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// This file implements a very simple allocator for external scanners running
// in WASM. Allocation is just bumping a static pointer and growing the heap
// as needed, and freeing is mostly a noop. But in the special case of freeing
// the last-allocated pointer, we'll reuse that pointer again.
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
extern void tree_sitter_debug_message(const char *, size_t);
#define PAGESIZE 0x10000
#define MAX_HEAP_SIZE (4 * 1024 * 1024)
typedef struct {
size_t size;
char data[0];
} Region;
static Region *heap_end = NULL;
static Region *heap_start = NULL;
static Region *next = NULL;
// Get the region metadata for the given heap pointer.
static inline Region *region_for_ptr(void *ptr) {
return ((Region *)ptr) - 1;
}
// Get the location of the next region after the given region,
// if the given region had the given size.
static inline Region *region_after(Region *self, size_t len) {
char *address = self->data + len;
char *aligned = (char *)((uintptr_t)(address + 3) & ~0x3);
return (Region *)aligned;
}
static void *get_heap_end() {
return (void *)(__builtin_wasm_memory_size(0) * PAGESIZE);
}
static int grow_heap(size_t size) {
size_t new_page_count = ((size - 1) / PAGESIZE) + 1;
return __builtin_wasm_memory_grow(0, new_page_count) != SIZE_MAX;
}
// Clear out the heap, and move it to the given address.
void reset_heap(void *new_heap_start) {
heap_start = new_heap_start;
next = new_heap_start;
heap_end = get_heap_end();
}
void *malloc(size_t size) {
Region *region_end = region_after(next, size);
if (region_end > heap_end) {
if ((char *)region_end - (char *)heap_start > MAX_HEAP_SIZE) {
return NULL;
}
if (!grow_heap(size)) return NULL;
heap_end = get_heap_end();
}
void *result = &next->data;
next->size = size;
next = region_end;
return result;
}
void free(void *ptr) {
if (ptr == NULL) return;
Region *region = region_for_ptr(ptr);
Region *region_end = region_after(region, region->size);
// When freeing the last allocated pointer, re-use that
// pointer for the next allocation.
if (region_end == next) {
next = region;
}
}
void *calloc(size_t count, size_t size) {
void *result = malloc(count * size);
memset(result, 0, count * size);
return result;
}
void *realloc(void *ptr, size_t new_size) {
if (ptr == NULL) {
return malloc(new_size);
}
Region *region = region_for_ptr(ptr);
Region *region_end = region_after(region, region->size);
// When reallocating the last allocated region, return
// the same pointer, and skip copying the data.
if (region_end == next) {
next = region;
return malloc(new_size);
}
void *result = malloc(new_size);
memcpy(result, ®ion->data, region->size);
return result;
}
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