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//===------------------------ fallback_malloc.cpp -------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is dual licensed under the MIT and the University of Illinois Open
// Source Licenses. See LICENSE.TXT for details.
//
//
// This file implements the a small heap for allocation of exception
// in the situation where malloc fails.
// http://www.codesourcery.com/public/cxx-abi/abi-eh.html (section 2.4.2)
//
//===----------------------------------------------------------------------===//
// A small, simple heap manager based (loosely) on
// the startup heap manager from FreeBSD, optimized for space.
//
// Manages a fixed-size memory pool, supports malloc and free only.
// No support for realloc.
//
// Allocates chunks in multiples of four bytes, with a four byte header
// for each chunk. The overhead of each chunk is kept low by keeping pointers
// as two byte offsets within the heap, rather than (4 or 8 byte) pointers.
namespace {
static pthread_mutex_t heap_mutex = PTHREAD_MUTEX_INITIALIZER;
class mutexor {
public:
mutexor ( pthread_mutex_t *m ) : mtx_(m) { pthread_mutex_lock ( mtx_ ); }
~mutexor () { pthread_mutex_unlock ( mtx_ ); }
private:
mutexor ( const mutexor &rhs );
mutexor & operator = ( const mutexor &rhs );
pthread_mutex_t *mtx_;
};
#define HEAP_SIZE 512
char heap [ HEAP_SIZE ];
typedef unsigned short heap_offset;
typedef unsigned short heap_size;
struct heap_node {
heap_offset next_node; // offset into heap
heap_size len; // size in units of "sizeof(heap_node)"
};
static const heap_node *list_end = (heap_node *) ( &heap [ HEAP_SIZE ] ); // one past the end of the heap
static heap_node *freelist = NULL;
heap_node *node_from_offset ( const heap_offset offset ) throw()
{ return (heap_node *) ( heap + ( offset * sizeof (heap_node))); }
heap_offset offset_from_node ( const heap_node *ptr ) throw()
{ return (((char *) ptr ) - heap) / sizeof (heap_node); }
void init_heap () throw() {
freelist = (heap_node *) heap;
freelist->next_node = offset_from_node ( list_end );
freelist->len = HEAP_SIZE / sizeof (heap_node);
}
// How big a chunk we allocate
size_t alloc_size (size_t len) throw()
{ return (len + sizeof(heap_node) - 1) / sizeof(heap_node) + 1; }
bool is_fallback_ptr ( void *ptr ) throw()
{ return ptr >= heap && ptr < ( heap + HEAP_SIZE ); }
void *fallback_malloc(size_t len) throw() {
heap_node *p, *prev;
const size_t nelems = alloc_size ( len );
mutexor mtx ( &heap_mutex );
if ( NULL == freelist )
init_heap ();
// Walk the free list, looking for a "big enough" chunk
for (p = freelist, prev = 0;
p && p != list_end; prev = p, p = node_from_offset ( p->next_node)) {
if (p->len > nelems) { // chunk is larger, shorten, and return the tail
heap_node *q;
p->len -= nelems;
q = p + p->len;
q->next_node = 0;
q->len = nelems;
return (void *) (q + 1);
}
if (p->len == nelems) { // exact size match
if (prev == 0)
freelist = node_from_offset(p->next_node);
else
prev->next_node = p->next_node;
p->next_node = 0;
return (void *) (p + 1);
}
}
return NULL; // couldn't find a spot big enough
}
// Return the start of the next block
heap_node *after ( struct heap_node *p ) throw() { return p + p->len; }
void fallback_free (void *ptr) throw() {
struct heap_node *cp = ((struct heap_node *) ptr) - 1; // retrieve the chunk
struct heap_node *p, *prev;
mutexor mtx ( &heap_mutex );
#ifdef DEBUG_FALLBACK_MALLOC
std::cout << "Freeing item at " << offset_from_node ( cp ) << " of size " << cp->len << std::endl;
#endif
for (p = freelist, prev = 0;
p && p != list_end; prev = p, p = node_from_offset (p->next_node)) {
#ifdef DEBUG_FALLBACK_MALLOC
std::cout << " p, cp, after (p), after(cp) "
<< offset_from_node ( p ) << ' '
<< offset_from_node ( cp ) << ' '
<< offset_from_node ( after ( p )) << ' '
<< offset_from_node ( after ( cp )) << std::endl;
#endif
if ( after ( p ) == cp ) {
#ifdef DEBUG_FALLBACK_MALLOC
std::cout << " Appending onto chunk at " << offset_from_node ( p ) << std::endl;
#endif
p->len += cp->len; // make the free heap_node larger
return;
}
else if ( after ( cp ) == p ) { // there's a free heap_node right after
#ifdef DEBUG_FALLBACK_MALLOC
std::cout << " Appending free chunk at " << offset_from_node ( p ) << std::endl;
#endif
cp->len += p->len;
if ( prev == 0 ) {
freelist = cp;
cp->next_node = p->next_node;
}
else
prev->next_node = offset_from_node(cp);
return;
}
}
// Nothing to merge with, add it to the start of the free list
#ifdef DEBUG_FALLBACK_MALLOC
std::cout << " Making new free list entry " << offset_from_node ( cp ) << std::endl;
#endif
cp->next_node = offset_from_node ( freelist );
freelist = cp;
}
#ifdef INSTRUMENT_FALLBACK_MALLOC
size_t print_free_list () {
struct heap_node *p, *prev;
heap_size total_free = 0;
if ( NULL == freelist )
init_heap ();
for (p = freelist, prev = 0;
p && p != list_end; prev = p, p = node_from_offset (p->next_node)) {
std::cout << ( prev == 0 ? "" : " ") << "Offset: " << offset_from_node ( p )
<< "\tsize: " << p->len << " Next: " << p->next_node << std::endl;
total_free += p->len;
}
std::cout << "Total Free space: " << total_free << std::endl;
return total_free;
}
#endif
}
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