That's not true

By: David Kanter (dkanter.delete@this.realworldtech.com), August 23, 2016 8:39 am
Room: Moderated Discussions
MH:

Essentially, μop caches operate as traditional
> caches which hold decoded instructions. However, they share some characteristics with loop caches. [b]In current
> commercial implementations, μop caches encode predicted branch paths.[/b] If branch paths differ from previously
> predicted paths, like loop caches the μop cache must be flushed and refilled.
> "

What does this mean?

uop caches don't encode predicted branch paths AFAIK. The uop cache is probed by the branch predictor, which means that the branch path is handled by the BP.

One of the big advantages of a trace cache (as opposed to a basic block cache) is that basic blocks are moved to have good locality in the cache. E.g., we have BB0, BB1...BBn. The common case is that BB0 jumps (via taken branch) to BB1, which jumps to BB2...BBn.

In a basic block cache each BB will be in a separate cache line - requiring n cache lines and n fetches. In a trace cache, multiple BBs mays be moved into the same cache line. E.g., BB0, BB1, BB2 are concatenated and reside in trace line 0. Thus, the same code requires fewer cache lines and fewer fetches.

I don't see how uop caches encode predicted branch paths? Can you explain?

David
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                    The (apparent) state of trace caches on modern CPUsMaynard Handley2016/08/23 06:48 AM
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                          The (wrong) state of trace caches on modern CPUsMaynard Handley2016/08/25 11:01 AM
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