Valve's patches add use of "ceil" without using libm, it works by accident with -O2 but fails with -Os. ld: dlls/winevulkan/vulkan.o: in function `fshack_vk_queue_present': vulkan.c:(.text+0x12f46): undefined reference to `ceil' --- a/dlls/winevulkan/Makefile.in +++ b/dlls/winevulkan/Makefile.in @@ -4,3 +4,3 @@ IMPORTS = user32 gdi32 advapi32 setupapi win32u -EXTRALIBS = $(PTHREAD_LIBS) +EXTRALIBS = -lm $(PTHREAD_LIBS)