System information (version)

• OpenCV => 3.3.0-dev
• Internal OpenCL 1.2 implementation.

Detailed description

In OpenCL the accuracy of builtin functions with the native_ prefix is implementation defined.
The improc corner.cl file is using native_sqrt in two places, line 219 & 211.

Because of this I'm seeing fails with an internal OpenCL implementation of the OCL_Imgproc/CornerMinEigenVal.Mat subtests in the opencv_test_imgproc unit tests due to not precise enough results. Changing native_sqrt to sqrt resolves this issue.

If there is a hard precision requirement here we shouldn't be using native builtins as it's not portable.