GCC Basic Performance Optimization User Guide ​

Introduction ​

Compiler performance optimization plays a vital role in enhancing application development efficiency, runtime performance, and maintainability. It is a significant research area in computer science and a critical component of the software development process. GCC for openEuler extends its general compilation optimization capabilities by improving backend performance techniques, including instruction optimization, vectorization enhancements, prefetching improvements, and data flow analysis optimizations.

Installation and Deployment ​

Software Requirements ​

OS: openEuler 25.03

Hardware Requirements ​

AArch64 architecture

Software Installation ​

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

yum install gcc

Usage ​

CRC Optimization ​

Description ​

Detects CRC software loop code and generates efficient hardware instructions.

Usage ​

Include the -floop-crc option during compilation.

Note: The -floop-crc option must be used alongside -O3 -march=armv8.1-a.

If-Conversion Enhancement ​

Description ​

Improves If-Conversion optimization by leveraging additional registers to minimize conflicts.

Usage ​

This optimization is part of the RTL if-conversion process. Enable it using the following compilation options:

-fifcvt-allow-complicated-cmps

--param=ifcvt-allow-register-renaming=[0,1,2] (The value controls the optimization scope.)

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

Multiplication Calculation Optimization ​

Description ​

Optimizes Arm-related instruction merging to recognize 32-bit complex combinations of 64-bit integer multiplication logic and produce efficient 64-bit instructions.

Usage ​

Enable the optimization using the -fuaddsub-overflow-match-all and -fif-conversion-gimple options.

Note: This optimization requires -O3 or higher optimization levels.

cmlt Instruction Generation Optimization ​

Description ​

Generates cmlt instructions for specific arithmetic operations, reducing the instruction count.

Usage ​

Enable the optimization using the -mcmlt-arith option.

Note: This optimization requires -O3 or higher optimization levels.

Vectorization Optimization Enhancement ​

Description ​

Identifies and simplifies redundant instructions generated during vectorization, enabling shorter loops to undergo vectorization.

Usage ​

Enable the optimization using the parameter --param=vect-alias-flexible-segment-len=1 (default is 0).

Note: This optimization requires -O3 or higher optimization levels.

Combined Optimization of min max and uzp1/uzp2 Instructions ​

Description ​

Identifies opportunities to optimize min max and uzp1/uzp2 instructions together, reducing the instruction count to enhance performance.

Usage ​

Enable min max optimization with the -fconvert-minmax option. The uzp1/uzp2 instruction optimization is automatically enabled at -O3 or higher levels.

Note: This optimization requires -O3 or higher optimization levels.

ldp/stp Optimization ​

Description ​

Detects poorly performing ldp/stp instructions and splits them into two separate ldr and str instructions.

Usage ​

Enable the optimization using the -fsplit-ldp-stp option. Control the search range with the parameter --param=param-ldp-dependency-search-range=[1,32] (default is 16).

Note: This optimization requires -O1 or higher optimization levels.

AES Instruction Optimization ​

Description ​

Identifies AES software algorithm instruction sequences and replaces them with hardware instructions for acceleration.

Usage ​

Enable the optimization using the -fcrypto-accel-aes option.

Note: This optimization requires -O3 or higher optimization levels.

Indirect Call Optimization ​

Description ​

Analyzes and optimizes indirect calls in the program, converting them into direct calls where possible.

Usage ​

Enable the optimization using the -ficp -ficp-speculatively options.

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

IPA-prefetch ​

Description ​

Detects indirect memory accesses in loops and inserts prefetch instructions to minimize latency.

Usage ​

Enable the optimization using the -fipa-prefetch -fipa-ic options.

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

-fipa-struct-reorg ​

Description ​

Optimizes memory layout by reorganizing the arrangement of structure members to improve cache hit rates.

Usage ​

Add the options -O3 -flto -flto-partition=one -fipa-struct-reorg to enable the optimization.

Note: The -fipa-struct-reorg option requires -O3 -flto -flto-partition=one to be enabled globally.

-fipa-reorder-fields ​

Description ​

Optimizes memory layout by reordering structure members from largest to smallest, reducing padding and improving cache hit rates.

Usage ​

Add the options -O3 -flto -flto-partition=one -fipa-reorder-fields to enable the optimization.

Note: The -fipa-reorder-fields option requires -O3 -flto -flto-partition=one to be enabled globally.

-ftree-slp-transpose-vectorize ​

Description ​

Enhances data flow analysis for loops with consecutive memory reads by inserting temporary arrays during loop splitting. During SLP vectorization, it introduces transposition analysis for grouped_stores.

Usage ​

Add the options -O3 -ftree-slp-transpose-vectorize to enable the optimization.

Note: The -ftree-slp-transpose-vectorize option requires -O3 to be enabled.

LLC-prefetch ​

Description ​

Analyzes the main execution paths of a program, performs memory access reuse analysis on the loops along the main path, calculates and sorts the top hot data, and inserts prefetch instructions to allocate the data to the LLC first, reducing LLC misses.

Usage ​

To enable the LLC feature, the -O2 or higher optimization level must be enabled, and the -fllc-allocate compilation option must be used.

Other related interfaces:

-fipa-struct-sfc ​

Description ​

Statically compresses structure members to reduce the overall memory size of a structure and improve the cache hit rate.

Usage ​

Add -O3 -flto -flto-partition=one -fipa-reorder-fields -fipa-struct-sfc to the options. On this basis, you can use -fipa-struct-sfc-bitfield and -fipa-struct-sfc-shadow to enable additional optimizations.

Note: The -fipa-struct-sfc option can be enabled only when -O3 -flto -flto-partition=one is globally enabled and -fipa-reorder-fields is enabled or -fipa-struct-reorg>=2 is set.