DEV-TOOLSApril 12, 20261 min read9 views

Optimization of 32-bit Unsigned Division by Constants on 64-bit Targets

Researchers have proposed a new method to optimize 32-bit unsigned division by constants on 64-bit CPUs, achieving up to 1.98x speedup on Apple M4. The method has been merged into LLVM, addressing long-standing inefficiencies in traditional compiler techniques.

Computer Science > Programming Languages

Title:Optimization of 32-bit Unsigned Division by Constants on 64-bit Targets

View PDF HTML (experimental)Abstract:Granlund and Montgomery proposed an optimization method for unsigned integer division by constants [3]. Their method (called the GM method in this paper) was further improved in part by works such as [1] and [7], and is now adopted by major compilers including GCC, Clang, Microsoft Compiler, and Apple Clang. However, for example, for x/7, the generated code is designed for 32-bit CPUs and therefore does not fully exploit 64-bit capabilities. This paper proposes an optimization method for 32-bit unsigned division by constants targeting 64-bit CPUs. We implemented patches for LLVM/GCC and achieved speedups of 1.67x on Intel Xeon w9-3495X (Sapphire Rapids) and 1.98x on Apple M4 (Apple M-series SoC) in the microbenchmark described later. The LLVM patch has already been merged into llvm:main [6], demonstrating the practical applicability of the proposed method.

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