I have a iMac 27" 5K 2018 with an AMD Radeon Pro 580 GPU.
Blender 2.8 cannot find a compatible GPU for Cycles render engine.
In Blender 2.79 my GPU worked.
What do I need to do so that my graphic card is defined correctly and works GPU?
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There is no OpenCL support for Blender 2.80 BETA on macOS and it likely won't come back in 2.80 RC or Final.
There was a growing payload of bugs in Cycles related on OpenCL on macOS platform, and those issues were caused by a compiler bug, which we have no control over.
Surely, it is sometimes possible to work compiler bugs around from a source, but we are facing some of the issues which are not solvable in this way. Also, such solutions are usually short-living,. since adding more features are often kicking compiler to provide buggy binary again.
In this case compiler will not get fixed since Apple decided to discontinue OpenCL on its platform.
So the decision was made to drop support of OpenCL, keep official features of Blender stable and predictable, and focus on things we have control over.
P.S. Older Blender releases are always available. Surely, this sounds like using an ancient software without neat features. But we can't push Cycles OpenCL on macOS measurably beyond that anyway.
-- With best regards, Sergey Sharybin
Apple is known for doing everything to make software development - especially 3D stuff - a living hell. They are doing their own stuff called Metal, instead of using a complete OpenGL/OpenCL compatible instruction set. With every new release functions are getting removed or are simply broken.
If you want to use Blender with an OpenCL render device consistently in the future, you might need to switch over to Linux or Windows.