On December 3rd, reports have surfaced indicating that AMD has quietly disabled the loop buffer functionality in its Zen 4 architecture processors following a recent BIOS update. The specific reasons behind this decision remain undisclosed.
The loop buffer is a part of the CPU frontend, designed to store a small number of fetched instructions. For small loops, instructions can be executed directly within the buffer, reducing power consumption and improving performance. However, after the BIOS update, performance monitoring data reveals that Zen 4 processors no longer dispatch any micro-operations from the loop buffer.
Impact on Performance
In benchmark tests, including the SPEC CPU2017, the performance impact of disabling the loop buffer is minimal. Whether dealing with integer or floating-point operations, the performance difference between enabling and disabling the loop buffer is less than 1%. Moreover, the performance improvement from simultaneous multithreading (SMT) remains unaffected.
In gaming benchmarks, specifically with Cyberpunk 2077, there was almost no noticeable difference in game performance when using VCache cores. However, when testing non-VCache cores, there was a 5% performance drop, though the exact cause remains unclear.
Lack of Official Statement
As of now, AMD has not issued any official statement or explanation regarding the disabling of the loop buffer function. This has led to widespread speculation about the potential reasons behind this decision. Sources suggest that AMD may have discovered an undisclosed hardware flaw, which could explain the change.
The lack of official communication from AMD has left the industry and tech community guessing about the motivations behind this alteration. Until further clarification is provided, the exact cause for disabling the loop buffer remains a mystery.
