From 0c1fbc4ea6ff5c74b83d3959eac0b355570bd439 Mon Sep 17 00:00:00 2001 From: Andrew Kelley Date: Fri, 19 Sep 2025 17:48:07 -0700 Subject: [PATCH 1/2] std: remove loop from growCapacity I measured this against master branch and found no statistical difference. Since this code is simpler and logically superior due to always leaving sufficient unused capacity when growing, it is preferred over status quo. --- lib/std/Io/Writer.zig | 2 +- lib/std/array_list.zig | 17 ++++++----------- lib/std/deque.zig | 14 +------------- lib/std/multi_array_list.zig | 23 +++++++++-------------- 4 files changed, 17 insertions(+), 39 deletions(-) diff --git a/lib/std/Io/Writer.zig b/lib/std/Io/Writer.zig index 7f1b1fd884..91028d60d0 100644 --- a/lib/std/Io/Writer.zig +++ b/lib/std/Io/Writer.zig @@ -2661,7 +2661,7 @@ pub const Allocating = struct { pub fn ensureTotalCapacity(a: *Allocating, new_capacity: usize) Allocator.Error!void { // Protects growing unnecessarily since better_capacity will be larger. if (a.writer.buffer.len >= new_capacity) return; - const better_capacity = ArrayList(u8).growCapacity(a.writer.buffer.len, new_capacity); + const better_capacity = ArrayList(u8).growCapacity(new_capacity); return ensureTotalCapacityPrecise(a, better_capacity); } diff --git a/lib/std/array_list.zig b/lib/std/array_list.zig index 9718d19c7e..96a0344442 100644 --- a/lib/std/array_list.zig +++ b/lib/std/array_list.zig @@ -172,7 +172,7 @@ pub fn AlignedManaged(comptime T: type, comptime alignment: ?mem.Alignment) 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 = Aligned(T, alignment).growCapacity(self.capacity, new_len); + const new_capacity = Aligned(T, alignment).growCapacity(new_len); const old_memory = self.allocatedSlice(); if (self.allocator.remap(old_memory, new_capacity)) |new_memory| { self.items.ptr = new_memory.ptr; @@ -408,7 +408,7 @@ pub fn AlignedManaged(comptime T: type, comptime alignment: ?mem.Alignment) type // Protects growing unnecessarily since better_capacity will be larger. if (self.capacity >= new_capacity) return; - const better_capacity = Aligned(T, alignment).growCapacity(self.capacity, new_capacity); + const better_capacity = Aligned(T, alignment).growCapacity(new_capacity); return self.ensureTotalCapacityPrecise(better_capacity); } @@ -1160,7 +1160,7 @@ pub fn Aligned(comptime T: type, comptime alignment: ?mem.Alignment) type { /// 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.ensureTotalCapacityPrecise(gpa, growCapacity(self.capacity, new_capacity)); + return self.ensureTotalCapacityPrecise(gpa, growCapacity(new_capacity)); } /// If the current capacity is less than `new_capacity`, this function will @@ -1359,17 +1359,12 @@ pub fn Aligned(comptime T: type, comptime alignment: ?mem.Alignment) type { return self.getLast(); } - const init_capacity = @as(comptime_int, @max(1, std.atomic.cache_line / @sizeOf(T))); + const init_capacity: 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. - pub fn growCapacity(current: usize, minimum: usize) usize { - var new = current; - while (true) { - new +|= new / 2 + init_capacity; - if (new >= minimum) - return new; - } + pub fn growCapacity(minimum: usize) usize { + return minimum +| (minimum / 2 + init_capacity); } }; } diff --git a/lib/std/deque.zig b/lib/std/deque.zig index 7130cd33fe..267b8a0afe 100644 --- a/lib/std/deque.zig +++ b/lib/std/deque.zig @@ -56,7 +56,7 @@ pub fn Deque(comptime T: type) type { /// Invalidates element pointers if additional memory is needed. pub fn ensureTotalCapacity(deque: *Self, gpa: Allocator, new_capacity: usize) Allocator.Error!void { if (deque.buffer.len >= new_capacity) return; - return deque.ensureTotalCapacityPrecise(gpa, growCapacity(deque.buffer.len, new_capacity)); + return deque.ensureTotalCapacityPrecise(gpa, std.ArrayList(T).growCapacity(new_capacity)); } /// If the current capacity is less than `new_capacity`, this function will @@ -243,18 +243,6 @@ pub fn Deque(comptime T: type) type { return index - head_len; } } - - const init_capacity: 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; - } - } }; } diff --git a/lib/std/multi_array_list.zig b/lib/std/multi_array_list.zig index e4eb60bd93..ec4b0b72e1 100644 --- a/lib/std/multi_array_list.zig +++ b/lib/std/multi_array_list.zig @@ -457,24 +457,19 @@ pub fn MultiArrayList(comptime T: type) type { /// 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)); + return self.setCapacity(gpa, growCapacity(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)); + const init_capacity: comptime_int = init: { + var max: comptime_int = 1; + for (fields) |field| max = @max(max, @sizeOf(field.type)); + break :init @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) { - new +|= new / 2 + init_capacity; - if (new >= minimum) - return new; - } + /// Given a lower bound of required memory capacity, returns a larger value + /// with super-linear growth. + pub fn growCapacity(minimum: usize) usize { + return minimum +| (minimum / 2 + init_capacity); } /// Modify the array so that it can hold at least `additional_count` **more** items. From 3cc0fc601af62adebf6b9f3bb1a241fa8a501d0b Mon Sep 17 00:00:00 2001 From: Andrew Kelley Date: Sat, 20 Sep 2025 14:38:01 -0700 Subject: [PATCH 2/2] std.json: delete test tightly coupled to ArrayList growth This test works by assuming that std.ArrayList will grow with a specific capacity increasing pattern, which is an invalid assumption. Delete the offending test. --- lib/std/json/scanner_test.zig | 27 --------------------------- lib/std/json/static_test.zig | 2 +- 2 files changed, 1 insertion(+), 28 deletions(-) diff --git a/lib/std/json/scanner_test.zig b/lib/std/json/scanner_test.zig index ae954a88f0..5b4bfa532a 100644 --- a/lib/std/json/scanner_test.zig +++ b/lib/std/json/scanner_test.zig @@ -431,33 +431,6 @@ test "skipValue" { try std.testing.expectError(error.SyntaxError, testSkipValue("[102, 111, 111}")); } -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 = 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 = Scanner.initCompleteInput(failing_allocator, input_string.items); - defer scanner.deinit(); - - if (do_ensure) { - try scanner.ensureTotalStackCapacity(nestings); - } - - try scanner.skipValue(); - try std.testing.expectEqual(Token.end_of_document, try scanner.next()); -} -test "ensureTotalStackCapacity" { - // Once to demonstrate failure. - try std.testing.expectError(error.OutOfMemory, testEnsureStackCapacity(false)); - // Then to demonstrate it works. - try testEnsureStackCapacity(true); -} - fn testDiagnosticsFromSource(expected_error: ?anyerror, line: u64, col: u64, byte_offset: u64, source: anytype) !void { var diagnostics = Diagnostics{}; source.enableDiagnostics(&diagnostics); diff --git a/lib/std/json/static_test.zig b/lib/std/json/static_test.zig index 735ccd82e1..805d2840a1 100644 --- a/lib/std/json/static_test.zig +++ b/lib/std/json/static_test.zig @@ -914,7 +914,7 @@ test "parse at comptime" { uptime: u64, }; const config = comptime x: { - var buf: [256]u8 = undefined; + var buf: [300]u8 = undefined; var fba = std.heap.FixedBufferAllocator.init(&buf); const res = parseFromSliceLeaky(Config, fba.allocator(), doc, .{}); // Assert no error can occur since we are