I don't understand how a microcode update could fix this. I assume microcode is used for slow operations like triginometric functions, and doesn't affect how registers are allocated or renamed. Or does the update simply disables some optimizations using "chicken bits"? And by the way, is there a list of such bits?
Everything a modern CPU runs is microcode. There are a few x86 instructions that translate to a single microcode instruction, but most are translated to several.
The designers leave themselves an ability to override any instruction using the microcode so they can patch any instruction. They don't use the microcode only to implement complex instructions that require loops.
