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Wait a sec, isn’t this backwards? The OS will restore thread state by loading all of RCX, which sounds like it should trigger the slow path.

I wonder whether IRET fixes it. Can you try MOV RCX, 1; push an IRET frame; IRET; SHLX to see if IRET fixes it?

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> Wait a sec, isn’t this backwards?

No, if you push and pop RCX it's fast again. I mentioned this in the blog post, `mov rcx, 1` is slow but `mov rcx, 1; push rcx; pop rcx` is fast.

I think I missed that one in the pile of options. That’s bizarre. I wonder if MOV from memory is like POP.

The slow state is when rcx has a special "hidden immediate" attached it, from the `mov rcx, 1` or `add rcx, 1`. If the last option on the register doesn't fit that narrow template, it is fast, so loading from memory in any way, like pop, xrestor, plan load, etc, will be in fast mode.

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