40 lines
1.7 KiB
Markdown
40 lines
1.7 KiB
Markdown
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C7 Alloc Hotpath Flattening (design memo)
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=========================================
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Goals
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-----
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- Make C7 alloc as close to a straight line as possible.
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- Minimise branches/indirections on the steady hit path (UC/TLS/Warm already stable).
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- Keep Box boundaries intact; isolate feature gates to one lookup.
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Current shape (simplified)
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--------------------------
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1. size→class LUT → `class_idx = 7` for 1024B path.
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2. Route/Policy checks (tiny_route_get, tiny_policy_get) → gate UC/Warm/Page.
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3. UC pop: hit path shares code with miss/refill, includes stats/guards.
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4. TLS/Warm engagement happens behind UC miss boundary.
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5. Multiple helper calls on the hit path (gate box, policy box, UC helpers).
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Target shape
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------------
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1. size→class LUT (unchanged).
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2. One policy snapshot: `const TinyClassPolicy* pol = tiny_policy_get(7);`
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3. One route decision: C7 fast path assumes Tiny→UC→TLS/Warm enabled.
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4. Hit path specialised:
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- Inline `tiny_unified_cache_pop_fast_c7()` that only touches the hot cache lines.
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- Stats optional/sampled (avoid atomic on every hit).
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- No feature/env reads.
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5. Miss path remains boxed and guarded; enters existing refill flow unchanged.
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Possible refactors
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------------------
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- Add `malloc_tiny_fast_c7_inline(...)` as a static inline used only when class==7.
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- Precompute `pol->warm_enabled/page_box_enabled` once per thread and reuse.
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- Split UC helpers into `*_hit_fast` vs `*_miss` to keep the hit CFG tiny.
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Trade-offs / checks
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-------------------
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- Keep the Box boundaries (Gate/Route/Policy) but allow an inline “fast lane” for C7.
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- Ensure Debug/Policy logging stays in the slow/miss path only.
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- Validate with IPC/ops after implementation; target +10–15% for C7-heavy mixes.
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