wasm: add WasmAllocator
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102
std/heap.zig
102
std/heap.zig
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@ -318,48 +318,96 @@ pub const FixedBufferAllocator = struct {
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}
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};
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extern fn @"llvm.wasm.memory.size.i32"(u32) u32;
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extern fn @"llvm.wasm.memory.grow.i32"(u32, u32) i32;
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/// This allocator tries to allocate the specified number of 64 KB pages and uses FixedBufferAllocator internally
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pub const WasmAllocator = blk: {
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if (builtin.arch != builtin.Arch.wasm32) {
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@compileError("only supported in wasm32");
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} else {
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const mem_grow = @"llvm.wasm.memory.grow.i32";
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const WASM_PAGE_SIZE = 64 * 1024; // 64 kilobytes
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break :blk struct {
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allocator: Allocator,
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fb_allocator: FixedBufferAllocator,
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pub const wasm_allocator = &wasm_allocator_state.allocator;
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var wasm_allocator_state = WasmAllocator{
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.allocator = Allocator{
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.reallocFn = WasmAllocator.realloc,
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.shrinkFn = WasmAllocator.shrink,
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},
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.start_ptr = undefined,
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.num_pages = 0,
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.end_index = 0,
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};
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pub fn init(num_pages: u32) !WasmAllocator {
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const prev_block = mem_grow(0, num_pages);
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if (prev_block == -1) {
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pub const WasmAllocator = struct {
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allocator: Allocator,
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start_ptr: [*]u8,
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num_pages: usize,
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end_index: usize,
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fn alloc(allocator: *Allocator, size: usize, alignment: u29) ![]u8 {
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const self = @fieldParentPtr(WasmAllocator, "allocator", allocator);
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const addr = @ptrToInt(self.start_ptr) + self.end_index;
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const adjusted_addr = mem.alignForward(addr, alignment);
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const adjusted_index = self.end_index + (adjusted_addr - addr);
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const new_end_index = adjusted_index + size;
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const required_memory = new_end_index - (self.num_pages * WASM_PAGE_SIZE);
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if (required_memory > 0) {
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var num_pages: u32 = @divTrunc(required_memory, WASM_PAGE_SIZE);
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if (@rem(required_memory, WASM_PAGE_SIZE) != 0) {
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num_pages += 1;
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}
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const prev_page = @"llvm.wasm.memory.grow.i32"(0, num_pages);
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if (prev_page == -1) {
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return error.OutOfMemory;
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}
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const buffer_slice = @intToPtr([*]u8, @intCast(usize, prev_block) * WASM_PAGE_SIZE)[0..(WASM_PAGE_SIZE * num_pages)];
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self.num_pages += num_pages;
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}
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return WasmAllocator{
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.allocator = Allocator{
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.reallocFn = realloc,
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.shrinkFn = shrink,
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},
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.fb_allocator = FixedBufferAllocator.init(buffer_slice),
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};
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const result = self.start_ptr[adjusted_index..new_end_index];
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self.end_index = new_end_index;
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return result;
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}
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// Check if memory is the last "item" and it aligns. That lets us expand or reclaim memory
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fn is_last_item(allocator: *Allocator, memory: []u8, alignment: u29) bool {
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const self = @fieldParentPtr(WasmAllocator, "allocator", allocator);
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return memory.ptr == self.start_ptr + self.end_index - memory.len and mem.alignForward(@ptrToInt(memory.ptr), alignment) == @ptrToInt(memory.ptr);
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}
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fn realloc(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) ![]u8 {
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const self = @fieldParentPtr(WasmAllocator, "allocator", allocator);
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return FixedBufferAllocator.realloc(&self.fb_allocator.allocator, old_mem, old_align, new_size, new_align);
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// Initialize start_ptr at the first realloc
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if (self.num_pages == 0) {
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self.start_ptr = @intToPtr([*]u8, @intCast(usize, @"llvm.wasm.memory.size.i32"(0)) * WASM_PAGE_SIZE);
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}
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if (is_last_item(allocator, old_mem, new_align)) {
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const start_index = self.end_index - old_mem.len;
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const new_end_index = start_index + new_size;
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if (new_end_index > self.num_pages * WASM_PAGE_SIZE) {
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_ = try alloc(allocator, new_end_index - self.end_index, new_align);
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}
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const result = self.start_ptr[start_index..new_end_index];
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self.end_index = new_end_index;
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return result;
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} else if (new_size <= old_mem.len and new_align <= old_align) {
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return error.OutOfMemory;
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} else {
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const result = try alloc(allocator, new_size, new_align);
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mem.copy(u8, result, old_mem);
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return result;
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}
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}
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fn shrink(allocator: *Allocator, old_mem: []u8, old_align: u29, new_size: usize, new_align: u29) []u8 {
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const self = @fieldParentPtr(WasmAllocator, "allocator", allocator);
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return FixedBufferAllocator.shrink(&self.fb_allocator.allocator, old_mem, old_align, new_size, new_align);
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}
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};
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// TODO: Use is_last_item or other heuristic here
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return old_mem[0..new_size];
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}
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};
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