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Direct3D renderer should set D3DCREATE_FPU_PRESERVE for not behaving vastly different to OpenGL/software rendering on doubles and break some libraries really badly.

Most notable affected example: Lua, which does the most unpredictable things which are really almost impossible to debug/find out for beginners who never heard this culprit exists.

Since I believe all renderers should behave the same on that doubles simply work as expected in a program, this should really be changed! (also this wasted a few days of my life wondering why everything in my program was so broken)

Please change src/render/direct3d/SDL_render_direct3d.c:441 from:

result = IDirect3D9_CreateDevice(data->d3d, data->adapter,
                                     D3DDEVTYPE_HAL,
                                     pparams.hDeviceWindow,
                                     (caps.
                                      DevCaps &
                                      D3DDEVCAPS_HWTRANSFORMANDLIGHT) ?
                                     D3DCREATE_HARDWARE_VERTEXPROCESSING :
                                     D3DCREATE_SOFTWARE_VERTEXPROCESSING,
                                     &pparams, &data->device);

to:

result = IDirect3D9_CreateDevice(data->d3d, data->adapter,
                                     D3DDEVTYPE_HAL,
                                     pparams.hDeviceWindow,
                                     D3DCREATE_FPU_PRESERVE | ((caps.
                                      DevCaps &
                                      D3DDEVCAPS_HWTRANSFORMANDLIGHT) ?
                                     D3DCREATE_HARDWARE_VERTEXPROCESSING :
                                     D3DCREATE_SOFTWARE_VERTEXPROCESSING),
                                     &pparams, &data->device);

Please note this currently makes any accelerated (non-software) SDL Texture renderers inoperable in Lua applications on some Windows configurations in combination with bug #1331

Of course there might be other ways to fix this I am not aware of, I am not a Windows/Direct3D guru. But if you can't think of any, please take this route soon.