Hi, all I want to propose and apply for the following GSoC project: Automatic Detection of Parallel Compilation Viability.
https://www.ime.usp.br/~belinass/Automatic_Detection_of_Parallel_Compilation_Viability.pdf Feedback is welcome :) Here is a markdown version of it: **Automatic Detection of Parallel Compilation Viability** [Giuliano Belinassi]{style="color: darkgreen"}\ Timezone: GMT$-$3:00\ University of São Paulo -- Brazil\ IRC: giulianob in \#gcc\ Email: [`giuliano.belina...@usp.br`](mailto:giuliano.belina...@usp.br)\ Github: <https://github.com/giulianobelinassi/>\ Date: About Me Computer Science Bachelor (University of São Paulo), currently pursuing a Masters Degree in Computer Science at the same institution. I've always been fascinated by topics such as High-Performance Computing and Code Optimization, having worked with a parallel implementation of a Boundary Elements Method software in GPU. I am currently conducting research on compiler parallelization and developing the [ParallelGcc](https://gcc.gnu.org/wiki/ParallelGcc) project, having already presented it in [GNU Cauldron 2019](https://www.youtube.com/watch?v=jd6R3IK__1Q). **Skills**: Strong knowledge in C, Concurrency, Shared Memory Parallelism, Multithreaded Debugging and other typical programming tools. Brief Introduction In [ParallelGcc](https://gcc.gnu.org/wiki/ParallelGcc), we showed that parallelizing the Intra Procedural optimizations improves speed when compiling huge files by a factor of 1.8x in a 4 cores machine, and also showed that this takes 75% of compilation time. In this project we plan to use the LTO infrastructure to improve compilation performance in the non-LTO case, with a tradeoff of generating a binary as good as if LTO is disabled. Here, we will automatically detect when a single file will benefit from parallelism, and proceed with the compilation in parallel if so. Use of LTO The Link Time Optimization (LTO) is a compilation technique that allows the compiler to analyse the program as a whole, instead of analysing and compiling one file at time. Therefore, LTO is able to collect more information about the program and generate a better optimization plan. LTO is divided in three parts: - *LGEN (Local Generation)*: Each file is translated to GIMPLE. This stage runs sequentially in each file and, therefore, in parallel in the project compilation. - *WPA (Whole Program Analysis)*: Run the Inter Procedural Analysis (IPA) in the entire program. This state runs serially in the project. - *LTRANS (Local Transformation)*: Execute all Intra Procedural Optimizations in each partition. This stage runs in parallel. Since WPA can bottleneck the compilation because it runs serially in the entire project, LTO was designed to produce faster binaries, not to produce binaries fast. Here, the proposed use of LTO to address this problem is to run the IPA for each Translation Unit (TU), instead in the Whole Program, and automatically detect when to partition the TU into multiple LTRANS to improve performance. The advantage of this approach is: - It can generate binaries as good as when LTO is disabled. - It is faster, as we can partition big files into multiple partitions and compile these partitions in parallel - It can interact with GNU Make Jobserver, improving CPU utilization. Planned Tasks I plan to use the GSoC time to develop the following topics: - Week \[1, 3\] -- April 27 to May 15:\ Update `cc1`, `cc1plus`, `f771`, ..., to partition the data after IPA analysis directly into multiple LTRANS partitions, instead of generating a temporary GIMPLE file. - Week \[4, 7\] -- May 18 to June 12:\ Update the `gcc` driver to take these multiple LTRANS partitions, then call the compiler and assembler for each of them, and merge the results into one object file. Here I will use the LTO LTRANS object streaming, therefore it should interact with GNU Make Jobserver. - Week 8 -- June 15 to 19: **First Evaluation**\ Deliver a non-optimized version of the project. Some programs ought to be compiled correctly, but probably there will be a huge overhead because so far there will not be any criteria about when to partition. Some tests are also planned for this evaluation. - Week \[9, 11\] -- June 22 to July 10:\ Implement a criteria about when to partition, and interactively improve it based on data. - Week 12 --- July 13 to 17: **Second Evaluation**\ Deliver a more optimized version of the project. Here we should filter files that would compile fast from files that would require partitioning, and therefore we should see some speedup. - Week \[13, 15\] --- July 20 to August 10:\ Develop adequate tests coverage and address unexpected issues so that this feature can be merged to trunk for the next GCC release. - Week 16: **Final evaluation**\ Deliver the final product as a series of patches for trunk.
