As far as my research has shown that there are two different extensions that can be used to force an early Z test in OpenGL 3.0. these are GL_ARB_conservative_depth and GL_ARB_shader_image_load_store. According to the spec they can be used to force early Z in glsl like so:
layout(depth_unchanged) out float gl_FragDepth;
My question is do these have the exact same effect? if so can I use them interchangeably, if not which one should I use?