From 0c1fbc4ea6ff5c74b83d3959eac0b355570bd439 Mon Sep 17 00:00:00 2001 From: Andrew Kelley Date: Fri, 19 Sep 2025 17:48:07 -0700 Subject: [PATCH] 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.