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Optimizing Indirections, using abstractions without remorse - Johannes Doerfert

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Indirections, either through memory, library interfaces, or function pointers, can easily induce major performance penalties as the current optimization pipeline is not able to look through them. The available inter-procedural-optimizations (IPO) are generally not well suited to deal with these issues as they require all code to be available and analyzable through techniques based on tracking value dependencies. Importantly, the use of class/struct objects and (parallel) runtime libraries commonly introduce indirections that prohibit basically all optimizations. In this talk, we introduce these problems with real-world examples and show how new analyses can mitigate them. We especially focus on:

- A field-sensitive, inter-procedural memory analysis that models simple communication through memory. - The integration of indirect, potentially hidden, call sites, e.g., in libraries like the OpenMP runtime library, into existing analysis and optimizations (function attribute detection, argument promotion, …). - Automatic and portable (non-LLVM specific) information transfer from library implementations through their interfaces to the user sites.

While our work originates in the optimization of parallel code, we want to show how the problems we encountered there are similar to existing ones in sequential programs. To this end, we try to augment the available analyses and optimizations rather than introducing new ones that are specific to parallel programs. As a consequence, we not only expect positive results for parallel code regions [1], but also hope to improve generic code that employs indirections or simply exhibit optimization opportunities similar to the ones that commonly arise for parallel programs.

The goal of this talk is to introduce possible solutions to several problems that commonly prevent optimization of code featuring indirections. As we want to introduce these solutions into the LLVM codebase, we hope to start a discussion on these issues as well as the caveats that we encountered while resolving them.

[1] [ Ссылка ]
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Videos Filmed & Edited by Bash Films: [ Ссылка ]

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