std.ArrayList: initial capacity based on cache line size

also std.MultiArrayList

Author: andrewrk

lib/std/array_list.zig

@@ -181,7 +181,7 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
             // a new buffer and doing our own copy. With a realloc() call,
             // the allocator implementation would pointlessly copy our
             // extra capacity.
-            const new_capacity = growCapacity(self.capacity, new_len);
+            const new_capacity = ArrayListAlignedUnmanaged(T, alignment).growCapacity(self.capacity, new_len);
             const old_memory = self.allocatedSlice();
             if (self.allocator.remap(old_memory, new_capacity)) |new_memory| {
                 self.items.ptr = new_memory.ptr;
@@ -446,7 +446,7 @@ pub fn ArrayListAligned(comptime T: type, comptime alignment: ?u29) type {
 
             if (self.capacity >= new_capacity) return;
 
-            const better_capacity = growCapacity(self.capacity, new_capacity);
+            const better_capacity = ArrayListAlignedUnmanaged(T, alignment).growCapacity(self.capacity, new_capacity);
             return self.ensureTotalCapacityPrecise(better_capacity);
         }
 
@@ -1062,14 +1062,12 @@ pub fn ArrayListAlignedUnmanaged(comptime T: type, comptime alignment: ?u29) typ
             self.capacity = 0;
         }
 
-        /// If the current capacity is less than `new_capacity`, this function will
-        /// modify the array so that it can hold at least `new_capacity` items.
+        /// Modify the array so that it can hold at least `new_capacity` items.
+        /// Implements super-linear growth to achieve amortized O(1) append operations.
         /// Invalidates element pointers if additional memory is needed.
-        pub fn ensureTotalCapacity(self: *Self, allocator: Allocator, new_capacity: usize) Allocator.Error!void {
+        pub fn ensureTotalCapacity(self: *Self, gpa: Allocator, new_capacity: usize) Allocator.Error!void {
             if (self.capacity >= new_capacity) return;
-
-            const better_capacity = growCapacity(self.capacity, new_capacity);
-            return self.ensureTotalCapacityPrecise(allocator, better_capacity);
+            return self.ensureTotalCapacityPrecise(gpa, growCapacity(self.capacity, new_capacity));
         }
 
         /// If the current capacity is less than `new_capacity`, this function will
@@ -1218,18 +1216,20 @@ pub fn ArrayListAlignedUnmanaged(comptime T: type, comptime alignment: ?u29) typ
             if (self.items.len == 0) return null;
             return self.getLast();
         }
-    };
-}
 
-/// Called when memory growth is necessary. Returns a capacity larger than
-/// minimum that grows super-linearly.
-fn growCapacity(current: usize, minimum: usize) usize {
-    var new = current;
-    while (true) {
-        new +|= new / 2 + 8;
-        if (new >= minimum)
-            return new;
-    }
+        const init_capacity = @as(comptime_int, @max(1, std.atomic.cache_line / @sizeOf(T)));
+
+        /// Called when memory growth is necessary. Returns a capacity larger than
+        /// minimum that grows super-linearly.
+        fn growCapacity(current: usize, minimum: usize) usize {
+            var new = current;
+            while (true) {
+                new +|= new / 2 + init_capacity;
+                if (new >= minimum)
+                    return new;
+            }
+        }
+    };
 }
 
 /// Integer addition returning `error.OutOfMemory` on overflow.

lib/std/json/scanner_test.zig

@@ -435,8 +435,13 @@ fn testEnsureStackCapacity(do_ensure: bool) !void {
     var fail_alloc = std.testing.FailingAllocator.init(std.testing.allocator, .{ .fail_index = 1 });
     const failing_allocator = fail_alloc.allocator();
 
-    const nestings = 999; // intentionally not a power of 2.
-    var scanner = JsonScanner.initCompleteInput(failing_allocator, "[" ** nestings ++ "]" ** nestings);
+    const nestings = 2049; // intentionally not a power of 2.
+    var input_string: std.ArrayListUnmanaged(u8) = .empty;
+    try input_string.appendNTimes(std.testing.allocator, '[', nestings);
+    try input_string.appendNTimes(std.testing.allocator, ']', nestings);
+    defer input_string.deinit(std.testing.allocator);
+
+    var scanner = JsonScanner.initCompleteInput(failing_allocator, input_string.items);
     defer scanner.deinit();
 
     if (do_ensure) {

lib/std/json/static_test.zig

@@ -916,7 +916,7 @@ test "parse at comptime" {
         uptime: u64,
     };
     const config = comptime x: {
-        var buf: [32]u8 = undefined;
+        var buf: [256]u8 = undefined;
         var fba = std.heap.FixedBufferAllocator.init(&buf);
         const res = parseFromSliceLeaky(Config, fba.allocator(), doc, .{});
         // Assert no error can occur since we are

lib/std/multi_array_list.zig

@@ -405,17 +405,27 @@ pub fn MultiArrayList(comptime T: type) type {
 
         /// Modify the array so that it can hold at least `new_capacity` items.
         /// Implements super-linear growth to achieve amortized O(1) append operations.
-        /// Invalidates pointers if additional memory is needed.
-        pub fn ensureTotalCapacity(self: *Self, gpa: Allocator, new_capacity: usize) !void {
-            var better_capacity = self.capacity;
-            if (better_capacity >= new_capacity) return;
+        /// Invalidates element pointers if additional memory is needed.
+        pub fn ensureTotalCapacity(self: *Self, gpa: Allocator, new_capacity: usize) Allocator.Error!void {
+            if (self.capacity >= new_capacity) return;
+            return self.setCapacity(gpa, growCapacity(self.capacity, new_capacity));
+        }
+
+        const init_capacity = init: {
+            var max = 1;
+            for (fields) |field| max = @as(comptime_int, @max(max, @sizeOf(field.type)));
+            break :init @as(comptime_int, @max(1, std.atomic.cache_line / max));
+        };
 
+        /// Called when memory growth is necessary. Returns a capacity larger than
+        /// minimum that grows super-linearly.
+        fn growCapacity(current: usize, minimum: usize) usize {
+            var new = current;
             while (true) {
-                better_capacity += better_capacity / 2 + 8;
-                if (better_capacity >= new_capacity) break;
+                new +|= new / 2 + init_capacity;
+                if (new >= minimum)
+                    return new;
             }
-
-            return self.setCapacity(gpa, better_capacity);
         }
 
         /// Modify the array so that it can hold at least `additional_count` **more** items.
@@ -838,7 +848,7 @@ test "union" {
 
     try testing.expectEqual(@as(usize, 0), list.items(.tags).len);
 
-    try list.ensureTotalCapacity(ally, 2);
+    try list.ensureTotalCapacity(ally, 3);
 
     list.appendAssumeCapacity(.{ .a = 1 });
     list.appendAssumeCapacity(.{ .b = "zigzag" });

lib/std/zig/string_literal.zig

@@ -377,7 +377,7 @@ test parseAlloc {
     const expectError = std.testing.expectError;
     const eql = std.mem.eql;
 
-    var fixed_buf_mem: [64]u8 = undefined;
+    var fixed_buf_mem: [512]u8 = undefined;
     var fixed_buf_alloc = std.heap.FixedBufferAllocator.init(&fixed_buf_mem);
     const alloc = fixed_buf_alloc.allocator();