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/*
* Copyright (c) 2001, 2008, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/
//
// Free block maintenance for Concurrent Mark Sweep Generation
//
// The main data structure for free blocks are
// . an indexed array of small free blocks, and
// . a dictionary of large free blocks
//
// No virtuals in FreeChunk (don't want any vtables).
// A FreeChunk is merely a chunk that can be in a doubly linked list
// and has a size field. NOTE: FreeChunks are distinguished from allocated
// objects in two ways (by the sweeper), depending on whether the VM is 32 or
// 64 bits.
// In 32 bits or 64 bits without CompressedOops, the second word (prev) has the
// LSB set to indicate a free chunk; allocated objects' klass() pointers
// don't have their LSB set. The corresponding bit in the CMSBitMap is
// set when the chunk is allocated. There are also blocks that "look free"
// but are not part of the free list and should not be coalesced into larger
// free blocks. These free blocks have their two LSB's set.
class FreeChunk VALUE_OBJ_CLASS_SPEC {
friend class VMStructs;
// For 64 bit compressed oops, the markOop encodes both the size and the
// indication that this is a FreeChunk and not an object.
volatile size_t _size;
FreeChunk* _prev;
FreeChunk* _next;
markOop mark() const volatile { return (markOop)_size; }
void set_mark(markOop m) { _size = (size_t)m; }
public:
NOT_PRODUCT(static const size_t header_size();)
// Returns "true" if the address indicates that the block represents
// a free chunk.
static bool indicatesFreeChunk(const HeapWord* addr) {
// Force volatile read from addr because value might change between
// calls. We really want the read of _mark and _prev from this pointer
// to be volatile but making the fields volatile causes all sorts of
// compilation errors.
return ((volatile FreeChunk*)addr)->isFree();
}
bool isFree() const volatile {
LP64_ONLY(if (UseCompressedOops) return mark()->is_cms_free_chunk(); else)
return (((intptr_t)_prev) & 0x1) == 0x1;
}
bool cantCoalesce() const {
assert(isFree(), "can't get coalesce bit on not free");
return (((intptr_t)_prev) & 0x2) == 0x2;
}
void dontCoalesce() {
// the block should be free
assert(isFree(), "Should look like a free block");
_prev = (FreeChunk*)(((intptr_t)_prev) | 0x2);
}
FreeChunk* prev() const {
return (FreeChunk*)(((intptr_t)_prev) & ~(0x3));
}
debug_only(void* prev_addr() const { return (void*)&_prev; })
debug_only(void* next_addr() const { return (void*)&_next; })
debug_only(void* size_addr() const { return (void*)&_size; })
size_t size() const volatile {
LP64_ONLY(if (UseCompressedOops) return mark()->get_size(); else )
return _size;
}
void setSize(size_t sz) {
LP64_ONLY(if (UseCompressedOops) set_mark(markOopDesc::set_size_and_free(sz)); else )
_size = sz;
}
FreeChunk* next() const { return _next; }
void linkAfter(FreeChunk* ptr) {
linkNext(ptr);
if (ptr != NULL) ptr->linkPrev(this);
}
void linkAfterNonNull(FreeChunk* ptr) {
assert(ptr != NULL, "precondition violation");
linkNext(ptr);
ptr->linkPrev(this);
}
void linkNext(FreeChunk* ptr) { _next = ptr; }
void linkPrev(FreeChunk* ptr) {
LP64_ONLY(if (UseCompressedOops) _prev = ptr; else)
_prev = (FreeChunk*)((intptr_t)ptr | 0x1);
}
void clearPrev() { _prev = NULL; }
void clearNext() { _next = NULL; }
void markNotFree() {
LP64_ONLY(if (UseCompressedOops) set_mark(markOopDesc::prototype());)
// Also set _prev to null
_prev = NULL;
}
// Return the address past the end of this chunk
HeapWord* end() const { return ((HeapWord*) this) + size(); }
// debugging
void verify() const PRODUCT_RETURN;
void verifyList() const PRODUCT_RETURN;
void mangleAllocated(size_t size) PRODUCT_RETURN;
void mangleFreed(size_t size) PRODUCT_RETURN;
void print_on(outputStream* st);
};
extern size_t MinChunkSize;
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