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172 lines
5.6 KiB
172 lines
5.6 KiB
/*
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* Yet Another Malloc
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* ya_block.c
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* Defines operations on blocks and boundary tags
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*/
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/*----------*/
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/* Includes */
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/*----------*/
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#include "ya_block.h"
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#include "ya_debug.h"
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/*---------*/
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/* Globals */
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/*---------*/
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intptr_t *heap_start = NULL; // with space for 2 words before
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intptr_t *heap_end = NULL; // first block outside heap
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/*----------------------*/
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/* Function definitions */
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/*----------------------*/
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#ifdef YA_DEBUG
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void block_print_range(intptr_t *start, intptr_t *end) {
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intptr_t *block;
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intptr_t block_size;
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for (block = start; block < end; block += block_size) {
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block_size = YA_SZ_BLK(block);
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printf("Block at %p size = %ld alloc = %d\n",
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block, block_size, YA_IS_ALLOC_BLK(block));
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}
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}
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#endif
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/* Initializes the block's boundary tags. */
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void block_init(intptr_t *block, intptr_t size) {
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block[-1] = size;
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block[size - 2] = size;
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}
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/* Sets the allocated bit in the block's boundary tags. */
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void block_alloc(intptr_t *block) {
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intptr_t block_size = YA_SZ_BLK(block);
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block[-1] |= 1;
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block[block_size-2] |= 1;
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}
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/* Erases the allocated bit in the block's boundary tags. */
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void block_free(intptr_t *block) {
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intptr_t block_size = YA_SZ_BLK(block);
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block[-1] &= -2;
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block[block_size-2] &= -2;
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}
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/* Returns the size in words of the smallest block that can
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* store n_bytes bytes. Takes alignment and boundary tags into account */
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intptr_t block_fit(size_t n_bytes) {
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intptr_t n_words = YA_ROUND_DIV(n_bytes, YA_SZ_WORD); // size in words
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// round to dword and make space for tags
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intptr_t size = 2 + YA_ROUND(n_words, 2);
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ya_debug("block_fit: requested = %ld, allocating = %ld * %ld = %ld\n",
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n_bytes, size, YA_SZ_WORD, size * YA_SZ_WORD);
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return size;
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}
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/* Tries to coalesce a block with its previous neighbor.
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* Returns a pointer to the coalesced block. */
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intptr_t *block_join_prev(intptr_t *block) {
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intptr_t prev_size = YA_SZ_TAG(block[-2]);
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intptr_t *prev = block - prev_size;
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if (prev <= heap_start || YA_IS_ALLOC_BLK(prev)) {
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return block;
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}
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intptr_t block_size = YA_SZ_BLK(block);
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block_init(prev, prev_size + block_size);
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ya_debug("block_join_prev: joining %p:%ld and %p:%ld -> %p:%ld\n",
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block, block_size, prev, prev_size, prev, prev_size + block_size);
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return prev;
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}
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/* Tries to colesce a block with its next neighbor.
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* Returns the unchanged pointer to the block. */
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intptr_t *block_join_next(intptr_t *block) {
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intptr_t block_size = YA_SZ_BLK(block);
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intptr_t *next = block + block_size;
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if (next >= heap_end || YA_IS_ALLOC_BLK(next)) {
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return block;
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}
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intptr_t next_size = YA_SZ_BLK(next);
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block_init(block, block_size + next_size);
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ya_debug("block_join_next: joining %p:%ld and %p:%ld -> %p:%ld\n",
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block, block_size, next, next_size, block, block_size + next_size);
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return block;
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}
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/* Tries to coalesce a block with its previous and next neighbors.
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* Returns a pointer to the coalesced block. */
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intptr_t *block_join(intptr_t *block) {
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if (block > heap_start) {
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block = block_join_prev(block);
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}
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return block_join_next(block);
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}
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/* Split the block [block_size] into [size, block_size - size] if possible
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* Returns a pointer to the second block or NULL if no split occurred. */
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intptr_t *block_split(intptr_t *block, intptr_t size) {
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intptr_t next_size = YA_SZ_BLK(block) - size;
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if (next_size < YA_MIN_SZ_BLK) {
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return NULL; // not enough space to warrant a split
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}
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block_init(block, size);
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block_init(block + size, next_size);
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return block + size;
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}
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/* Try to find a free block at least size words big by walking the boundary
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* tags. If no block is found the heap is grown adequately.
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* Returns a pointer to the block or NULL in case of failure. */
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intptr_t *block_find(intptr_t size) {
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intptr_t *block;
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intptr_t block_size;
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for (block = heap_start; block < heap_end; block += block_size) {
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block_size = YA_SZ_BLK(block);
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if (!YA_IS_ALLOC_BLK(block) && size <= block_size) {
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return block;
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}
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}
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// could not find block, extend heap
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return heap_extend(size);
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}
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/* Initializes the heap by calling sbrk to allocate some starter memory.
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* Sets heap_start and heap_end to their appropriate values.
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* Returns the pointer to the start of the heap or NULL in case of failure. */
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intptr_t *heap_init() {
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intptr_t size_w = YA_SZ_CHUNK / YA_SZ_WORD;
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void *ptr = sbrk(YA_SZ_WORD * (size_w + 2));
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if (ptr == (void*) -1) {
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heap_start = NULL;
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heap_end = NULL;
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return NULL;
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}
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heap_start = ptr; // cast to intptr_t *
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heap_start += 2; // space for the first block[-1] + dword alignment
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heap_end = heap_start + size_w;
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block_init(heap_start, size_w);
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ya_debug("heap_init: start = %p, end = %p, size_w = %ld\n",
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heap_start, heap_end, size_w);
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return heap_start;
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}
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/* Extends the heap by at least size_w words by calling sbrk.
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* Returns a pointer to the last (free) block or NULL in case of failure. */
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intptr_t *heap_extend(intptr_t size_w) {
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intptr_t size = YA_ROUND(size_w * YA_SZ_WORD, YA_SZ_CHUNK);
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size_w = size / YA_SZ_WORD;
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ya_debug("heap_extend: size_w = %ld\n", size_w);
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void *ptr = sbrk(size);
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if (ptr == (void *) - 1) {
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return NULL;
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}
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intptr_t *block = ptr; // == old heap_end
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heap_end = block + size_w;
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block_init(block, size_w);
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ya_debug("heap_extend: old end = %p, new end = %p, size_w = %ld\n",
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block, heap_end, size_w);
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ya_print_blocks();
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return block_join(ptr);
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}
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