https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116854
--- Comment #18 from Thiago Macieira <thiago at kde dot org> ---
(In reply to Uroš Bizjak from comment #16)
> --quote--
> Note, that clearing the RDRAND CPUID bit does not prevent a processor
> that normally supports the RDRAND instruction from executing it. So any
> code that determined the support based on family and model won't #UD.
> --/quote--
FYI, Qt has some code to test the quality of the RNG at runtime because of this
very issue.
QT_FUNCTION_TARGET(BASELINE_RDRND) Q_DECL_COLD_FUNCTION
static bool checkRdrndWorks() noexcept
{
/*
* Some AMD CPUs (e.g. AMD A4-6250J and AMD Ryzen 3000-series) have a
* failing random generation instruction, which always returns
* 0xffffffff, even when generation was "successful".
*
* This code checks if hardware random generator generates four consecutive
* equal numbers. If it does, then we probably have a failing one and
* should disable it completely.
*
* https://bugreports.qt.io/browse/QTBUG-69423
*/
However, this check is bypassed if __RDRND__ was defined by the compiler.
