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/* -----------------------------------------------------------------------------
(c) The University of Glasgow 2001
Arena allocation. Arenas provide fast memory allocation at the
expense of fine-grained recycling of storage: memory may be
only be returned to the system by freeing the entire arena, it
isn't possible to return individual objects within an arena.
Do not assume that sequentially allocated objects will be adjacent
in memory.
Quirks: this allocator makes use of the RTS block allocator. If
the current block doesn't have enough room for the requested
object, then a new block is allocated. This means that allocating
large objects will tend to result in wasted space at the end of
each block. In the worst case, half of the allocated space is
wasted. This allocator is therefore best suited to situations in
which most allocations are small.
-------------------------------------------------------------------------- */
#include "Rts.h"
#include "RtsUtils.h"
#include "Arena.h"
#include <stdlib.h>
// Each arena struct is allocated using malloc().
struct _Arena {
bdescr *current;
StgWord *free; // ptr to next free byte in current block
StgWord *lim; // limit (== last free byte + 1)
// We like to keep track of how many blocks we've allocated for
// Storage.c:memInventory().
static long arena_blocks = 0;
// Begin a new arena
Arena *
newArena( void )
Arena *arena;
arena = stgMallocBytes(sizeof(Arena), "newArena");
arena->current = allocBlock_lock();
arena->current->link = NULL;
arena->free = arena->current->start;
arena->lim = arena->current->start + BLOCK_SIZE_W;
return arena;
// The minimum alignment of an allocated block.
#define MIN_ALIGN 8
/* 'n' is assumed to be a power of 2 */
#define ROUNDUP(x,n) (((x)+((n)-1))&(~((n)-1)))
#define B_TO_W(x) ((x) / sizeof(W_))
// Allocate some memory in an arena
void *
arenaAlloc( Arena *arena, size_t size )
void *p;
nat size_w;
nat req_blocks;
bdescr *bd;
// round up to nearest alignment chunk.
size = ROUNDUP(size,MIN_ALIGN);
// size of allocated block in words.
size_w = B_TO_W(size);
if ( arena->free + size_w < arena->lim ) {
// enough room in the current block...
p = arena->free;
arena->free += size_w;
return p;
} else {
// allocate a fresh block...
req_blocks = (lnat)BLOCK_ROUND_UP(size) / BLOCK_SIZE;
bd = allocGroup_lock(req_blocks);
arena_blocks += req_blocks;
bd->gen_no = 0;
bd->step = NULL;
bd->flags = 0;
bd->free = bd->start;
bd->link = arena->current;
arena->current = bd;
arena->free = bd->free + size_w;
arena->lim = bd->free + bd->blocks * BLOCK_SIZE_W;
return bd->start;
// Free an entire arena
arenaFree( Arena *arena )
bdescr *bd, *next;
for (bd = arena->current; bd != NULL; bd = next) {
next = bd->link;
arena_blocks -= bd->blocks;
ASSERT(arena_blocks >= 0);
unsigned long
arenaBlocks( void )
return arena_blocks;
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