Sunny Cove wide

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), December 15, 2018 10:52 am
Room: Moderated Discussions
anon (spam.delete.delete.delete@this.this.this.spam.com) on December 15, 2018 6:02 am wrote:
>
> Yes, renaming is still renaming but obviously it's vastly easier
> with just one architectural register instead of 16.
> Linus' argument seems to be "both are renaming, both use multiple physical registers,
> therefore the complexity is the same" and I think we both don't agree with that.

That wasn't my argument at all. I was arguing that renaming definitely happens, and I was also arguing that it wasn't at all as simple as you were trying to claim - exactly because eflags is not one single register at all.

Sure, you can read it as one register, but it doesn't act like one.

Do you think POWER has just one CR register? Or are there eight groups of four bits that you can address and write independently using the normal instructions, and that you can just read or write as a group using a couple of special instructions?

You really can and do address the bits in %eflags individually, and it really does matter. The fact is, conditional branches read a very limited subset of bits, often just a single one. And yes, it very much matters both for renaming and for the issue of instruction fusion.

For example, you can actually fuse sequences like "dec + jne", but you can not easily fuse "dec + jb". Why? Because "dec" very much writes ZF, but does not write CF, and "jne" only reads ZF but "jb" reads CF.

Now, when it comes to instruction fusion, you can just have a hardcoded trivial table or equivalent, and I'm not claiming the above is at all complicated. But it's an example of how this is not about some one register thing, and how it matters for simple cases.

Saying "let's ignore the partial register writes" is silly. That's a very big part of the whole eflags renaming, exactly because eflags isn't really one single register. Claiming that eflags is simple because it's a single register ignores reality.


Linus
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