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//===------------------------ fallback_malloc.ipp -------------------------===//
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// The LLVM Compiler Infrastructure
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// This file is dual licensed under the MIT and the University of Illinois Open
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// Source Licenses. See LICENSE.TXT for details.
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// This file implements the "Exception Handling APIs"
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// http://www.codesourcery.com/public/cxx-abi/abi-eh.html
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//===----------------------------------------------------------------------===//
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// A small, simple heap manager based (loosely) on
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// the startup heap manager from FreeBSD, optimized for space.
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// Manages a fixed-size memory pool, supports malloc and free only.
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// No support for realloc.
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// Allocates chunks in multiples of four bytes, with a four byte header
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// for each chunk. The overhead of each chunk is kept low by keeping pointers
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// as two byte offsets within the heap, rather than (4 or 8 byte) pointers.
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static pthread_mutex_t heap_mutex = PTHREAD_MUTEX_INITIALIZER;
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mutexor ( pthread_mutex_t *m ) : mtx_(m) { pthread_mutex_lock ( mtx_ ); }
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~mutexor () { pthread_mutex_unlock ( mtx_ ); }
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mutexor ( const mutexor &rhs );
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mutexor & operator = ( const mutexor &rhs );
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pthread_mutex_t *mtx_;
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char heap [ HEAP_SIZE ];
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typedef unsigned short heap_offset;
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typedef unsigned short heap_size;
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heap_offset next_node; // offset into heap
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heap_size len; // size in units of "sizeof(heap_node)"
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static const heap_node *list_end = (heap_node *) ( &heap [ HEAP_SIZE ] ); // one past the end of the heap
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static heap_node *freelist = NULL;
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heap_node *node_from_offset ( const heap_offset offset )
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{ return (heap_node *) ( heap + ( offset * sizeof (heap_node))); }
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heap_offset offset_from_node ( const heap_node *ptr )
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{ return static_cast<heap_offset>(static_cast<size_t>(((char *) ptr ) - heap) / sizeof (heap_node)); }
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freelist = (heap_node *) heap;
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freelist->next_node = offset_from_node ( list_end );
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freelist->len = HEAP_SIZE / sizeof (heap_node);
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// How big a chunk we allocate
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size_t alloc_size (size_t len)
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{ return (len + sizeof(heap_node) - 1) / sizeof(heap_node) + 1; }
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bool is_fallback_ptr ( void *ptr )
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{ return ptr >= heap && ptr < ( heap + HEAP_SIZE ); }
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void *fallback_malloc(size_t len) {
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const size_t nelems = alloc_size ( len );
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mutexor mtx ( &heap_mutex );
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if ( NULL == freelist )
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// Walk the free list, looking for a "big enough" chunk
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for (p = freelist, prev = 0;
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p && p != list_end; prev = p, p = node_from_offset ( p->next_node)) {
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if (p->len > nelems) { // chunk is larger, shorten, and return the tail
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q->len = static_cast<heap_size>(nelems);
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return (void *) (q + 1);
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if (p->len == nelems) { // exact size match
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freelist = node_from_offset(p->next_node);
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prev->next_node = p->next_node;
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return (void *) (p + 1);
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return NULL; // couldn't find a spot big enough
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// Return the start of the next block
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heap_node *after ( struct heap_node *p ) { return p + p->len; }
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void fallback_free (void *ptr) {
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struct heap_node *cp = ((struct heap_node *) ptr) - 1; // retrieve the chunk
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struct heap_node *p, *prev;
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mutexor mtx ( &heap_mutex );
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#ifdef DEBUG_FALLBACK_MALLOC
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std::cout << "Freeing item at " << offset_from_node ( cp ) << " of size " << cp->len << std::endl;
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for (p = freelist, prev = 0;
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p && p != list_end; prev = p, p = node_from_offset (p->next_node)) {
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#ifdef DEBUG_FALLBACK_MALLOC
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std::cout << " p, cp, after (p), after(cp) "
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<< offset_from_node ( p ) << ' '
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<< offset_from_node ( cp ) << ' '
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<< offset_from_node ( after ( p )) << ' '
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<< offset_from_node ( after ( cp )) << std::endl;
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if ( after ( p ) == cp ) {
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#ifdef DEBUG_FALLBACK_MALLOC
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std::cout << " Appending onto chunk at " << offset_from_node ( p ) << std::endl;
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p->len += cp->len; // make the free heap_node larger
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else if ( after ( cp ) == p ) { // there's a free heap_node right after
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#ifdef DEBUG_FALLBACK_MALLOC
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std::cout << " Appending free chunk at " << offset_from_node ( p ) << std::endl;
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cp->next_node = p->next_node;
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prev->next_node = offset_from_node(cp);
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// Nothing to merge with, add it to the start of the free list
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#ifdef DEBUG_FALLBACK_MALLOC
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std::cout << " Making new free list entry " << offset_from_node ( cp ) << std::endl;
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cp->next_node = offset_from_node ( freelist );
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#ifdef INSTRUMENT_FALLBACK_MALLOC
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size_t print_free_list () {
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struct heap_node *p, *prev;
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heap_size total_free = 0;
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if ( NULL == freelist )
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for (p = freelist, prev = 0;
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p && p != list_end; prev = p, p = node_from_offset (p->next_node)) {
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std::cout << ( prev == 0 ? "" : " ") << "Offset: " << offset_from_node ( p )
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<< "\tsize: " << p->len << " Next: " << p->next_node << std::endl;
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total_free += p->len;
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std::cout << "Total Free space: " << total_free << std::endl;
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} // end unnamed namespace