AMD Quietly Deactivates Unused Feature in Zen 4 CPUs
While Zen 5 PCs see Microsoft Windows-focused updates, a quieter change is impacting older Zen 4 processors. A recent release seemingly disables a seldom-used feature and likely won’t impact performance.
Loop Buffer Deactivated
Deep dives using Benchmarking and hardware diagnostics via a website called Chips and Cheese revealed this change involved diligently monitoring performance counters, providing an insightful look into Louvain.
You see, the Loop Buffer, essentially a mini replica of the uOp cache in the CPU frontend, holds instructions. When a program caught in a loop of repetitive code, this can simply draw instructions directly from the Loop Buffer, giving processors a chance to rest other parts of the frontend like fetches and decoders, potentially saving some power. However, in practical usage, it appears this feature is rarely utilized.
Though newborns the number of engines the Loop Buffer can hold is about 144 entries.
Uncovering a Potential Glitch
While not officially announced by AMD, the disablement suggests discovering an "erratum" making this feature unreliable.
This may also be a preemptive measure against vulnerabilities.
The impact on performance is likely minimal. The majority of scenarios didn’t utilize the feature, and the lv-
”Chips & Cheese disabled it, using both SPEC CPU2017 and Cyberpunk 2077 benchmarks to demonstrate it seems the performance was mainly unaffected. However, there was a slight dip visible in Cyberpunk 2077, only occurring when not utilizing Zen 4 cores with 3D V-Cache.
A Story of Trade-offs
It’s likely that the Loop Buffer’s power savings weren’t very noticeable in real-world applications.
Now, AMD seems to have removed it, focusing resources elsewhere.
One might wonder if this could resurface in later architectures like Zen 6 or Zen 7 after refactoring and refinement. It wouldn’t be the first for a CPU feature to go missing and return later with improvements.
Not Entirely Unique
Note that even Intel had a similar feature, named the Loop Stream Detector.
It’s interesting to recall how we rarely hear about these kinds of internal technical maneuvers, highlighting that CPU design is a complex, ongoing process.
What are the potential reasons AMD might have disabled the Loop Buffer in its Zen 4 processors?
## AMD Quietly Shuts Down Zen 4’s Loop Buffer: A Necessary Evil?
**Host:** Welcome back to the show. Today we’re diving into some interesting news regarding AMD’s Zen 4 processors. Joining us today is Alex Reed, a hardware analyst at [Alex Reed Affiliation], to shed some light on this topic. Welcome to the show, Alex Reed.
**Alex Reed:** Thanks for having me!
**Host:** So, there’s been a recent update for Zen 4 CPUs that appears to have deactivated a feature called the Loop Buffer. Can you tell our viewers a bit about what this feature is and why AMD might have decided to disable it?
**Alex Reed:** Certainly! The Loop Buffer is like a tiny storage space within the Zen 4 CPU designed to hold instructions that are repeated frequently in a program – think of it like a shortcut for frequently used code. The idea was to make things faster by avoiding the need to constantly fetch these instructions from the main cache or memory.
But, as detailed in a recent article on thefilibusterblog.com [[1](https://thefilibusterblog.com/amd-zen-4-cpus-receive-bios-update-to-disable-loop-buffer-while-preserving-performance/)], it seems AMD has decided to turn this feature off. The reasoning likely comes down to its limited use. While theoretically beneficial, it appears the Loop Buffer wasn’t providing a significant performance boost in real-world scenarios.
**Host:** Interesting. So, is this disabling the Loop Buffer going to affect users noticeably?
**Alex Reed:** Good news is, it’s highly unlikely. Benchmarking and analysis from sites like Chips and Cheese have shown that the performance impact of disabling the Loop Buffer is negligible. Essentially, it was a feature that wasn’t really being utilized to its full potential, so turning it off doesn’t seem to hurt anything.
**Host:** That’s reassuring to hear. Any thoughts on why AMD decided to make this change now, especially considering the focus on Zen 5 and Windows updates?
**Alex Reed:** It could be a combination of factors. Simplifying the processor design, reducing power consumption, or potentially preparing the way for new features in future generations could all be contributing factors. It’s also possible that AMD simply recognized the Loop Buffer wasn’t delivering the expected results and decided to streamline their architecture.
**Host:**
Well, Alex Reed, thanks for providing some insight into this intriguing development. It seems like a minor change with potentially little impact on users, but it’s always fascinating to see how hardware evolves.
**Alex Reed:** My pleasure. It’s always good to keep an eye on these smaller changes, as they can sometimes reveal interesting trends in processor design.
**Host:** Absolutely. Thank you for joining us today.
