The 7D supports auto ISO in the 100-3200 range, as well as Safety Shift.
In Tv mode, you can set 1/500 s and auto ISO. If so, the camera will use ISO 100 and vary the aperture, as long as the aperture range doesn't give underexposure. If it does, it will rise the ISO up to 3200. When that doesn't help, it will lengthen the exposure time to avoid underexposure.
If the smallest aperture at ISO 100 still gives overexposure, it will make the exposure time shorter.
Av mode is similar, just opposite.
In P mode, it will adjust both aperture and shutter speed to make maximum use of the combined range, down to a shutter speed of about 1/focal length. There it will hold the shutter speed and rise ISO to 3200. After that, it will make the exposure time longer.