Chris Lattner: Compilers, LLVM, Swift, TPU, and ML Accelerators | Lex Fridman Podcast #21 | Summary and Q&A

May 13, 2019
Lex Fridman Podcast
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Chris Lattner: Compilers, LLVM, Swift, TPU, and ML Accelerators | Lex Fridman Podcast #21


Chris Ladner, a senior director at Google, talks about his expertise in compiler technologies and his involvement in projects such as CPU, GPU, and TPU accelerators for machine learning. LLVM, the compiler infrastructure project he created, has had a significant impact on various languages and hardware. He also discusses his work on Swift and the challenges of optimizing code.

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Key Insights

  • 👨‍💻 Compilers allow programmers to write code at a high level of abstraction, which is then converted into machine code for specific hardware.
  • ❓ LLVM has provided a common optimization infrastructure for various programming languages, resulting in improved performance and hardware support.
  • 👶 Swift was developed as a new language to address the limitations of Objective-C, with a focus on safety, modern tooling, and a developer-friendly experience.
  • 🙈 The interplay between hardware and software is essential in developing efficient systems, as seen in the collaboration between TPUs and machine learning algorithms.


the following is a conversation with Chris flattener currently he's a senior director of Google working on several projects including CPU GPU TPU accelerators for tensorflow swift for tensorflow and all kinds of machine learning compiler magic going on behind the scenes he's one of the top experts in the world on compiler technologies which means h... Read More

Questions & Answers

Q: What is the purpose of a compiler?

Compilers bridge the gap between human-written code and machine-executable code, enabling programmers to write at a higher level of abstraction. They convert the code into a format that can be executed on specific hardware.

Q: What are the main components of a compiler?

A typical compiler consists of a front-end, which is language-specific and handles parsing and creating an abstract syntax tree, an optimizer, which applies various transformations to improve performance, and a back-end, which generates the final machine code based on the target hardware.

Q: How has LLVM impacted the development of different programming languages?

LLVM has standardized the middle and last parts of the compiler, allowing different languages like Swift, C, C++, Objective-C, and Rust to use the same optimization infrastructure. This has led to improved performance and code generation support for various hardware platforms.

Q: What were the main challenges in developing Swift as a new programming language?

Swift started as an attempt to improve upon Objective-C by introducing better error messages, compile-time performance, and modern tooling. The challenge was convincing the Apple community, which had a strong attachment to Objective-C, that Swift could provide a better developer experience. Swift's focus on type safety and its progressive disclosure of complexity played key roles in gaining acceptance.


In this conversation, Lex Friedman talks with Chris Lattner, a senior director at Google and one of the top experts in compiler technologies. They discuss Lattner's journey with programming languages, the creation of LLVM and Clang, the challenges of optimizing code, the impact of Java and JavaScript, the use of machine learning in optimization, and the success of LLVM in the industry.

Questions & Answers

Q: What was the first program Lattner ever wrote and when?

Lattner's first program was written when he was a kid, typing out programs from a basic programming book and trying to figure out why they didn't work.

Q: Which programming language did Lattner connect with the most?

Lattner started with BASIC and then moved on to GW-BASIC, QBasic, and QuickBASIC. He eventually learned Pascal, C, and C++, but also explored other languages like Smalltalk. He chose the path of low-level languages and assembly instead of higher-level functional languages like Lisp.

Q: When did Lattner start learning C and C++?

Lattner learned C and C++ in high school as a way to do more powerful things than what Pascal could do. He found Pascal confusing due to pointers and the syntax of C and C++ took some time to grasp. While Pascal had memory management, C and C++ made Lattner think more about how things were laid out in memory.

Q: What is a compiler and what is it used for?

A compiler is a tool that allows humans to write code at a level of abstraction they are comfortable with and then translates that code into machine-readable form for the hardware to execute. It takes code written by humans and converts it into a format that machines can understand and execute. Compilers bridge the gap between programming languages and hardware.

Q: Can Lattner explain what LLVM is and how he became its creator and lead developer?

LLVM (Low-Level Virtual Machine) is a compiler infrastructure and a community. It is a bunch of code that people reuse to build compilers, and it allows different languages to compile through it. Lattner became involved with LLVM during his time as a graduate student at the University of Illinois, where he started as a research project. He later brought it to Apple and worked on products like OpenGL, the C compiler (Clang), and Swift, building a team of amazing compiler engineers along the way. After gaining momentum, others, including Google and Nvidia, started contributing to LLVM.

Q: Is LLVM a standard or an implementation?

LLVM is an implementation. It is a bunch of code that people reuse to build their compilers. It is also a community of hundreds of people collaborating on shared infrastructure. LLVM has successfully brought competitors in the industry together to collaborate on building a great shared infrastructure.

Q: How does the LLVM community collaborate on such a complex project?

The LLVM community includes hundreds of people collaborating and sharing their expertise. Despite being competitors in the market, companies like Google, Apple, AMD, Intel, Nvidia, and more come together to make LLVM better because it is in their commercial interest to have a great infrastructure to build on. The community collaborates on code ownership, code review, and the overall direction of the project.

Q: Did Lattner expect LLVM to become as successful as it is today?

When Lattner started working on LLVM, he never expected it to become as big as it is today. His original goal was to get in and out with a non-thesis master's degree in a year and then get back to work. But he ended up staying and having fun building cool stuff, learning interesting things, and working with a team. He does not regret staying and helping create LLVM.

Q: What are some of the challenges in optimizing code with LLVM?

One of the challenges in optimizing code is register allocation, which involves deciding which values get put in registers at what points in the program. Another challenge is scheduling, which involves rearranging instructions to keep the processor's pipelines full and prevent stalling. These are just a few examples, but there are many complex algorithms and techniques involved in optimizing code.

Q: Has machine learning been applied to code optimization?

Machine learning has been applied to code optimization in research systems and small problem spaces, such as optimizing matrix multiplication for GPUs. However, machine learning techniques are still being explored and more research is needed to effectively apply them to the field of compiler optimization.


The conversation with Chris Lattner provides insights into the world of compilers and optimizations. LLVM, created by Lattner during his time at the University of Illinois, is an implementation and a community of people collaborating on shared infrastructure. LLVM has brought competitors in the industry together to create a powerful compiler framework. The challenges of optimizing code involve complex algorithms, such as register allocation and scheduling. Machine learning has the potential to enhance code optimization, but further research is needed. Overall, LLVM has revolutionized the field of compiler technologies and its impact is seen in various industries.

Summary & Key Takeaways

  • Chris Ladner discusses his journey as a compiler expert, starting with his first programming experiences with languages like BASIC and Pascal.

  • He explains the purpose of compilers, which enable humans to write code at a higher level of abstraction and then convert it into machine code.

  • Ladner details the creation and significance of LLVM, a compiler infrastructure project that allows different languages to use the same optimization and code generation platform, resulting in improved performance and hardware support.

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