Why Microsoft, GCC, and Clang Use the Itanium C ABI

The choice of the Itanium C ABI (Application Binary Interface) by GCC and Clang, while Microsoft's Visual C (MSVC) uses a different approach, is a complex history rooted in the needs of different software ecosystems and compiler teams. This article delves into the reasons behind this choice and its implications for software development.

The CABI Landscape: GCC and Clang

The Itanium C ABI was designed in the late 1990s as a response to the emerging Itanium (IA-64) architecture. At that time, various Unix vendors were considering migrating to Itanium. This led to the development of the CABI (C Application Binary Interface) to standardize the ABI for C on the Itanium architecture. The GCC (GNU Compiler Collection) team chose to adopt this as their default ABI, leading to its widespread use in the Unix and Linux environments.

The Born Stack and Its Challenges

When Bjarne Stroustrup first implemented the C programming language, he designed a straightforward yet inefficient ABI. Over time, this ABI was refined, but other C compiler vendors, including Microsoft in the 1990s, developed their own ABIs to address specific inefficiencies.

The GCC team's adoption of the Itanium ABI was driven by the technological landscape of the time. They recognized that an efficient and standardized ABI would be beneficial for their compiler and the broader ecosystem. Although Itanium itself did not achieve widespread adoption, the GCC ABI has since become a staple in Unix and Linux environments.

The Adoption by Clang

Clang, the open-source C, C , and Objective-C compiler, emerged from Apple's acquisition of NeXT, which used GCC as its system compiler. When Apple transitioned to using Clang, they also adopted the Itanium ABI to ensure compatibility with GCC-produced object code. This binary compatibility was crucial for maintaining the ecosystem and ensuring seamless integration in software projects.

Microsoft's Choice: Continuity and Control

Microsoft's decision not to adopt the Itanium ABI for MSVC was largely a matter of maintaining continuity with their existing 32-bit ABI. This ABI was already efficient and had established behavior relied upon by a significant amount of the software ecosystem. Changing to the Itanium ABI would have required a comprehensive rewrite of many existing applications, which was not a feasible option.

When Microsoft needed to develop 64-bit and ARM ABIs, they chose to maintain their existing ABI to avoid disrupting their dominant position in the market. Keeping the control and alignment with their own platform was more aligned with their strategic goals.

Backwards Compatibility and Evolution

Backwards compatibility plays a crucial role in the decision-making process of compiler teams. Microsoft, for instance, aimed to maintain compatibility with their legacy code base, which was why they did not switch to the Itanium ABI. GCC and Clang, on the other hand, prioritized standardization and efficiency, leading them to adopt the Itanium ABI.

The diversity of ABIs is a testament to the evolving nature of software development and the complexity of ensuring compatibility across different platforms and ecosystems. While backward compatibility is important, it is not always the primary consideration for all compiler teams.

Conclusion

The choice to use the Itanium C ABI by GCC and Clang, while MSVC chooses a different path, is a reflection of the historical development of these ecosystems and the strategic decisions made by their respective teams. The Itanium ABI has become a key component of the Unix and Linux development landscape, while Microsoft's choice aligns with their goals of maintaining control and compatibility within their ecosystem.