Thread

AMD's random number generator can't generate a 0?

288 points · 220 comments · BruceEel

jstanley · 1790072957 · focus · HN ↗
This is not the first RNG bug on Zen 2, I recall after I first got mine that some application or other would quit immediately at startup because rdrand always returned -1, i.e. all 1s. It was fixed with a microcode update.
Do we now learn that they fixed "always generate all 1s" with "never generate all 0s"??
EDIT: I've been unable to reproduce the problem on my CPU, FWIW. It's a Ryzen 5 3600.
EDIT2: OK, update, I can reproduce it with rdrand16, rdrand32 is fine but rdrand16 can never generate all 0s. So my CPU does have this problem!
1. RandomOnyx · 1790074761 · focus · HN ↗
  Does rdrand32 and then taking the lowest 16 bits of its result yield any zeroes?
  Basically I'm wondering if it's a bug in the version of the instruction that writes to a 16-bit reg, or a bug in the underlying RNG
  1. jstanley · 1790074816 · focus · HN ↗
    Yes it does. rdrand32()%65535 was my first attempt, and generated zeroes at about the expected rate, that's why I initially erroneously thought my CPU did not have this problem.
    goalieca · 1790075079 · focus · HN ↗
    You should be using &0xFFFF for masking. Your mod is off by 1 too.
    jstanley · 1790077686 · focus · HN ↗
    You're right, the code was correct but my comment above is wrong.

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