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Optimize arena_alloc for the fast path

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gingerBill
2026-06-09 16:05:29 +01:00
parent bcff6688a1
commit e6334c65c3
1 changed files with 29 additions and 35 deletions
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64
src/common_memory.cpp
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@@ -80,16 +80,6 @@ gb_internal void virtual_memory_dealloc(MemoryBlock *block);
gb_internal void *arena_alloc(Arena *arena, isize min_size, isize alignment);
gb_internal void arena_free_all(Arena *arena);
gb_internal isize arena_align_forward_offset(Arena *arena, isize alignment) {
isize alignment_offset = 0;
isize ptr = cast(isize)(arena->curr_block->base + arena->curr_block->used);
isize mask = alignment-1;
if (ptr & mask) {
alignment_offset = alignment - (ptr & mask);
}
return alignment_offset;
}
gb_internal void thread_init_arenas(Thread *t) {
t->permanent_arena = gb_alloc_item(heap_allocator(), Arena);
t->temporary_arena = gb_alloc_item(heap_allocator(), Arena);
@@ -102,38 +92,42 @@ gb_internal void thread_init_arenas(Thread *t) {
}
gb_internal void *arena_alloc(Arena *arena, isize min_size, isize alignment) {
GB_ASSERT(gb_is_power_of_two(alignment));
usize u_alignment = cast(usize)alignment;
GB_ASSERT(!(u_alignment & (u_alignment-1)));
GB_ASSERT(arena->custom_arena || arena->parent_thread == get_current_thread());
isize size = 0;
if (arena->curr_block != nullptr) {
size = min_size + arena_align_forward_offset(arena, alignment);
isize const mask = alignment - 1;
MemoryBlock *block = arena->curr_block;
// NOTE(bill): Fast path, allocation fits in the current block.
if (block != nullptr) {
isize offset = (-cast(isize)(block->base + block->used)) & mask;
isize size = min_size + offset;
if (block->used + size <= block->size) {
u8 *ptr = block->base + block->used + offset;
block->used += size;
return ptr; // pre-zeroed by virtual_memory_alloc
}
}
if (arena->curr_block == nullptr || (arena->curr_block->used + size) > arena->curr_block->size) {
size = align_formula_isize(min_size, alignment);
arena->minimum_block_size = gb_max(DEFAULT_MINIMUM_BLOCK_SIZE, arena->minimum_block_size);
isize block_size = gb_max(size, arena->minimum_block_size);
MemoryBlock *new_block = virtual_memory_alloc(block_size, true);
new_block->prev = arena->curr_block;
arena->curr_block = new_block;
}
MemoryBlock *curr_block = arena->curr_block;
GB_ASSERT((curr_block->used + size) <= curr_block->size);
u8 *ptr = curr_block->base + curr_block->used;
ptr += arena_align_forward_offset(arena, alignment);
curr_block->used += size;
GB_ASSERT(curr_block->used <= curr_block->size);
// NOTE(bill): Slow path: grow the arena with a fresh block.
isize size = align_formula_isize(min_size, alignment);
arena->minimum_block_size = gb_max(DEFAULT_MINIMUM_BLOCK_SIZE, arena->minimum_block_size);
isize block_size = gb_max(size, arena->minimum_block_size);
// NOTE(bill): memory will be zeroed by default due to virtual memory
return ptr;
MemoryBlock *new_block = virtual_memory_alloc(block_size, true);
new_block->prev = arena->curr_block;
arena->curr_block = new_block;
isize offset = (-cast(isize)(new_block->base + new_block->used)) & mask;
u8 *ptr = new_block->base + new_block->used + offset;
new_block->used += size;
GB_ASSERT(new_block->used <= new_block->size);
return ptr; // pre-zeroed by virtual_memory_alloc
}
gb_internal void *platform_virtual_memory_alloc_internal(isize total_size, bool commit);
gb_internal bool platform_virtual_memory_commit_internal(void *data, isize commit_amount);