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Resolving clang: error: unsupported option ‘-fopenmp’ on macOS

Published on January 31, 2024

OpenMP is a popular API that supports multi-platform shared-memory multiprocessing programming. When working with certain benchmarks, users may encounter issues related to OpenMP support and narrowing conversions, especially on macOS systems where the default Clang compiler lacks OpenMP support.

The Problem:

error: clang: error: unsupported option '-fopenmp'

The error message indicates that the default Clang compiler is unable to recognize the ‘-fopenmp’ option.

Solution Steps:

  1. Install LLVM with OpenMP using Homebrew:
brew install llvm

This ensures that LLVM, including Clang with OpenMP support, is installed on the system.

  1. Update PATH and Compiler Variables: Set up the environment to prioritize the Homebrew LLVM installation:
export PATH="/usr/local/opt/llvm/bin:$PATH"
export CC="/usr/local/opt/llvm/bin/clang"
export CXX="/usr/local/opt/llvm/bin/clang++"

This ensures that the Homebrew-installed Clang with OpenMP is used during compilation.

  1. Retry the Installation process 
./your-app

Narrowing Conversion Issue:

In some cases, users might encounter narrowing conversion errors during compilation. These errors are related to attempting to initialize variables with values that cannot be safely narrowed from one type to another.

Solution for Narrowing Conversion Issue:

  1. Modify the Code: If you have access to the benchmark’s source code, inspect and modify the relevant lines to avoid narrowing conversions. Adjust the type of the variable or the assigned value accordingly.
  2. Compiler Flags: As a temporary measure, you can use compiler flags to suppress narrowing conversion warnings:
./your-app CXXFLAGS="-Wno-c++11-narrowing"

adding this flag should resolve the issue

CXXFLAGS=”-Wno-c++11-narrowing”

However, it’s recommended to address the issue directly in the code for long-term stability.

  1. Check for Updates: Ensure that you are using the latest version of the benchmark. Check for updates that might have addressed narrowing conversion issues.

By following these steps, users can successfully resolve issues related to OpenMP support and narrowing conversions with LLVM on macOS. This guide provides a clear path to overcome these challenges.

Remember to check for updates, as software packages are frequently improved and updated. If you encounter further issues or have questions, don’t hesitate to seek assistance from relevant communities or forums.

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