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From: Stefan Monnier <monnier@iro.umontreal.ca>
Newsgroups: comp.arch
Subject: Re: Reverse engineering of Intel branch predictors
Date: Mon, 11 Nov 2024 17:10:14 -0500
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>> Hmm... but in order not to have bubbles, your prediction structure still
>> needs to give you a predicted target address (rather than a predicted
>> index number), right?
> Yes, but you use the predicted index number to find the predicted
> target IP.

Hmm... but that would require fetching that info from memory.
Can you do that without introducing bubbles?
If you're lucky it's in the L1 Icache, but that still takes a couple
cycles to get, doesn't it?
Or do you have a dedicated "jump table cache" as part of your jump
prediction tables?  [ Even if you do, it still means your prediction
has to first predict an index and then look it up in the table, which
increases its latency.  I don't know what kind of latency is used in
current state of the art predictors, but IIUC any increase in latency
can be quite costly.  ]


        Stefan