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 important in both computer science and software development. Based on the general compilation optimization capability, GCC for openEuler enhances middle- 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 24.03 LTS SP3

Hardware Requirements ​

AArch64 architecture

Software Installation ​

Install GCC and related components as needed. The following uses GCC as an example:

yum install gcc

How to Use ​

Optimization for CRC ​

Description ​

GCC identifies cyclic redundancy check (CRC) code and generates efficient hardware instructions.

How to Use ​

Add -floop-crc during compilation.

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

If-conversion Enhancement ​

Description ​

If-conversion optimization is enhanced by using more registers to reduce conflicts.

How to Use ​

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

-fifcvt-allow-complicated-cmps

--param=ifcvt-allow-register-renaming=[0,1,2], where the numbers are used to control the optimization scope

Note: This optimization 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.

Optimization for Multiplication ​

Description ​

Arm instructions are combined to convert low-order 32-bit multiplications into high-order 64-bit multiplication instructions.

How to Use ​

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

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

Optimization for CMLT Instruction Generation ​

Description ​

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

How to Use ​

Use the -mcmlt-arith option.

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

Optimization for Vectorization ​

Description ​

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

How to Use ​

Use --param=vect-alias-flexible-segment-len=1. The default value is 0.

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

Optimization for min max and uzp1/uzp2 Instructions ​

Description ​

The min max and uzp1/uzp2 instructions are optimized to reduce the total instructions and improve performance.

How to Use ​

Use the -fconvert-minmax option to enable min max optimization. uzp1/uzp2 instruction optimization is enabled by default at a level higher than -O3.

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

Optimization for LDP and STP ​

Description ​

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

How to Use ​

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.

Optimization for AES Instruction ​

Description ​

The AES software instruction sequences are identified and accelerated using hardware instructions.

How to Use ​

Use the -fcrypto-accel-aes option.

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

Optimization for Indirect Calls ​

Description ​

Indirect calls in programs are identified, analyzed, and then optimized into direct calls.

How to Use ​

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, and a prefetch instruction is inserted to reduce the delay.

How to Use ​

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

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

-fipa-struct-reorg ​

Description ​

This option optimizes memory layout. The structure members are rearranged in memory to improve the cache hit rate.

How to Use ​

Add -O3 -flto -flto-partition=one -fipa-struct-reorg to the option.

Note: The -fipa-struct-reorg option can be enabled only when -O3 -flto -flto-partition=one is enabled globally.

-fipa-reorder-fields ​

Description ​

The memory space layout is optimized by arranging structure members from largest to smallest based on their size. This reduces padding caused by alignment boundaries, decreases overall memory usage, and improves the cache hit rate.

How to Use ​

Add -O3 -flto -flto-partition=one -fipa-reorder-fields to the option.

Note: The -fipa-reorder-fields option can be enabled only when -O3 -flto -flto-partition=one is enabled globally.

-ftree-slp-transpose-vectorize ​

Description ​

In the loop splitting phase, temporary arrays are introduced to partition the loop, which enhances data-flow analysis for loops that read continuous memory. In the superword-level parallelism (SLP) vectorization phase, SLP analysis is performed for transposing grouped_stores.

How to Use ​

Add -O3 -ftree-slp-transpose-vectorize to the option.

Note: The -ftree-slp-transpose-vectorize option can be enabled only when -O3 is enabled.

LLC-prefetch ​

Description ​

In main execution paths of programs, memory-reuse patterns in loops are analyzed to identify and rank top hot data. The prefetch instruction is introduced to allocate the data to last-level cache (LLC), reducing LLC misses.

How to Use ​

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

Other related interfaces:

-fipa-struct-sfc ​

Description ​

This option is used to statically compress structure members to reduce the structure size and improve the cache hit rate.

How to Use ​

Add -O3 -flto -flto-partition=one -fipa-reorder-fields -fipa-struct-sfc to the option. You can use -fipa-struct-sfc-bitfield and -fipa-struct-sfc-shadow for further optimization.

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

-fipa-struct-dfc ​

Description ​

This option is used to dynamically compress structure members by cloning the program path and heuristically minimizing the structure size. At runtime, it improves the cache hit rate by checking execution paths and selecting the optimal one.

How to Use ​

Add -O3 -flto -flto-partition=one -fipa-reorder-fields -fipa-struct-dfc to the option. You can use -fipa-struct-dfc-bitfield and -fipa-struct-dfc-shadow for further optimization.

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

-fipa-alignment-propagation ​

Description ​

This option is used to analyze and propagate the address-alignment values for local variables, optimizing the bitwise AND operations.

How to Use ​

Add -O3 -fipa-alignment-propagation to the option.

Note: The -fipa-alignment-propagation option can be enabled only when -O3 is enabled.

-fipa-localize-array ​

Description ​

This option is used to convert the global pointer variables allocated by calloc to local variables.

How to Use ​

Add -O3 -fipa-localize-array to the option.

Note: The -fipa-localize-array option can be enabled only when -O3 is enabled.

-fipa-array-dse ​

Description ​

This option is used to analyze the transfer of arrays between functions and the usage of the arrays in the called functions, removing redundant array writes.

How to Use ​

Add -O3 -fipa-array-dse to the option.

Note: The -fipa-array-dse option can be enabled only when -O3 is enabled.

-ffind-with-sve ​

Description ​

This option is used to identify std::find function calls and attempt to optimize them using SVE instructions.

How to Use ​

Add -ffind-with-sve to the option.

-floop-sve-mode-opt ​

Description ​

By analyzing static code characteristics, special scenarios can be identified. When the conditions are met, additional optimization opportunities leveraging the SVE instruction set are introduced, resulting in improved performance.

How to Use ​

Add -O3 -floop-sve-mode-opt to the option.

Note: The -floop-sve-mode-opt option can be enabled only when -O3 is enabled and SVE is included in the -march setting.

SVE Memcall Inline Optimization ​

Description ​

This optimization replaces memcpy and memset calls with inline SVE loops, reducing library call overhead and improving the performance of small memory operations. It does not support memmove and is unsuitable for workloads with frequent large memory operations.

How to Use ​

Add -march=armv8-a+sve -maarch64-sve-memcall-inlining to the compilation options.

The related options and parameters are as follows:

Note: Adjust the threshold based on the size distribution of memcpy and memset operations in the actual workload so that large memory operations fall back to library calls.