LibreOffice On Windows Will Now Hard Require Clang For Performance Reasons

Written by Michael Larabel in LibreOffice on 8 May 2020 at 09:28 AM EDT. 39 Comments
Last month we reported on LibreOffice now preferring its new rendering code be built with LLVM Clang over alternative compilers. When falling back to CPU-based software rasterization, the Clang-generated code performs much better than alternative compilers given Google's own emphasis with Skia on being Clang-focused. LibreOffice 7.0 is now beginning a hard requirement on Clang when building for Windows.

Collabora's Luboš Luňák made the change today in LibreOffice Git to hard require the Clang compiler for Skia on Windows. Skia rendering is the default on Windows and in its CPU-based rasterization mode "performs much worse when compiled under MSVC" compared to Clang.

Ideally most LibreOffice 7.0 users will be using the Vulkan acceleration available with Skia, but for those on older/unsupported drivers or hardware, this CPU-based acceleration is important. Below is a video showing the performance difference of Clang (top half of the screen) compared to Microsoft MSVC built LibreOffice on the bottom.

Due to that dramatic performance difference, LibreOffice is now requiring Clang for building on Windows. If/when Skia is exclusively used on other platforms, it's possible we will see the Clang hardware requirement on those platforms like Linux while for right now it's just a recommendation -- that is unless the Skia code or GCC compiler see better optimizations for greater Skia performance outside of Clang.

LibreOffice 7.0 with its Skia rendering code and Vulkan support is due out in August.
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