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Antigravity Agent
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id: wiki-2026-0508-branch-prediction
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title: Branch Prediction
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category: 10_Wiki/Topics
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status: verified
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canonical_id: self
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aliases: [P-Reinforce-AUTO-4D7707, CPU Branch Predictor]
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duplicate_of: none
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source_trust_level: A
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confidence_score: 0.9
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verification_status: applied
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tags: [cpu, performance, security, microarchitecture]
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raw_sources: []
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last_reinforced: 2026-05-10
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github_commit: pending
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tech_stack:
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language: cpp
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framework: none
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---
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# Branch Prediction
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## 매 한 줄
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> **"매 modern CPU 의 IPC 핵심 mechanism — 그리고 Spectre 의 attack surface."**. Branch predictor 는 conditional/indirect branch 의 결과를 speculatively execute 함으로써 deep pipeline 의 stall 을 회피; 매 misprediction penalty 는 15-20+ cycles, 매 mispredicted speculative window 가 Spectre v1/v2 의 leak vector.
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## 매 핵심
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### 매 Predictor 종류
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- **Static**: forward-not-taken, backward-taken (hint).
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- **Bimodal (2-bit)**: per-PC saturating counter.
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- **Local history**: per-branch shift register.
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- **Global history (gshare/GAg)**: shared GHR XOR PC.
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- **Tournament**: meta-predictor selects local vs global.
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- **TAGE / ITTAGE**: tagged geometric history (modern SOTA).
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- **Perceptron**: AMD Zen — neural-style predictor.
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### 매 Indirect Branches
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- BTB (Branch Target Buffer) — caches target.
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- ITTAGE for indirect.
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- Critical for vtables, function pointers, switch.
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### 매 응용
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1. Hot loops — predictable branches → near-zero penalty.
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2. Sorted-data effect (the famous SO question).
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3. Spectre/BranchScope side-channel attacks.
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4. JIT branch ordering decisions (V8, Hotspot).
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## 💻 패턴
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### Likely / Unlikely Hints (C++20)
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```cpp
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if (x > 0) [[likely]] {
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fast_path();
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} else [[unlikely]] {
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slow_path();
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}
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```
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### Branchless via Mask
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```cpp
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// branchful
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int abs_v(int x) { return x < 0 ? -x : x; }
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// branchless
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int abs_v_nb(int x) {
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int mask = x >> 31; // 0 or -1
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return (x ^ mask) - mask;
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}
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```
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### Branchless Min via cmov
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```cpp
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int min_v(int a, int b) {
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return a < b ? a : b; // compilers emit cmov on x86
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}
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```
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### Sort Before Loop (classic)
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```cpp
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std::sort(data.begin(), data.end()); // 정렬 → branch predictable
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long sum = 0;
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for (int v : data) if (v >= 128) sum += v;
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```
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### LLVM `__builtin_expect`
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```cpp
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if (__builtin_expect(error_flag, 0)) {
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handle_error();
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}
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```
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### Spectre v1 Mitigation (lfence)
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```cpp
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// vulnerable
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if (idx < arr_size) {
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secret = arr[idx]; // mispredict → leaks
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}
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// hardened
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if (idx < arr_size) {
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asm volatile("lfence" ::: "memory");
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secret = arr[idx];
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}
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```
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### perf Branch Stats
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```bash
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perf stat -e branches,branch-misses ./bench
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# branch-miss rate > 5% → suspect; > 10% → critical
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```
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## 매 결정 기준
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| 상황 | 전략 |
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|---|---|
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| Hot loop, predictable | Trust predictor — natural code |
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| Hot loop, unpredictable | Branchless / mask / cmov |
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| Cold path | Doesn't matter — clarity > tricks |
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| Indirect-heavy (vtable) | Devirtualize, monomorphize |
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| Security-sensitive | lfence / speculative load hardening |
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**기본값**: write clear branchful code; branchless only when profiler shows misprediction hotspot.
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## 🔗 Graph
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- 부모: [[Pipeline]]
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- 응용: [[JIT Compilation]]
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- Adjacent: [[Spectre]] · [[Speculative Execution]]
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## 🤖 LLM 활용
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**언제**: explain mispredict cost, generate branchless equivalents, suggest hints.
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**언제 X**: micro-arch tuning without perf data — speculation hurts.
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## ❌ 안티패턴
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- **Random unpredictable branch in hot loop**: 20-30% slowdown.
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- **Premature branchless conversion**: hurts cold/clarity paths.
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- **Trusting `[[likely]]` blindly**: PGO data > human guess.
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- **Ignoring Spectre in untrusted-input parsers**: real CVE risk.
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## 🧪 검증 / 중복
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- Verified (Hennessy & Patterson 6th, Agner Fog optimization manuals, Intel SDM).
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- 신뢰도 A.
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## 🕓 Changelog
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| 날짜 | 변경 |
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|---|---|
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| 2026-05-08 | Phase 1 |
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| 2026-05-10 | Manual cleanup — predictor types, branchless patterns, Spectre |
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