Structural Code Representation Learning for Auto-Vectorization

Yao Xiao, Nesreen Ahmed, Mihai Capotă, Guixiang Ma, Theodore L. Willke, Shahin Nazarian, Paul Bogdan

Abstract: The single instruction multiple data (SIMD) capability in modern processors is critical to improving the performance of current compute-intensive programs. SIMD allows architectures to exploit the natural data parallelism that exists in a wide-range of real applications (e.g., games, signal processing, etc) by executing a single instruction on multiple data items simultaneously. Modern compilers use vectorization techniques to exploit the SIMD capability, by detecting data parallelism in scalar source code and transforming a group of scalar instructions into vector-based instructions. In this work, we focus on one of the most common vectorization techniques called \emph{loop-based vectorization}, which targets loops and optimize their performance by grouping multiple occurrences of the same operation across loop iterations into single SIMD instructions. This is achieved by setting two key parameters: (1) the vectorization factor (VF), and (2) the interleaving factor (IF). Unfortunately, vectorizing loop computations effectively is a key challenging problem for both programmers and compilers due to the large search space. For example, manual vectorization of each loop puts a huge burden on the programmer, is more error-prone, and/or requires expert knowledge of both the software and the architecture. Alternatively, current compilers use fixed-cost models based on expert heuristics to make automatic vectorization decisions. However, these models often ignore the data dependencies, as well as the underlying computation graph. In this paper, we propose a data-driven graph-based learning framework for automatic vectorization, called \emph{autograph}, which takes an input program, extracts the loops, then learns a structured representation to automatically predict the correct VF/IF factors. Our proposed framework utilizes deep reinforcement learning to learn an optimal policy (observations to actions) from an intelligent agent in a SIMD environment, and automatically injects the predicted vectorization pragmas into the input program. We conducted an extensive evaluation on multiple benchmark datasets and comparisons with state-of-the-art baselines. Our experimental results show that the proposed framework can achieve up to 1.02x-2.26x and 1.06x-4.27x performance improvement, compared to state-of-the-art baseline and LLVM -O3 respectively.

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