_end);
 120   freelist->len = static_cast<heap_size>(list_end - freelist);
 121 }
 122
 123 //  How big a chunk we allocate
 124 size_t alloc_size(size_t len) {
 125   return (len + sizeof(heap_node) - 1) / sizeof(heap_node) + 1;
 126 }
 127
 128 bool is_fallback_ptr(void* ptr) {
 129   return ptr >= heap && ptr < (heap + HEAP_SIZE);
 130 }
 131
 132 void* 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(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(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
 188 heap_node* after(struct heap_node* p) { return p + p->len; }
 189
 190 void 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(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(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(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(freelist);
 232   freelist = cp;
 233 }
 234
 235 #ifdef INSTRUMENT_FALLBACK_MALLOC
 236 size_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
 254 namespace __cxxabiv1 {
 255
 256 struct __attribute__((aligned)) __aligned_type {};
 257
 258 void* __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(size);
 272 }
 273
 274 void* __calloc_with_fallback(size_t count, size_t size) {
 275   void* ptr = ::calloc(count, size);
 276   if (NULL != ptr)
 277     return ptr;
 278   // if calloc fails, fall back to emergency stash
 279   ptr = fallback_malloc(size * count);
 280   if (NULL != ptr)
 281     ::memset(ptr, 0, size * count);
 282   return ptr;
 283 }
 284
 285 void __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
 297 void __free_with_fallback(void* ptr) {
 298   if (is_fallback_ptr(ptr))
 299     fallback_free(ptr);
 300   else
 301     ::free(ptr);
 302 }
 303
 304 } // namespace __cxxabiv1