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M bench/PERF_LOG.md

@@ 84,6 84,54 @@ macOS arm64.
 
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+## 2026-05-18 — u39: table.new(0, N) for result tables (REVERTED)
+
+**Task:** [tarantool-protobuf-u39] Decoder: emit `table.new(0, N)` for
+result tables. The thesis was that pre-sizing the result table's hash
+part to the message's field count would avoid the rehash that
+fresh-`{}` tables pay as fields are added.
+
+**Change attempted:** Header now `require('table.new')` with a pcall
+fallback so plain-Lua loads still work; `emitInlineDecode` allocates the
+result via `table_new(0, len(m.Fields))` instead of `{}`. 745/745 tests
+pass. JIT 37/37.
+
+**Bench (Person full decode, msgs/s, median-of-3 vs post-cch):**
+
+| size  | post-cch  | u39       | Δ        |
+|-------|-----------|-----------|----------|
+| 10B   | 2,495,633 | 2,111,197 | **-15.4%** |
+| 100B  | 2,131,196 | 1,754,540 | **-17.7%** |
+| 1KB   | 115,310   | 117,270   | +1.7%    |
+| 10KB  | 15,831    | 15,627    | -1.3%    |
+| 100KB | 1,620     | 1,617     | -0.2%    |
+
+**Outcome: reverted.** Small-payload decode collapsed. The
+`table_new(0, N)` call (through an upvalue, with arguments) is
+several times the cost of a `{}` literal, and at 10B/100B the decode
+loop only populates 2-4 fields — well under the LuaJIT default hash
+size where the first rehash would land. The rehash savings never
+materialize because the small case doesn't reach them; meanwhile the
+extra call cost is paid every decode.
+
+Large payloads (1KB+) saw no meaningful gain either — by then decode
+time is dominated by tag decoding, string extraction, and varint
+parsing, not table allocation. The `drm` memory entry already
+documented "136 B alloc floor is not the throughput bottleneck"; this
+result confirms it from the opposite direction.
+
+**Lesson:** allocation-shape optimizations are only worth it when the
+workload spends real time in allocation. The bench corpus does not
+exercise that mode; if a workload ever does (lots of tiny messages, or
+messages with very wide field sets), revisit then. Sticking with `{}`
+also keeps generated code compatible with plain Lua (no LuaJIT-only
+require) — small platform-portability win.
+
+**Commit:** none — change reverted. PERF_LOG entry is the only
+artifact.
+
+---
+
 ## 2026-05-18 — cch: local counter for repeated-field append
 
 **Task:** [tarantool-protobuf-cch] Decoder: use local counter instead of