Phase 18 v1: Hot Text Isolation — NO-GO (I-cache regression)

## Summary

Phase 18 v1 attempted layout optimization using section splitting + GC:
- `-ffunction-sections -fdata-sections -Wl,--gc-sections`

Result: **Catastrophic I-cache regression**
- Throughput: -0.87% (48.94M → 48.52M ops/s)
- I-cache misses: +91.06% (131K → 250K)
- Variance: +80% (σ=0.45M → σ=0.81M)

Root cause: Section-based splitting without explicit hot symbol ordering
fragments code locality, destroying natural compiler/LTO layout.

## Build Knob Safety

Makefile updated to separate concerns:
- `HOT_TEXT_ISOLATION=1` → attributes only (safe, but no perf gain)
- `HOT_TEXT_GC_SECTIONS=1` → section splitting (currently NO-GO)

Both kept as research boxes (default OFF).

## Verdict

Freeze Phase 18 v1:
- Do NOT use section-based linking without strong ordering strategy
- Keep hot/cold attributes as placeholder (currently unused)
- Proceed to Phase 18 v2: BENCH_MINIMAL compile-out

Expected impact v2: +10-20% via instruction count reduction
- GO threshold: +5% minimum, +8% preferred
- Only continue if instructions clearly drop

## Files

New:
- docs/analysis/PHASE18_HOT_TEXT_ISOLATION_1_AB_TEST_RESULTS.md

Modified:
- Makefile (build knob safety isolation)
- CURRENT_TASK.md (Phase 18 v1 verdict)
- docs/analysis/PHASE18_HOT_TEXT_ISOLATION_1_NEXT_INSTRUCTIONS.md

## Lessons

1. Layout optimization is extremely fragile without ordering guarantees
2. I-cache is first-order performance factor (IPC=2.30 is memory-bound)
3. Compiler defaults may be better than manual section splitting
4. Next frontier: instruction count reduction (stats/ENV removal)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/analysis/PHASE18_HOT_TEXT_ISOLATION_1_AB_TEST_RESULTS.md

@@ -0,0 +1,55 @@
+# Phase 18 v1: Hot Text Isolation / Layout Control — A/B Test Results
+
+**Date**: 2025-12-15  
+**Verdict**: ❌ **NO-GO**（I-cache regression）
+
+---
+
+## Executive Summary
+
+Phase 18 v1 attempted a low-risk layout improvement using section splitting + GC:
+- `-ffunction-sections -fdata-sections -Wl,--gc-sections`
+
+Result was worse than baseline:
+- throughput slightly regressed
+- **I-cache misses nearly doubled**
+- variance increased significantly
+
+Conclusion: section-based linking **without strong ordering/clustering** is harmful for this codebase/toolchain at present.
+
+---
+
+## A/B Results (Mixed)
+
+| Metric | Baseline | Optimized | Delta |
+|--------|----------|-----------|-------|
+| Throughput | 48.94M ops/s | 48.52M ops/s | **-0.87%** |
+| I-cache misses | 131K | 250K | **+91.06%** |
+| Instructions | 41.29B | 41.32B | +0.09% |
+| Variance (σ) | 0.45M | 0.81M | **+80%** |
+
+---
+
+## Root Cause (most likely)
+
+The section-splitting flags increase fragmentation and can destroy the natural locality produced by:
+- compiler + LTO layout decisions
+- existing hot/cold separation by function boundaries
+
+Without an explicit “hot symbol ordering” mechanism (link-order file / linker script / PGO call-graph sort), the remaining `.text.*` fragments may become poorly clustered, increasing front-end stalls.
+
+This matches observations:
+- instructions did not decrease
+- I-cache misses increased dramatically
+- throughput regressed modestly (front-end disruption + variability)
+
+---
+
+## Decision
+
+Freeze Phase 18 v1:
+- Treat `--gc-sections` path as **NO-GO** in this environment unless combined with explicit link ordering.
+
+Next:
+- Phase 18 v2: **BENCH_MINIMAL compile-out** (reduce instruction footprint directly).
+

docs/analysis/PHASE18_HOT_TEXT_ISOLATION_1_NEXT_INSTRUCTIONS.md

@@ -58,16 +58,17 @@ Rollback: restore include in `core/hakmem.c` and drop new TU.
 ### Patch 3 (optional): bench-only section GC
 
 Makefile knob:
-- `HOT_TEXT_ISOLATION=0/1`
+- `HOT_TEXT_GC_SECTIONS=0/1`（research-only）
 
 When `=1`, add for bench builds:
-- `-DHAKMEM_HOT_TEXT_ISOLATION=1`
 - `-ffunction-sections -fdata-sections`
-- `LDFLAGS += -Wl,--gc-sections`
+- `-Wl,--gc-sections`
 
 Notes:
 - Keep it bench-only first (do not touch shared lib build until proven stable).
-- If toolchain rejects `--gc-sections` or results are unstable → skip this patch.
+- **Phase 18 v1 outcome**: This exact flag set caused an I-cache regression in this repo/toolchain.
+  - Ref: `docs/analysis/PHASE18_HOT_TEXT_ISOLATION_1_AB_TEST_RESULTS.md`
+  - Therefore, **Patch 3 is NO-GO for now** unless combined with explicit hot symbol ordering.
 
 ---