Optimize C and C++ code using compiler autovectorization techniques
Introduction
An introduction to autovectorization
Autovectorization using the restrict keyword
Autovectorization limits
Autovectorization and conditionals
Autovectorization on Arm
More autovectorization on Arm
Next Steps
Optimize C and C++ code using compiler autovectorization techniques
Who is this for?
This is an advanced topic for C/C++ developers who are interested in taking advantage of autovectorization in compilers.
What will you learn?
Upon completion of this Learning Path, you will be able to:
- Modify loops to take advantage of autovectorization in compilers
Prerequisites
Before starting, you will need the following:
- An Arm computer running Linux and a recent version of Clang or the GNU compiler (gcc) installed.
Summary
This summary was drafted with an approved AI-assisted workflow and reviewed by Arm contributors before publication. Human technical review remains part of the process so the final page reflects engineering rigor, accuracy, and Arm editorial standards.
You’ll enable and inspect compiler autovectorization for C and C++ loops on Arm Linux with GCC or Clang. You’ll build focused examples, apply the C99
restrict qualifier, and inspect assembly to verify vectorization. Then, you’ll examine countable loops and branches, refactor conditionals when needed, and use an integer dot product to recognize Arm SIMD instructions in generated code.Frequently asked questions
These FAQs were drafted with an approved AI-assisted workflow and reviewed by Arm contributors before publication. Human technical review remains part of the process so the final page reflects engineering rigor, accuracy, and Arm editorial standards.
Compile with optimization, then disassemble the binary with
objdump -D. In the target function, look for wide loads, stores, and arithmetic that process multiple elements per iteration instead of scalar steps.Confirm that the loop is countable and free of loop-carried dependencies or branches. If the loop includes conditionals, restructure or move the conditionals so the hot path is branch-free.
Yes. The examples use
gcc, but Clang can compile the same sources and generate assembly for inspection.Compile the example sources, such as
addvec.c and addvec_neon.c, then disassemble the addvec binary. Inspect the addvec function for vectorized operations after applying restrict.Build
dotprod.c and inspect the assembly for dotprod. Look for vectorized integer operations that process multiple elements per iteration with vector loads and stores.