https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94087
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Last reconfirmed| |2020-05-14
--- Comment #10 from Richard Biener <rguenth at gcc dot gnu.org> ---
So it looks like the rdseed usage is new in GCC 10 libstdc++ and it prevails
over the previous rdrand support if supported on your CPU.
I can reproduce this on a CPU with rdseed support and libstdc++ from GCC 10.
The code invoked looks correct to me:
20: 83 e8 01 sub $0x1,%eax
23: 74 12 je 37
<_ZNSt12_GLOBAL__N_112__x86_rdseedEPv+
0x37>
25: f3 90 pause
27: 0f c7 fa rdseed %edx
2a: 89 11 mov %edx,(%rcx)
2c: 73 f2 jae 20
<_ZNSt12_GLOBAL__N_112__x86_rdseedEPv+
0x20>
the number of tries libstdc++ does is 100. Note rdrand doesn't exhibit this
issue.
So it might very well be a hardware limitation. Btw, the reproducer can be
"enhanced" by providing the method of operation:
std::random_device rd("rdseed");
that makes sure it will fail in a different way on a not capable CPU
(Intel Broadwell or later or AMD Zen).
