GCC Base Performance Optimization Guide ​

Overview ​

The optimization of compiler base performance is crucial to improving the development efficiency, running performance, and maintainability of applications. It is an important research direction in computer science and one of the key steps in the process of software development. Based on the general compilation optimization capability, GCC for openEuler enhances mid- and back-end performance optimization technologies, including instruction optimization, vectorization enhancement, prefetch enhancement, and data flow analysis enhancement.

Installation and Deployment ​

Software Requirements ​

OS: openEuler 22.03 LTS SP4

Hardware Requirements ​

AArch64 architecture

Software Installation ​

Install GCC and related components as required. For example, install GCC:

yum install gcc

Usage ​

CRC Optimization ​

Description ​

Cyclic redundancy check (CRC) code is identified to generate efficient hardware instructions.

Usage ​

Add the -floop-crc option during compilation.

Note: -floop-crc must be used together with -O3 -march=armv8.1-a.

IF-conversion Enhancement ​

Description ​

IF-conversion is enhanced to use more registers to reduce conflicts.

Usage ​

This enhancement is part of the IF-conversion optimization of the Register Transfer Language (RTL). Enable the enhancement by using the following options.

-fifcvt-allow-complicated-cmps

-param=ifcvt-allow-register-renaming=[0,1,2] The default value is 0. The number is used to control the optimization scope.

Note: This enhancement requires the -O2 optimization level and must be used together with --param=max-rtl-if-conversion-unpredictable-cost=48 and --param=max-rtl-if-conversion-predictable-cost=48.

Multiplication Optimization ​

Description ​

Arm instructions are combined to convert low-order multiplications into high-order multiplication instructions.

Usage ​

Use the -fuaddsub-overflow-match-all and -fif-conversion-gimple options.

Note: This optimization requires the -O3 or higher optimization level and must be used together with -ftree-fold-phiopt option.

CMLT Instruction Generation ​

Description ​

CMLT instructions are generated for some elementary arithmetic operations to reduce the number of instructions.

Usage ​

Use the -mcmlt-arith option.

Note: This optimization requires the -O3 or higher optimization level.

Vectorization Enhancement ​

Description ​

Redundant instructions generated during vectorization are identified and simplified, and shorter arrays can be vectorized.

Usage ​

Use --param=tree-forwprop-perm=1 and --param=vect-alias-flexible-segment-len=1. The default values are 0.

Note: This optimization requires the -O3 or higher optimization level.

maxmin and UZP1/UZP2 Instruction Optimization ​

Description ​

The maxmin and UZP1/UZP2 instructions are optimized to reduce the total instructions and improve performance.

Usage ​

Use the -fconvert-minmax option. UZP1/UZP2 instruction optimization is enabled by default at a level higher than -O3.

Note: This optimization requires the -O3 or higher optimization level.

LDP and STP Optimization ​

Description ​

Each LDP and STP instruction with poor performance is split into two LDR and STR instructions.

Usage ​

Use the -fsplit-ldp-stp option. Use --param=param-ldp-dependency-search-range= [1,32] to control the search range. The default value is 16.

Note: This optimization requires the -O1 or higher optimization level.

AES Instruction Optimization ​

Description ​

The AES algorithm code is identified to accelerate instructions using hardware.

Usage ​

Use the -fcrypto-accel-aes option.

Note: This optimization requires the -O3 or higher optimization level.

Indirect Call Optimization ​

Description ​

Indirect calls in programs are identified and analyzed to convert them into direct calls.

Usage ​

Use the -ficp -ficp-speculatively option.

Note: This optimization must be used together with -O2 -flto -flto-partition=one.

IPA-prefetch ​

Description ​

Indirect memory accesses in a loop are identified to insert a prefetch instruction, thereby reducing the delay of indirect memory accesses.

Usage ​

Use the -fipa-prefetch -fipa-ic option.

Note: This optimization must be used together with -O3 -flto.

LLC-prefetch ​

Description ​

GCC for openEuler analyzes main execution paths in programs, performs memory multiplexing analysis on loops on the primary path, calculates and sorts top hot data, and inserts prefetch instructions to pre-allocate data to LLCs, reducing LLC misses.

Usage ​

Use the -fllc-allocate option. The -O2 or higher optimization level is required.

Other related interfaces: