Fabian GiesenMay 21, 2025

What's that mysterious workaround?

Core Huff6 decode step is described in https://fgiesen.wordpress.com/2023/10/29/entropy-decoding-in-oodle-data-x86-64-6-stream-huffman-decoders/

A customer managed to get a fairly consistent repro for transient decode errors by overclocking an i7-14700KF by about 5% from stock settings ("performance" multiplier 56->59).

It took weeks of back and forth and forensic debugging to figure out what actually happens, but TL;DR: the observed decode errors are all consistent with a single instruction misbehaving.

Entropy decoding in Oodle Data: x86-64 6-stream Huffman decoders

It’s been a while! Last time, I went over how the 3-stream Huffman decoders in Oodle Data work. The 3-stream layout is what we originally went with. It gives near-ideal performance on the las…

The ryg blog
Show threadFabian GiesenMay 21, 2025

This instruction:
mov [rDest + <index>], ch

under these conditions, when overclocked a bit, once the machine has "warmed up", seems to have around a 1/10000 chance of actually storing the contents of CL instead of CH to memory.

(this was "fun" to debug.)

The workaround: when we detect Raptor Lake CPUs, we now do

shr ecx, 8
mov [rDest + <index>], cl

instead. This takes more FE and uop bandwidth, but this loop is mainly latency-limited, and this is off the critical path.

Show threadVery Human RobotMay 22, 2025

@rygorous How about "actually, we don't support overclocking?"

(I get that customers will complain no matter what, so this is probably the optimal solution though ...)

Show threadFabian GiesenMay 22, 2025
@StompyRobot oh they do that without overclocking too, OC just makes for a more consistent repro
Show threadKim Spence-Jones 🇬🇧😷May 22, 2025
@rygorous @StompyRobot
😱
Show threadFabian Giesen
@KimSJ @StompyRobot to reiterate, this is a known HW problem that made the rounds last year and has been confirmed by Intel https://community.intel.com/t5/Blogs/Tech-Innovation/Client/Intel-Core-13th-and-14th-Gen-Desktop-Instability-Root-Cause/post/1633239 as a physical problem with the circuit that causes it to degrade over time. To some extent all chips have this kind of thing but normally this happens on a timescale of several years of intensive use, not weeks or months
Intel Core 13th and 14th Gen Desktop Instability Root Cause Update

Following extensive investigation of the Intel® Core™ 13th and 14th Gen desktop processor Vmin Shift Instability issue, Intel can now confirm the root cause diagnosis for the issue. This post will cover Intel’s understanding of the root cause, as well as additional mitigations and next steps for Int...

May 22, 2025 at 9:40amWeb