关键词: LLVM编译器, 编译优化, 循环不变量外提, 寄存器溢出, 循环代价分析

Abstract: Loop-invariant code motion(LICM) is a commonly used compilation optimization algorithm for loop structures in programs.By moving the invariant calculations in the loop body to outside the loop,the algorithm reduces the overhead of duplicate calculations,thus improving program execution speed.However,in LLVM compiler,the traditional LICM algorithm hoists all loop-invariants outside the loop body,which will lead to register overflow when the number of loop-invariant reaches a certain level.It will introduce additional memory access cost in the loop,resulting in a negative optimization effect on the loop.To address this issue,a loop cost analysis algorithm is introduced based on the traditional LLVM LICM algorithm.This algorithm evaluates the running cost of loop-invariant code inside the loop and the overflow cost that may be caused by moving the code outside the loop,and assesses the benefits of moving the code outside the loop.Only the loop-invariant code that produces positive benefits is moved outside the loop,effectively reducing the overhead of duplicate calculations in the loop while avoiding the risk of introducing additional costs.Theproposed new optimization algorithm achieves more than 17% performance improvement compared to the traditional LICM algorithm in typical use cases for the domestic SW831 processor platform under millions of loops.Comprehensive optimization effect evaluations are conducted using the SPEC CPU 2017 benchmark test suite(SPECspeed2017 Integer Suite),Perl interpreter DKbench benchmark test suite,and Python interpreter pyperformance benchmark test suite.The results show that compared with the traditional LICM algorithm,the proposed algorithm has 0.4%,0.63% and 1% performance improvement respectively.

Key words: LLVM compiler, Compilation optimization, Loop-invariant code motion, Register overflow, Loop cost analysis