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C++ Modules in ROOT and Beyond
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Abstract
C++ Modules come in C++20 to fix the long-standing build scalability problems in the language. They provide an io-efficient, on-disk representation capable to reduce build times and peak memory usage. ROOT employs the C++ modules technology further in the ROOT dictionary system to improve its performance and reduce the memory footprint. ROOT with C++ Modules was released as a technology preview in fall 2018, after intensive development during the last few years. The current state is ready for production, however, there is still room for performance optimizations. In this talk, we show the roadmap for making the technology default in ROOT. We demonstrate a global module indexing optimization which allows reducing the memory footprint dramatically for many workflows. We will report user feedback on the migration to ROOT with C++ Modules.
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ROOT comes with a C++ compliant interpreter cling. Cling needs to understand the content of the libraries in order to interact with them. Exposing the full shared library descriptors to the interpreter at runtime translates into increased memory footprint. ROOT's exploratory programming concepts allow implicit and explicit runtime shared library loading. It requires the interpreter to load the library descriptor. Re-parsing of descriptors' content has a noticeable effect on the runtime performance. Present state-of-art lazy parsing technique brings the runtime performance to reasonable levels but proves to be fragile and can introduce correctness issues. An elegant solution is to load information from the descriptor lazily and in a non-recursive way.
The LLVM community advances its C++ Modules technology providing an io-efficient, on-disk representation capable to reduce build times and peak memory usage. The feature is standardized as a C++ technical specification. C++ Modules are a flexible concept, which can be employed to match CMS and other experiments' requirement for ROOT: to optimize both runtime memory usage and performance. Cling technically " inherits " the feature, however tweaking it to ROOT scale and beyond is a complex endeavor. The paper discusses the status of the C++ Modules in the context of ROOT, supported by few preliminary performance results. It shows a step-by-step migration plan and describes potential challenges which could appear.
Cling is an interactive C++ interpreter, built on top of Clang and LLVM compiler infrastructure. Like its predecessor Cint, Cling realizes the read-print-evaluate-loop concept, in order to leverage rapid application development. Implemented as a small extension to LLVM and Clang, the interpreter reuses their strengths such as the praised concise and expressive compiler diagnostics. We show how to match the interpreter concept to the compiler library and generalize common set of requirements for building up an interactive interpreter. We reason the design and implementation decisions as solution to the challenge of implementing interpreter behaviour as an extension of the compiler library. We present the new features, e.g. how C++11 will come to Cling and how Cint-specific extensions are being adopted. We clarify the state of integration in the ROOT framework and the induced change set. We explain how ROOT dictionaries are simplified due to the new interpreter.
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