[wongarsu]

One advantage of not exposing microcode is that newer processors can add support for new microcode instructions and map existing X86 instructions to them. In a sense there's a tiny JIT in the CPU that turns X86 into processor-optimized code.

The disadvantage is of course that this is complex to do in silicon, and the CPU might lack some insights that the compiler had. As I understand it Itanium was HP's and Intel's attempt to give a lot more power to the compiler, with an instruction set that better matches what's going on under the hood. But we all know how that ended: performance was lackluster and the Itanic was nothing but a waste of money for everyone involved.

GPUs have successfully moved the microcode translation one layer up, you generally compile to an intermediate ISA (let's call it a bytecode) and when you load the program (or shader) the GPU driver translates it to GPU-specific instructions. But that model doesn't easily translate to CPUs.