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gpa: Add helper to calculate size class of empty buckets

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Empty buckets have their `alloc_cursor` set to `slot_count` to allow the
size class to be calculated later. This happens deep within the free
function.

This adds a helper and a test to verify that the size class of empty
buckets is indeed recoverable.
This commit is contained in:
Hampus Fröjdholm
2024-05-18 11:43:42 +02:00
parent 6a65561e3e
commit 61f1b2db70
1 changed files with 24 additions and 1 deletions
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25
lib/std/heap/general_purpose_allocator.zig
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@@ -297,6 +297,12 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
const stack_addresses = bucket.stackTracePtr(size_class, slot_index, trace_kind);
collectStackTrace(ret_addr, stack_addresses);
}
/// Only valid for buckets within `empty_buckets`, and relies on the `alloc_cursor`
/// of empty buckets being set to `slot_count` when they are added to `empty_buckets`
fn emptyBucketSizeClass(bucket: *BucketHeader) usize {
return @divExact(page_size, bucket.alloc_cursor);
}
};
pub fn allocator(self: *Self) Allocator {
@@ -447,7 +453,7 @@ pub fn GeneralPurposeAllocator(comptime config: Config) type {
self.backing_allocator.free(bucket.page[0..page_size]);
}
// alloc_cursor was set to slot count when bucket added to empty_buckets
self.freeBucket(bucket, @divExact(page_size, bucket.alloc_cursor));
self.freeBucket(bucket, bucket.emptyBucketSizeClass());
self.bucket_node_pool.destroy(node);
}
self.empty_buckets.root = null;
@@ -1418,6 +1424,23 @@ test "double frees" {
try std.testing.expect(!gpa.large_allocations.contains(@intFromPtr(large.ptr)));
}
test "empty bucket size class" {
const GPA = GeneralPurposeAllocator(.{ .safety = true, .never_unmap = true, .retain_metadata = true });
var gpa = GPA{};
defer std.testing.expect(gpa.deinit() == .ok) catch @panic("leak");
const allocator = gpa.allocator();
// allocate and free to create an empty bucket
const size_class: usize = @as(usize, 1) << 6;
const small = try allocator.alloc(u8, size_class);
allocator.free(small);
// the metadata tracking system relies on alloc_cursor of empty buckets
// being set to the slot count so that we can get back the size class.
const empty_bucket = GPA.searchBucket(&gpa.empty_buckets, @intFromPtr(small.ptr), null).?;
try std.testing.expect(empty_bucket.emptyBucketSizeClass() == size_class);
}
test "bug 9995 fix, large allocs count requested size not backing size" {
// with AtLeast, buffer likely to be larger than requested, especially when shrinking
var gpa = GeneralPurposeAllocator(.{ .enable_memory_limit = true }){};