In a remarkable revelation, computer scientists from UC San Diego and Purdue University have discovered an undocumented security feature in Intel processors that offers robust protection against notorious vulnerabilities like Spectre.

This groundbreaking study, titled “Half&Half: Demystifying Intel’s Directional Branch Predictors for Fast, Secure Partitioned Execution,” has provided valuable insights into the inner workings of Intel’s processors, particularly their conditional branch predictors.

This newfound knowledge has significant implications for computer security, offering enhanced protection against critical attacks.

Understanding Branch Predictors and Security Vulnerabilities

Modern processors utilize branch predictors to optimize processing speed by anticipating the outcome of conditional branches.

However, existing processors share the branch predictor among all threads and processes, leading to severe security vulnerabilities.

Malicious actors can exploit these vulnerabilities to extract confidential data, such as passwords and encryption keys, by observing branch outcomes.

Spectre attacks, in particular, leverage the branch predictor to inject data and extract sensitive information from memory.

Reverse-Engineering Intel’s Branch Predictors

The researchers successfully reverse-engineered Intel’s flagship processors, unraveling the structures, sizes, and lookup functions of their conditional branch predictors, even those introduced over a decade ago.

Intel’s branch predictor consists of four tables, with complex hashing techniques based on data collected from previous branch instances.

Surprisingly, the researchers discovered that modifying a single bit of the branch address could effectively partition the branch predictor into two parts.

Half&Half Approach: Eliminating Data Leakage and Thwarting Spectre Attacks

The discovery made by Hosein Yavarzadeh, a PhD student in Computer Science and Engineering at UC San Diego, has led to a groundbreaking revelation in the field of cybersecurity. By implementing a minor alteration in code generation, it is now possible to concurrently execute two threads on a single processor core. This innovative approach effectively eliminates data leakage through the branch predictor and provides a robust defense against Spectre attacks.

Professor Dean Tullsen, a notable figure in the CSE department, expressed his astonishment at this remarkable finding. He emphasized the intricate nature of the indexing functions involved and the seemingly isolated behavior of a single bit without any interdependencies. Furthermore, Professor Tullsen remarked on the unexpected nature of this security-enhancing capability, as it has been present in every major Intel processor for over a decade, largely unnoticed in terms of its potential security implications.

Kazem Taram, an esteemed professor at Purdue University and an alumnus of the CSE department, shared his enthusiasm regarding the newfound understanding of branch predictors.

These components, particularly the conditional branch predictor, have long been shrouded in secrecy, posing a significant challenge for both security and performance researchers to decipher their inner workings.

The groundbreaking research conducted by the authors of the Half&Half paper has shed light on the functionality and intricate structure of branch predictors. This newfound knowledge not only has immense implications for enhancing security measures but also paves the way for further advancements in the field of cybersecurity.

Balancing Security and Performance

Previous software techniques aimed at achieving conditional branch isolation between threads incurred significant performance overhead.

In contrast, the Half&Half approach offers the same level of security protection against branch predictor leaks with a minimal performance cost of only 2 to 5 percent.

This achievement was made possible through a straightforward modification to compiler code generation.

The Half&Half approach represents a substantial breakthrough in computer security, demonstrating how a small change can yield significant results.

Implications and Future Prospects

The discovery of this hidden security feature in Intel processors presents a significant advancement in computer security.

It sheds light on the previously mysterious nature of branch predictors, which were regarded as the most challenging component to reverse-engineer.

The newfound insights into the functionality and detailed structure of branch predictors open doors for both security and performance researchers, offering opportunities for further advancements in safeguarding sensitive data from malicious attacks like Spectre.


The Half&Half approach serves as a powerful tool in the fight against critical vulnerabilities, providing robust protection against attacks like Spectre. With a minimal performance cost and straightforward implementation, this hidden security feature in Intel processors holds great promise for enhancing computer security and safeguarding sensitive information. As researchers delve deeper into the intricacies of processors, we can expect further breakthroughs that will fortify the defenses against evolving cyber threats.