[neuecc]

Summary

ArrayPoolListWriteBuffer kept its chain of rented arrays inline: 16 byte[] references and 17 lengths, about 240 bytes of struct. GC references can never be left uninitialized, so every construction zeroed the whole thing even when the message never left the scratch span, and Dispose walked it again. For tiny messages (the byte[] Serialize<T>(T) entry point) that was most of the buffer's lifetime cost.

The chain now lives in a PooledSegment[] (array + finished length pairs) rented from ArrayPool<PooledSegment>.Shared on the first spill. The struct is ~64 bytes (two spans, three ints, a long and one reference), BytesWritten is a running total instead of a loop, and ToArray / WriteTo take a single-copy path when nothing spilled. No heap allocation is introduced: the table is pooled exactly like the byte[] segments.

CompatibleArrayPoolListWriteBuffer and BufferSegments share the same table, which removes the netstandard switch-based inline storage and the length normalization in the compatible variant.

Measurements

net10.0, x64 (i7-13700KF), 1 KB stackalloc scratch, warmup 8 / iteration 15.

Scenario	Before	After	SpanWriteBuffer (floor)
ctor + Dispose	3.62 ns	0.31 ns	-
+ write 1 byte	3.27 ns	1.89 ns	0.29 ns
+ ToArray (1 byte)	7.33 ns	4.63 ns	3.04 ns
ToArray (32 bytes, 4 × 8 B)	12.84 ns	9.19 ns	-
2 KB (one spill)	124 ns	100 ns	-
200 KB (three segments)	53.8 µs	53.1 µs	-

Steady-state allocation measured with GC.GetAllocatedBytesForCurrentThread across 200 KB × 1000 (62 Gen2 GCs), 2 KB × 100k and 64 B × 100k: 0 extra bytes on every path.

The remaining gap to SpanWriteBuffer is the struct staying on the stack because GetSpanSlow takes ref this; that is the floor of this shape.

Notes

• Rent order in GetSpanSlow: the new segment is rented before any field changes, so a failed Rent leaves the buffer untouched.
• BufferSegments borrows the rented table. The existing contract (valid until the next write or Dispose) is unchanged, but a view used after Dispose now reads a table that may have been re-rented rather than hitting a null array.
• New tests in ArrayPoolListWriteBufferTest cover scratch-only, single and multiple spills, empty scratch, Dispose reset / reuse and the compatible variant. 143 tests pass.

🤖 Generated with Claude Code