AMD disables loop buffer feature in Zen 4 CPUs with no major performance hit

In a nutshell: AMD has quietly disabled the Loop Buffer feature in its processors through an AGESA microcode update. This unannounced change impacts all Zen 4 CPUs, including the Ryzen 7000 series and the latest Epyc server models.

The Loop Buffer feature was designed to enhance performance and efficiency on Zen 4 CPUs, but its less-than-optimal implementation and lack of detailed documentation meant developers were unable to fully leverage its potential. Reports suggest that its removal does not impact overall performance, as the Ryzen 7000 CPUs can operate just as effectively without it. This is largely thanks to Zen 4’s Op Cache, which handles tasks the Loop Buffer was intended to manage.

According to Chips and Cheese, which first identified the change, the feature was disabled between the BIOS 1.21 (AGESA 1.0.0.6 patch) and BIOS 3.10 (AGESA 1.2.0.2a) releases. The discovery was made while testing the AMD Ryzen 9 7950X3D CPU on an ASRock B650 PG Lightning motherboard.

Chips and Cheese tested the Ryzen 9 7950X3D using SPEC CPU2017 benchmarks to evaluate any potential impact on performance. The results showed less than a one percent performance drop for both integer and floating-point workloads, with no impact on SMT performance. In a built-in Cyberpunk 2077 benchmark, disabling the Loop Buffer had no effect when the game ran on the V-Cache die, but a five percent performance drop was observed when running on the non-V-Cache die.

The Loop Buffer is a widely used feature in modern processors, employed by major chip designers like Intel, AMD, and Arm. It enables CPUs to store looped instruction sets in a small, dedicated memory region within the chip. These loops, consisting of repetitive code blocks, are executed multiple times during program operations. By storing these sets locally, the Loop Buffer allows the CPU to bypass cache or memory fetch operations, improving performance and efficiency.

At Hot Chips 2024, AMD engineers reportedly described the Loop Buffer in Zen 4 CPUs as “primarily a power optimization” feature rather than a performance-enhancing tool. This claim appears to hold up, as the Chips and Cheese tests suggest the feature had a minimal impact on performance in the Ryzen processors.