kc3-lang/libjpeg-turbo

2025-06-06T13:37:48

Build: Don't modify CMAKE_C_FLAGS_* if specified The build system originally force-enabled the maximum optimization level (-O3) with GCC because it significantly improved the performance of the C Huffman encoder on x86 platforms. Since libjpeg-turbo 1.5.x, the Huffman encoder has been SIMD-accelerated on x86 and Arm platforms, and in my testing on various Intel CPUs, the difference between -O2 and -O3 is no longer statistically significant. However, on certain Arm CPUs, I observe that grayscale decompression slows down by 16-27% with -O2 vs. -O3. Although modern versions of CMake use -O3 as the default optimization level for Release builds, -O2 is still the default optimization level for RelWithDebInfo builds. Thus, I am reluctant to change the default behavior of our build system. However, referring to #815, some users observe better performance with -O2 vs. -O3 on other Arm CPUs, so the build system needs to allow users to override the default behavior. Closes #815