3 ms·

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

by amluto 2y ago

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?

tavianator 2y ago
> 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.
- amluto 2y ago
  I think I missed that one in the pile of options. That’s bizarre. I wonder if MOV from memory is like POP.
  - deleted 2y ago
    [deleted]
BeeOnRope 2y ago
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.