GeneralPurposeAllocator.searchBucket: check current bucket before searching the list

Follow up to #17383. This is a minor optimization that only matters when a small allocation is resized/free'd soon after it is allocated.

The only real difference I was able to observe with this was via a synthetic benchmark that allocates a full bucket and then frees all but one of the slots, over and over in a loop:

Debug build:

Benchmark 1 (9 runs): gpa-degen-master.exe
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           575ms ± 5.19ms     569ms …  583ms          0 ( 0%)        0%
  peak_rss           43.8MB ± 1.37KB    43.8MB … 43.8MB          1 (11%)        0%
Benchmark 2 (10 runs): gpa-degen-search-cur.exe
  measurement          mean ± σ            min … max           outliers         delta
  wall_time           532ms ± 5.55ms     520ms …  539ms          0 ( 0%)        ⚡-  7.5% ±  0.9%
  peak_rss           43.8MB ± 65.2KB    43.8MB … 44.0MB          1 (10%)          +  0.0% ±  0.1%

ReleaseFast build:

Benchmark 1 (129 runs): gpa-degen-master-release.exe
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          38.9ms ± 1.12ms    36.7ms … 42.4ms          8 ( 6%)        0%
  peak_rss           23.2MB ± 2.39KB    23.2MB … 23.2MB          0 ( 0%)        0%
Benchmark 2 (151 runs): gpa-degen-search-cur-release.exe
  measurement          mean ± σ            min … max           outliers         delta
  wall_time          33.2ms ±  999us    31.9ms … 36.3ms         20 (13%)        ⚡- 14.7% ±  0.6%
  peak_rss           23.2MB ± 2.26KB    23.2MB … 23.2MB          0 ( 0%)          +  0.0% ±  0.0%

1 files changed, 14 insertions, 10 deletions

diff --git a/lib/std/heap/general_purpose_allocator.zig b/lib/std/heap/general_purpose_allocator.zig
index 2870dcee5a..6dc6df3998 100644
--- a/lib/std/heap/general_purpose_allocator.zig
+++ b/lib/std/heap/general_purpose_allocator.zig
@@ -541,10 +541,14 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
         fn searchBucket(
             buckets: *Buckets,
             addr: usize,
+            current_bucket: ?*BucketHeader,
         ) ?*BucketHeader {
-            const search_page = mem.alignBackward(usize, addr, page_size);
+            const search_page: [*]align(page_size) u8 = @ptrFromInt(mem.alignBackward(usize, addr, page_size));
+            if (current_bucket != null and current_bucket.?.page == search_page) {
+                return current_bucket;
+            }
             var search_header: BucketHeader = undefined;
-            search_header.page = @ptrFromInt(search_page);
+            search_header.page = search_page;
             const entry = buckets.getEntryFor(&search_header);
             return if (entry.node) |node| node.key else null;
         }
@@ -712,7 +716,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
             var bucket_index = math.log2(size_class_hint);
             var size_class: usize = size_class_hint;
             const bucket = while (bucket_index < small_bucket_count) : (bucket_index += 1) {
-                if (searchBucket(&self.buckets[bucket_index], @intFromPtr(old_mem.ptr))) |bucket| {
+                if (searchBucket(&self.buckets[bucket_index], @intFromPtr(old_mem.ptr), self.cur_buckets[bucket_index])) |bucket| {
                     break bucket;
                 }
                 size_class *= 2;
@@ -720,7 +724,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                 if (config.retain_metadata) {
                     if (!self.large_allocations.contains(@intFromPtr(old_mem.ptr))) {
                         // object not in active buckets or a large allocation, so search empty buckets
-                        if (searchBucket(&self.empty_buckets, @intFromPtr(old_mem.ptr))) |bucket| {
+                        if (searchBucket(&self.empty_buckets, @intFromPtr(old_mem.ptr), null)) |bucket| {
                             // bucket is empty so is_used below will always be false and we exit there
                             break :blk bucket;
                         } else {
@@ -830,7 +834,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
             var bucket_index = math.log2(size_class_hint);
             var size_class: usize = size_class_hint;
             const bucket = while (bucket_index < small_bucket_count) : (bucket_index += 1) {
-                if (searchBucket(&self.buckets[bucket_index], @intFromPtr(old_mem.ptr))) |bucket| {
+                if (searchBucket(&self.buckets[bucket_index], @intFromPtr(old_mem.ptr), self.cur_buckets[bucket_index])) |bucket| {
                     break bucket;
                 }
                 size_class *= 2;
@@ -838,7 +842,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
                 if (config.retain_metadata) {
                     if (!self.large_allocations.contains(@intFromPtr(old_mem.ptr))) {
                         // object not in active buckets or a large allocation, so search empty buckets
-                        if (searchBucket(&self.empty_buckets, @intFromPtr(old_mem.ptr))) |bucket| {
+                        if (searchBucket(&self.empty_buckets, @intFromPtr(old_mem.ptr), null)) |bucket| {
                             // bucket is empty so is_used below will always be false and we exit there
                             break :blk bucket;
                         } else {
@@ -1387,10 +1391,10 @@ test "double frees" {
     const index: usize = 6;
     const size_class: usize = @as(usize, 1) << 6;
     const small = try allocator.alloc(u8, size_class);
-    try std.testing.expect(GPA.searchBucket(&gpa.buckets[index], @intFromPtr(small.ptr)) != null);
+    try std.testing.expect(GPA.searchBucket(&gpa.buckets[index], @intFromPtr(small.ptr), gpa.cur_buckets[index]) != null);
     allocator.free(small);
-    try std.testing.expect(GPA.searchBucket(&gpa.buckets[index], @intFromPtr(small.ptr)) == null);
-    try std.testing.expect(GPA.searchBucket(&gpa.empty_buckets, @intFromPtr(small.ptr)) != null);
+    try std.testing.expect(GPA.searchBucket(&gpa.buckets[index], @intFromPtr(small.ptr), gpa.cur_buckets[index]) == null);
+    try std.testing.expect(GPA.searchBucket(&gpa.empty_buckets, @intFromPtr(small.ptr), null) != null);
 
     // detect a large allocation double free
     const large = try allocator.alloc(u8, 2 * page_size);
@@ -1408,7 +1412,7 @@ test "double frees" {
     // check that flushing retained metadata doesn't disturb live allocations
     gpa.flushRetainedMetadata();
     try std.testing.expect(gpa.empty_buckets.root == null);
-    try std.testing.expect(GPA.searchBucket(&gpa.buckets[index], @intFromPtr(normal_small.ptr)) != null);
+    try std.testing.expect(GPA.searchBucket(&gpa.buckets[index], @intFromPtr(normal_small.ptr), gpa.cur_buckets[index]) != null);
     try std.testing.expect(gpa.large_allocations.contains(@intFromPtr(normal_large.ptr)));
     try std.testing.expect(!gpa.large_allocations.contains(@intFromPtr(large.ptr)));
 }