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Windows Defender likes to scan new files, including those that tests create. So if we don’t exclude the test folder there’s Which can change their last accessed times. There’s at least one test known to fail because of this, “LLVM :: ThinLTO/X86/cache.ll”. To add this exclusion exclude the folder follow https://support.microsoft.com/en-us/windows/add-an-exclusion-to-windows-security-811816c0-4dfd-af4a-47e4-c301afe13b26 and add “<llvm-build-dir>/test”. (you shouldn’t need to restart defender or the machine, it takes effect automatically)