Thread

1. 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. peri-cl · 1790077737 · focus · HN ↗

      Zen 4 reporting in. I'm unable to reproduce it (7840U).

         $ ./a.out | rg '\b\-?\d\b' | sort -n | uniq -c
         15281 -2
         15192 -1
         15273 0
         15243 1
         15269 2
      

      I used the GCC intrinsic ( _rdrand16_step ),

          #include <immintrin.h>
          
          short rdrand16() {     // gcc -mrdrnd
              short ret;
              while (1 != _rdrand16_step(&ret)) { }    
              return ret;
          }

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