Zen 4 user reproduces no issue with RDRAND16
3 Sep 22 7:48 AM · 1d ago · 2 comments · 1 source · development 3 of 4
A Zen 4 user (7840U) runs a test with GCC intrinsics and confirms zero values generate normally on that architecture, showing the bug is not universal across all Zen generations.
jstanley Zen 2 CPU owner, bug reproducer0x000xca0xfe Hardware/cryptography developerAMD CPU manufacturer
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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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I have a couple questions:Looks like they tried 16-bit numbers. Does the odd behavior happen also on 32 and 64 (might take a long time to check - I'd start scratching my head after a couple hundred years of no zeroes) ones? Is the zero masking as some other fixed number, increasing its output count? Is RDRAND implemented as multiple reads of an…
All 4 developments of AMD's RDRAND16 instruction can't generate zero on some Zen… →
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