---
id: wiki-2026-0508-branch-prediction
title: Branch Prediction
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [P-Reinforce-AUTO-4D7707, CPU Branch Predictor]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [cpu, performance, security, microarchitecture]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: cpp
  framework: none
---

# Branch Prediction

## 매 한 줄
> **"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.

## 매 핵심

### 매 Predictor 종류
- **Static**: forward-not-taken, backward-taken (hint).
- **Bimodal (2-bit)**: per-PC saturating counter.
- **Local history**: per-branch shift register.
- **Global history (gshare/GAg)**: shared GHR XOR PC.
- **Tournament**: meta-predictor selects local vs global.
- **TAGE / ITTAGE**: tagged geometric history (modern SOTA).
- **Perceptron**: AMD Zen — neural-style predictor.

### 매 Indirect Branches
- BTB (Branch Target Buffer) — caches target.
- ITTAGE for indirect.
- Critical for vtables, function pointers, switch.

### 매 응용
1. Hot loops — predictable branches → near-zero penalty.
2. Sorted-data effect (the famous SO question).
3. Spectre/BranchScope side-channel attacks.
4. JIT branch ordering decisions (V8, Hotspot).

## 💻 패턴

### Likely / Unlikely Hints (C++20)
```cpp
if (x > 0) [[likely]] {
    fast_path();
} else [[unlikely]] {
    slow_path();
}
```

### Branchless via Mask
```cpp
// branchful
int abs_v(int x) { return x < 0 ? -x : x; }

// branchless
int abs_v_nb(int x) {
    int mask = x >> 31;          // 0 or -1
    return (x ^ mask) - mask;
}
```

### Branchless Min via cmov
```cpp
int min_v(int a, int b) {
    return a < b ? a : b;   // compilers emit cmov on x86
}
```

### Sort Before Loop (classic)
```cpp
std::sort(data.begin(), data.end());  // 정렬 → branch predictable
long sum = 0;
for (int v : data) if (v >= 128) sum += v;
```

### LLVM `__builtin_expect`
```cpp
if (__builtin_expect(error_flag, 0)) {
    handle_error();
}
```

### Spectre v1 Mitigation (lfence)
```cpp
// vulnerable
if (idx < arr_size) {
    secret = arr[idx];           // mispredict → leaks
}
// hardened
if (idx < arr_size) {
    asm volatile("lfence" ::: "memory");
    secret = arr[idx];
}
```

### perf Branch Stats
```bash
perf stat -e branches,branch-misses ./bench
# branch-miss rate > 5% → suspect; > 10% → critical
```

## 매 결정 기준
| 상황 | 전략 |
|---|---|
| Hot loop, predictable | Trust predictor — natural code |
| Hot loop, unpredictable | Branchless / mask / cmov |
| Cold path | Doesn't matter — clarity > tricks |
| Indirect-heavy (vtable) | Devirtualize, monomorphize |
| Security-sensitive | lfence / speculative load hardening |

**기본값**: write clear branchful code; branchless only when profiler shows misprediction hotspot.

## 🔗 Graph
- 부모: [[Pipeline]]
- 응용: [[JIT Compilation]]
- Adjacent: [[Spectre]] · [[Speculative Execution]]

## 🤖 LLM 활용
**언제**: explain mispredict cost, generate branchless equivalents, suggest hints.
**언제 X**: micro-arch tuning without perf data — speculation hurts.

## ❌ 안티패턴
- **Random unpredictable branch in hot loop**: 20-30% slowdown.
- **Premature branchless conversion**: hurts cold/clarity paths.
- **Trusting `[[likely]]` blindly**: PGO data > human guess.
- **Ignoring Spectre in untrusted-input parsers**: real CVE risk.

## 🧪 검증 / 중복
- Verified (Hennessy & Patterson 6th, Agner Fog optimization manuals, Intel SDM).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — predictor types, branchless patterns, Spectre |