Matt Godbolt, author of godbolt.org Compiler Explorer, discusses disassembling language designs including reference counting optimizations, destructors and unwinding, and defending the decision of NaN != NaN. They also explore the benefits of Dwarf Debug Format, stack unwinding, memory allocation strategies, and the logic behind NaN in programming. Additionally, they touch on reference counting implementation and advantages of atomic instructions for thread sharing.
Compiler Explorer disassembles and maps compiled programs in various languages, providing mappings between high-level code and assembly instructions.
The podcast explores language design optimizations, including reference counting, destructors, unwinding, and NaN design decisions.
The discussion covers memory management strategies, such as immutability in the Rock language, and reducing reference counting overhead through static analysis and size-based optimizations.
Deep dives
The Compiler Explorer Tool for Disassembling Programs
The podcast episode discusses the Godbolt.org compiler explorer tool, which disassembles and maps compiled programs in various languages. It allows users to input code in a high-level programming language and view the corresponding assembly code. The tool also provides mappings between functions, individual lines of code, statements, and assembly instructions. This mapping is made possible through the compiler's output formats and debug information, which specify the correspondence between source code lines and assembly instructions.
Disassembling Language Designs and Optimizations
The episode delves into the disassembly of language designs and optimizations. It explores reference counting optimizations, destructors, unwinding, and the design decision related to NaN (not a number) values. The podcast highlights that Compiler Explorer is built on the compilers' output formats and their knowledge of how to attribute assembly instructions to specific lines of source code. The episode also emphasizes that many of the features and behaviors of the tool are driven by the compiler itself, with Compiler Explorer being a front end to the compilers.
Understanding the Debug Format Dwarf
The podcast covers the Dwarf debugging format, which allows compilers to explain to debuggers how to decode aspects of compiled files. The hosts discuss the intricacies of the Dwarf format, which supports features like specifying variable locations, complicated memory address expressions, and unwinding instructions for exception handling. While the exact details of Dwarf were not extensively covered, the episode acknowledges the complexity of the format and its ability to describe various facets of compiled files.
Memory Management and Exception Handling in Rock
The conversation turns towards memory management and exception handling in the Rock programming language. The episode highlights that Rock is a purely functional language where everything is semantically immutable. This immutability eliminates the need for reference counting cycle detection since cyclic data structures cannot be created in the language. The hosts further discuss how Rock manages memory and exceptions through different strategies, including using separate heaps for memory management and leveraging unique types to enforce mutation safety.
Opportunities for Statically Eliminating Reference Counts
The podcast episode explores the idea of eliminating reference counts statically to reduce overhead. The hosts discuss the potential for borrow annotations and borrow inference to determine when reference counting is unnecessary. They highlight the advantage of reference counting over tracing garbage collectors, as it allows for more opportunities to make the compiler smarter and eliminate reference counts. The hosts also mention tricks like page protection for long-lived objects and the potential to make the compiler smarter. Overall, the discussion revolves around the goal of reducing reference counting overhead through static analysis and intelligent optimizations.
Improving Performance with Size-based Optimizations
Another topic discussed in the podcast is the potential for size-based optimizations to improve performance. The hosts touch upon ideas like tight packing reference counts in a separate page to enhance cache coherency and reduce memory access time. They also explore the concept of leveraging read-only sections of the binary or global sections for storing reference counts. The discussion extends to potential trade-offs and strategies for efficient memory management. The hosts mention techniques used in other languages like C++ and Rust, such as passing references instead of actual shared references. The goal is to find ways to eliminate or reduce the need for runtime reference counting, making the language more efficient and cache-friendly.
Richard talks with Matt Godbolt, author of the godbolt.org Compiler Explorer, about how certain aspects of the Compiler Explorer work, as well as "disassembling" language designs themselves - talking about reference counting optimizations, destructors and unwinding, and even defending the infamous design decision of NaN != NaN.
