Analyzing and addressing false interactions during compiler optimization phase ordering

Abstract

Compiler optimization phase ordering is a fundamental, pervasive, and long-standing problem for optimizing compilers. This problem is caused by interacting optimization phases producing different codes when applied in different orders. Producing the best phase ordering code is very important in performance-oriented and cost-constrained domains, such as embedded systems. In this work, we analyze the causes of the phase ordering problem in our compiler, Very Portable Optimizer (VPO), and report our observations. We devise new techniques to eliminate, what we call, false phase interactions in our compiler. We find that reducing such false phase interactions significantly prunes the phase order search space. We also develop and study algorithms to find the best average performance that can be delivered by a single phase sequence over our benchmark set and discuss the challenges in resolving this important problem. Our results show that there is no single sequence in VPO that can achieve the optimal phase ordering performance across all functions.ANALYZING FALSE INTERACTIONS DURING OPTIMIZATION PHASE ORDERING 645 set. Therefore, in this work, we also explore this problem of automatically finding the best average performance that can be achieved by any single compiler phase sequence in our compiler and discuss the challenges in addressing this issue. Our study empirically shows the non-existence of a single phase sequence that can achieve optimal code for all functions.Although phase ordering is a pervasive issue across all or most compilers, we must necessarily restrict this study to a single compiler. The presence of an exhaustive framework for phase ordering research, along with our intimate knowledge of its internal organization, made VPO the best choice for us to conduct this research. However, we believe that our motivation and methodology for exploring and addressing phase interaction issues should be more generally applicable to other compilers. Most of all, we believe that even though different compilers may have different optimization phases, implementations, and phase interactions, this study shows the potential and benefit of understanding such interactions to guide future implementation decisions that minimize the phase ordering problem and generate higher-quality code in compilers. The major contributions of this research work are as follows:ANALYZING FALSE INTERACTIONS DURING OPTIMIZATION PHASE ORDERING 647 § In theory, compiler optimizations can undo changes made by preceding phases and introduce back-edges in the directed graph. However, for our set of 244 benchmark functions, the exhaustive search algorithm only produced 'acyclic' graphs (no back-edges), and so we call them DAGs in this paper. The acyclic nature of the graphs is not a requirement for the exhaustive search algorithm. ANALYZING FALSE INTERACTIONS DURING OPTIMIZATION PHASE ORDERING 651 ** Applying conservative copy propagation explicitly as a distinct reorderable increases the size of the search space by over 77%, on average, and reduces...

REDUCER: Elimination of Repetitive Codes for Accelerated Iterative Compilation

“…Several works [13,22,21,20,19] have emphasized on code comparisons for removing redundant executions. [22,21,20,19] were based on VPO (Very Portable Optimizer) compiler back end, which performed all the analyses and optimizations on a single low-level representation called Register Transfer Lists (RTLs).…”

“…Several works [13,22,21,20,19] have emphasized on code comparisons for removing redundant executions. [22,21,20,19] were based on VPO (Very Portable Optimizer) compiler back end, which performed all the analyses and optimizations on a single low-level representation called Register Transfer Lists (RTLs). It detected the iden-tical function instances by performing three checks including instructions count, instructions byte-sum, and the CRC (Cyclic Redundancy Code) checksum on the bytes of the RTLs.…”

“…However, these techniques consider the identical codes as separate optimization, hence increasing the overall search space due to redundant testings. Although [20,22,19], emphasizes on reducing the search overhead by detecting identical codes. But, no such method has been practically adopted by LLVM compilation techniques, for detecting identical optimization sequences.…”

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“…编译优化序列选择涉及到众多的编译器, 包括 GCC [26] , LLVM [23] , VPO [35] 等. 图 8 给出了论文 库中研究者所使用的编译器统计结果, 按照编译器 GCC, LLVM, VPO, ReflectC, ICC, WCC 使用次数 排序结果为 20, 13, 7, 4, 4, 2.…”

Translating the ARM Neon and VFP instructions in a binary translator

“…Taking advantage of the SIMD capability, such as Neon/VFP, is important in media applications. Jantz et al consider re‐ordering the optimization phases for generating SIMD instructions. Pryanishnikov et al use interprocedural analysis to produce SIMD code that access data aligned properly in order to increase the performance of the resulting SIMD program.…”