branch-prediction-and-speculation

Branch Prediction and Speculation

Purpose

Explain how modern CPUs predict branches, execute speculatively, recover on mispredict, and why speculation created side channels (Spectre/Meltdown) — linking performance tuning with security-aware low-level coding.

When to Use

• Hot loop branchy code underperforms expectations
• Evaluating likely/unlikely or branchless refactors
• Understanding kernel retpoline, IBRS, and similar mitigations
• Secure coding around secret-dependent branches

Workflow

1. Branch predictor basics

Fetch sees conditional branch
├── Predict direction (taken / not-taken)
├── Speculatively execute predicted path
└── On resolve:
    ├── Correct → commit, ~0 penalty (deep pipelines still cost on mispredict)
    └── Wrong → squash, refill from correct PC (10–20+ cycles typical)

Patterns: backward branches often predicted taken (loops); forward not-taken.

2. Performance patterns

/* Predictable — tight loop backward branch */
for (int i = 0; i < n; i++)
    sum += a[i];

/* Unpredictable — data-dependent */
if (data[i] > threshold)  /* hard to predict */
    rare_path();

Techniques: branchless cmov/select, lookup tables, sorting data to reduce branches, splitting hot/cold paths.

/* Branchless min (integer) */
int m = a < b ? a : b;  /* compiler may lower to cmov */

3. Compiler hints (use sparingly)

#define likely(x)   __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)

if (unlikely(ptr == NULL))
    return -EINVAL;

Measure with perf — hints are not magic on modern predictors.

4. Speculative execution and side channels

CPUs may execute instructions before branch outcome is known. If speculated path touches secret-dependent memory, cache state can leak (Spectre variant 1).

Mitigations (high level):

• Kernel: retpoline, IBRS/IBPB, STIBP (see kernel cpu_show_mitigations)
• Compiler: speculative load hardening (-mspeculative-load-hardening on Clang)
• Code: constant-time crypto — no secret-dependent branches or indices