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.

Toolchain Applicability ​

The features in this guide apply to the system GCC (gcc 12.3.1) and to gcc-toolset-14 with the openEuler optimization stack merged (see Alternative GCC 14 User Guide), with the following exceptions.

Supported by the system GCC only:

• -mcmlt-arith
• -fconvert-minmax
• -fsplit-ldp-stp and --param=param-ldp-dependency-search-range
• -fllc-allocate and its associated parameters
• -ftree-slp-transpose-vectorize
• --param=vect-alias-flexible-segment-len

The following system GCC options are retired in gcc-toolset-14. Their positive forms are rejected with an error; remove or replace them in build scripts:

Note: The -fno- forms of the first two options are silently accepted.

Installation and Deployment ​

Software Requirements ​

OS: openEuler 24.09

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.

-fif-split ​

Description ​

Splits a composite condition of the form "variable compared with a constant || other condition" and clones the branch body, so IPA constant propagation can propagate the constant into function calls.

Usage ​

Add the -fif-split option.

Note: The -fif-split option requires -O3 to be enabled.

Vectorization Analysis Parameters ​

Description ​

Three independent vectorization analysis switches.

Usage ​

-ftree-slp-late ​

Description ​

Runs an additional SLP vectorization pass after reassociation.

Usage ​

Add the -ftree-slp-late option. Disabled by default.

-floop-elim ​

Description ​

Eliminates redundant loops.

Usage ​

Add the -floop-elim option.

Note: Runs as part of phiopt and depends on -fssa-phiopt (enabled by default at -O1 and higher).

-floop-sve-mode-opt ​

Description ​

Vectorizes certain loops in SVE mode.

Usage ​

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

Note: Effective only on AArch64 with SVE enabled.

-farray-widen-compare ​

Description ​

Rewrites qualifying byte-wise compare loops to compare eight bytes at a time.

Usage ​

Add the -farray-widen-compare option.

Note: Requires -O3; effective only on little-endian targets; only the LP64 data model is supported. The compared arrays must remain readable up to the loop bound: the widened 8-byte loads may read bytes past an early exit that the byte-wise loop would never access, so violating this precondition risks out-of-bounds reads.

-fbuiltin-will-return ​

Description ​

Treats built-ins that cannot loop, throw, or exit (currently only __builtin_prefetch) as calls that always return.

Usage ​

Add the -fbuiltin-will-return option.

-mmul-widen128 ​

Description ​

Recognizes 64-bit to 128-bit widening multiplication idioms and generates efficient instruction sequences.

Usage ​

Add the -mmul-widen128 option.

Note: AArch64 only. The corresponding system GCC option is -fmerge-mull.

Vector Math Library and Floating-Point Control ​

Description ​

Vector math library support and floating-point behavior control.

Usage ​

• -fsimdmath: declares vector variants of math functions (_ZGV*) and injects libmathlib at link time.
• -ffp-model=: floating-point model control; values: normal/fast/precise/except/strict.
• -mdaz-ftz: sets the FTZ/DAZ flags in the floating-point control register at startup, flushing subnormal values to zero.

Note: The corresponding system GCC spellings of -ffp-model= and -mdaz-ftz are -fp-model= and -fftz.

CFGO / CSPGO ​

Description ​

CFGO enables a bundle of openEuler optimizations alongside PGO profile use; CSPGO instruments a second time after inlining so profile counts carry call context.

Usage ​

• CFGO instrumentation: -fcfgo-profile-generate[=<dir>]; use: -fcfgo-profile-use[=<dir>].
• CSPGO instrumentation: -fcfgo-csprofile-generate[=<dir>]; use: -fcfgo-csprofile-use[=<dir>]; directory: -fcfgo-csprofile-dir=<dir>.

Note: The CFGO bundle does not include -fselective-scheduling. The option is still accepted, but combining it with profile instrumentation (-fcfgo-profile-generate/-fcfgo-csprofile-generate) carries a known mis-compilation risk (statements observed silently dropped on AArch64 with no diagnostic); do not combine them.

oeAware Co-Optimization ​

Description ​

Emits an ELF section read by the oeAware runtime tuning framework.

Usage ​

Add the -foeaware-policy option; -foeaware-policy=[1,7] selects the optimization policy (default: 1).

AutoBOLT ​

Description ​

Writes function and branch information (derived from AutoFDO/PGO profiles) into an ELF section at compile time; after linking, bolt-plugin collects it and performs BOLT binary optimization.

Usage ​

Add the -fauto-bolt option; -fauto-bolt=<dir> specifies the profile data directory (default: current directory).

Note: AArch64 only. Not supported with -flto (compilation error) and mutually exclusive with -fbolt-use; it therefore cannot be combined with the Struct-Reorg family options in this guide, which require -flto.