Some Meaningful Performance Benefits For Clang + LTO Built Linux Kernels

Written by Michael Larabel in Software on 30 December 2025 at 01:50 PM EST. Page 2 of 5. 17 Comments.

Before getting to the exciting results, just to set realistic expectations, for many workloads there was little to no benefit from switching the compiler building the Linux kernel:

👁 LevelDB benchmark with settings of Benchmark: Fill Sync. Clang 21 Built Kernel + LTO was the fastest.

👁 LevelDB benchmark with settings of Benchmark: Sequential Fill. Clang 21 Built Kernel + LTO was the fastest.

👁 vkpeak benchmark with settings of fp16-matrix. Clang 21 Built Kernel + LTO was the fastest.

👁 vkpeak benchmark with settings of int8-dotprod. GCC 15 Built Kernel was the fastest.

👁 ParaView benchmark with settings of Test: Wavelet Volume, Frames: 3000, Resolution: 1920 x 1080. Clang 21 Built Kernel was the fastest.

👁 Cryptsetup benchmark with settings of AES-XTS 512b Encryption. Clang 21 Built Kernel + LTO was the fastest.

👁 Llama.cpp benchmark with settings of Backend: CPU BLAS, Model: Qwen3-8B-Q8_0, Test: Prompt Processing 512. Clang 21 Built Kernel + LTO was the fastest.

👁 Llama.cpp benchmark with settings of Backend: Vulkan, Model: Llama-3.1-Tulu-3-8B-Q8_0, Test: Text Generation 128. GCC 15 Built Kernel was the fastest.

👁 Llama.cpp benchmark with settings of Backend: Vulkan, Model: Llama-3.1-Tulu-3-8B-Q8_0, Test: Prompt Processing 1024. Clang 21 Built Kernel + LTO was the fastest.

👁 ClickHouse benchmark with settings of 100M Rows Hits Dataset, First Run / Cold Cache. Clang 21 Built Kernel was the fastest.

👁 Timed LLVM Compilation benchmark with settings of Build System: Ninja. Clang 21 Built Kernel + LTO was the fastest.

👁 Timed PHP Compilation benchmark with settings of Time To Compile. Clang 21 Built Kernel + LTO was the fastest.

Even for I/O workloads or those involving the AMDGPU kernel graphics driver, involving the LLVM Clang compiler or the Full LTO kernel build hadn't significantly affected the results compared to the straight-up kernel build with GCC 15. Both GCC and LLVM/Clang offer very competitive x86_64 performance these days.

Out of 163 benchmarks ran on these three kernel builds, in about two dozen benchmarks there ended up being a significant difference though to performance. So let's jump to where the kernel builds did provide some meaningful performance benefits from Clang and Clang + Full LTO.

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URL: https://www.phoronix.com/review/linux-kernel-llvm-clang-lto/2

⇱ Some Meaningful Performance Benefits For Clang + LTO Built Linux Kernels Review - Phoronix

Some Meaningful Performance Benefits For Clang + LTO Built Linux Kernels