(iOS)Problem when use HDPI

sed SDL: SDL 2.0.4(2015.7.7)

Used iOS: iOS 8.4.0

Using device: iPhone5S(HDPI: 1136 ? 640)

Call SDL_CreateWindow(), w=568, h = 320 and flag includes
SDL_WINDOW_ALLOW_HIGHDPI. The result is poor display quality. It is worst
than non-HDPI.

Call SDL_CreateWindow(), w=1136, h = 640 and flag includes
SDL_WINDOW_ALLOW_HIGHDPI. The result is HDPI display quality. But next call
SDL_GetWindowSize, result is 568x320!

Which calling sequence is right when HDPI?