branch-prediction-and-speculation

Name: branch-prediction-and-speculation
Author: mohitmishra786

141stars19forks

How to install branch-prediction-and-speculation?

mohitmishra786/low-level-dev-skills/branch-prediction-and-speculation

$npx -y skills add mohitmishra786/low-level-dev-skills --skill branch-prediction-and-speculation

Installs into the current project.

Summary

Branch prediction and speculation skill for CPU security and performance. Use when explaining branch predictors, mispredict penalties, speculative execution, Spectre/Meltdown mitigations, or branchless patterns. Activates on queries about branch prediction, speculative execution,

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SKILL.md

1	# Branch Prediction and Speculation
2
3	## Purpose
4
5	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.
6
7	## When to Use
8
9	- Hot loop branchy code underperforms expectations
10	- Evaluating `likely`/`unlikely` or branchless refactors
11	- Understanding kernel `retpoline`, IBRS, and similar mitigations
12	- Secure coding around secret-dependent branches
13
14	## Workflow
15
16	### 1. Branch predictor basics
17
18	```
19	Fetch sees conditional branch
20	├── Predict direction (taken / not-taken)
21	├── Speculatively execute predicted path
22	└── On resolve:
23	├── Correct → commit, ~0 penalty (deep pipelines still cost on mispredict)
24	└── Wrong → squash, refill from correct PC (10–20+ cycles typical)
25	```
26
27	Patterns: backward branches often predicted taken (loops); forward not-taken.
28
29	### 2. Performance patterns
30
31	```c
32	/* Predictable — tight loop backward branch */
33	for (int i = 0; i < n; i++)
34	sum += a[i];
35
36	/* Unpredictable — data-dependent */
37	if (data[i] > threshold) /* hard to predict */
38	rare_path();
39	```
40
41	Techniques: branchless `cmov`/select, lookup tables, sorting data to reduce branches, splitting hot/cold paths.
42
43	```c
44	/* Branchless min (integer) */
45	int m = a < b ? a : b; /* compiler may lower to cmov */
46	```
47
48	### 3. Compiler hints (use sparingly)
49
50	```c
51	#define likely(x) __builtin_expect(!!(x), 1)
52	#define unlikely(x) __builtin_expect(!!(x), 0)
53
54	if (unlikely(ptr == NULL))
55	return -EINVAL;
56	```
57
58	Measure with `perf` — hints are not magic on modern predictors.
59
60	### 4. Speculative execution and side channels
61
62	CPUs may execute instructions before branch outcome is known. If speculated path touches secret-dependent memory, cache state can leak (Spectre variant 1).
63
64	Mitigations (high level):
65
66	- Kernel: retpoline, IBRS/IBPB, STIBP (see kernel `cpu_show_mitigations`)
67	- Compiler: speculative load hardening (`-mspeculative-load-hardening` on Clang)
68	- Code: constant-time crypto — no secret-dependent branches or indices
69
70	Meltdown (Intel): user load from kernel mapping — fixed by KPTI (separate page tables).
71
72	### 5. Measurement
73
74	```bash
75	perf stat -e branches,branch-misses ./app
76	```
77
78	High `branch-misses` ratio → investigate hot branches.
79
80	### 6. Agent usage
81
82	```
83	/branch-prediction-and-speculation Make this comparison function constant-time against Spectre-style leakage
84	```
85
86	## Common Problems
87
88	\| Symptom \| Cause \| Fix \|
89	\|---------\|-------\|-----\|
90	\| Loop slower than expected \| Mispredicted exit \| Peel iterations; branchless tail \|
91	\| `likely` no help \| Predictor already good \| Profile first \|
92	\| Secret leak in crypto \| Branches on secret bytes \| Constant-time algorithms \|
93	\| Mitigation regression \| KPTI/retpoline overhead \| Accept or isolate secrets \|
94	\| "Branchless" slower \| CMOV still executes both \| Benchmark on target CPU \|
95
96	## Related Skills
97
98	- `skills/computer-architecture/cpu-pipelines-and-hazards` — control hazards
99	- `skills/security/kernel-security` — KPTI, CET, speculation mitigations
100	- `skills/low-level-programming/cpu-cache-opt` — cache timing channels
101	- `skills/profilers/hardware-counters` — branch-misses event
102	- `skills/runtimes/binary-hardening` — userspace hardening overlap

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Type

branch-prediction-and-speculation

141stars19forks

View on GitHub

How to install branch-prediction-and-speculation?

mohitmishra786/low-level-dev-skills/branch-prediction-and-speculation

$npx -y skills add mohitmishra786/low-level-dev-skills --skill branch-prediction-and-speculation

Installs into the current project.

Summary

Files · 1

View on GitHub

SKILL.md

1	# Branch Prediction and Speculation
2
3	## Purpose
4
5	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.
6
7	## When to Use
8
9	- Hot loop branchy code underperforms expectations
10	- Evaluating `likely`/`unlikely` or branchless refactors
11	- Understanding kernel `retpoline`, IBRS, and similar mitigations
12	- Secure coding around secret-dependent branches
13
14	## Workflow
15
16	### 1. Branch predictor basics
17
18	```
19	Fetch sees conditional branch
20	├── Predict direction (taken / not-taken)
21	├── Speculatively execute predicted path
22	└── On resolve:
23	├── Correct → commit, ~0 penalty (deep pipelines still cost on mispredict)
24	└── Wrong → squash, refill from correct PC (10–20+ cycles typical)
25	```
26
27	Patterns: backward branches often predicted taken (loops); forward not-taken.
28
29	### 2. Performance patterns
30
31	```c
32	/* Predictable — tight loop backward branch */
33	for (int i = 0; i < n; i++)
34	sum += a[i];
35
36	/* Unpredictable — data-dependent */
37	if (data[i] > threshold) /* hard to predict */
38	rare_path();
39	```
40
41	Techniques: branchless `cmov`/select, lookup tables, sorting data to reduce branches, splitting hot/cold paths.
42
43	```c
44	/* Branchless min (integer) */
45	int m = a < b ? a : b; /* compiler may lower to cmov */
46	```
47
48	### 3. Compiler hints (use sparingly)
49
50	```c
51	#define likely(x) __builtin_expect(!!(x), 1)
52	#define unlikely(x) __builtin_expect(!!(x), 0)
53
54	if (unlikely(ptr == NULL))
55	return -EINVAL;
56	```
57
58	Measure with `perf` — hints are not magic on modern predictors.
59
60	### 4. Speculative execution and side channels
61
62	CPUs may execute instructions before branch outcome is known. If speculated path touches secret-dependent memory, cache state can leak (Spectre variant 1).
63
64	Mitigations (high level):
65
66	- Kernel: retpoline, IBRS/IBPB, STIBP (see kernel `cpu_show_mitigations`)
67	- Compiler: speculative load hardening (`-mspeculative-load-hardening` on Clang)
68	- Code: constant-time crypto — no secret-dependent branches or indices
69
70	Meltdown (Intel): user load from kernel mapping — fixed by KPTI (separate page tables).
71
72	### 5. Measurement
73
74	```bash
75	perf stat -e branches,branch-misses ./app
76	```
77
78	High `branch-misses` ratio → investigate hot branches.
79
80	### 6. Agent usage
81
82	```
83	/branch-prediction-and-speculation Make this comparison function constant-time against Spectre-style leakage
84	```
85
86	## Common Problems
87
88	\| Symptom \| Cause \| Fix \|
89	\|---------\|-------\|-----\|
90	\| Loop slower than expected \| Mispredicted exit \| Peel iterations; branchless tail \|
91	\| `likely` no help \| Predictor already good \| Profile first \|
92	\| Secret leak in crypto \| Branches on secret bytes \| Constant-time algorithms \|
93	\| Mitigation regression \| KPTI/retpoline overhead \| Accept or isolate secrets \|
94	\| "Branchless" slower \| CMOV still executes both \| Benchmark on target CPU \|
95
96	## Related Skills
97
98	- `skills/computer-architecture/cpu-pipelines-and-hazards` — control hazards
99	- `skills/security/kernel-security` — KPTI, CET, speculation mitigations
100	- `skills/low-level-programming/cpu-cache-opt` — cache timing channels
101	- `skills/profilers/hardware-counters` — branch-misses event
102	- `skills/runtimes/binary-hardening` — userspace hardening overlap

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6 picks

Type