hakmem/docs/analysis/C7_HOTPATH_FLATTENING.md

C7 Alloc Hotpath Flattening (design memo)

Goals

• Make C7 alloc as close to a straight line as possible.
• Minimise branches/indirections on the steady hit path (UC/TLS/Warm already stable).
• Keep Box boundaries intact; isolate feature gates to one lookup.

Current shape (simplified)

1. size→class LUT → class_idx = 7 for 1024B path.
2. Route/Policy checks (tiny_route_get, tiny_policy_get) → gate UC/Warm/Page.
3. UC pop: hit path shares code with miss/refill, includes stats/guards.
4. TLS/Warm engagement happens behind UC miss boundary.
5. Multiple helper calls on the hit path (gate box, policy box, UC helpers).

Target shape

1. size→class LUT (unchanged).
2. One policy snapshot: const TinyClassPolicy* pol = tiny_policy_get(7);
3. One route decision: C7 fast path assumes Tiny→UC→TLS/Warm enabled.
4. Hit path specialised:
   • Inline tiny_unified_cache_pop_fast_c7() that only touches the hot cache lines.
   • Stats optional/sampled (avoid atomic on every hit).
   • No feature/env reads.
5. Miss path remains boxed and guarded; enters existing refill flow unchanged.

Possible refactors

• Add malloc_tiny_fast_c7_inline(...) as a static inline used only when class==7.
• Precompute pol->warm_enabled/page_box_enabled once per thread and reuse.
• Split UC helpers into *_hit_fast vs *_miss to keep the hit CFG tiny.

Trade-offs / checks

• Keep the Box boundaries (Gate/Route/Policy) but allow an inline “fast lane” for C7.
• Ensure Debug/Policy logging stays in the slow/miss path only.
• Validate with IPC/ops after implementation; target +10–15% for C7-heavy mixes.