libstdc++: Use RDRAND as fallback if RDSEED keeps failing (PR 94087)

It's not difficult for multiple threads to drain the entropy available
to the RDSEED instruction, at which point we throw an exception. This
change will try to use RDRAND after RDSEED fails repeatedly, and only
throw if RDRAND also fails repeatedly. This doesn't guarantee a random
value can always be read, but reduces the likelihood of failure when
using the RDSEED instruction.

	PR libstdc++/94087
	* src/c++11/random.cc (__x86_rdseed): Allow fallback function to be
	passed in.
	(__x86_rdseed_rdrand): New function that uses rdseed with rdrand
	fallback.
	(random_device::_M_init): Use __x86_rdseed_rdrand when both
	instructions are available.
	* testsuite/26_numerics/random/random_device/94087.cc: New test.

1 files changed, 24 insertions, 2 deletions

diff --git a/libstdc++-v3/src/c++11/random.cc b/libstdc++-v3/src/c++11/random.cc
index 236eccf..62ed274 100644
--- a/libstdc++-v3/src/c++11/random.cc
+++ b/libstdc++-v3/src/c++11/random.cc
@@ -97,7 +97,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
 #if USE_RDSEED
     unsigned int
     __attribute__ ((target("rdseed")))
-    __x86_rdseed(void*)
+    __x86_rdseed(void* fallback)
     {
       unsigned int retries = 100;
       unsigned int val;
@@ -105,12 +105,25 @@ namespace std _GLIBCXX_VISIBILITY(default)
       while (__builtin_ia32_rdseed_si_step(&val) == 0)
 	{
 	  if (--retries == 0)
-	    std::__throw_runtime_error(__N("random_device: rdseed failed"));
+	    {
+	      if (auto f = reinterpret_cast<unsigned int(*)(void*)>(fallback))
+		return f(nullptr);
+	      std::__throw_runtime_error(__N("random_device: rdseed failed"));
+	    }
 	  __builtin_ia32_pause();
 	}
 
       return val;
     }
+
+#if USE_RDRAND
+    unsigned int
+    __attribute__ ((target("rdseed,rdrnd")))
+    __x86_rdseed_rdrand(void*)
+    {
+      return __x86_rdseed(reinterpret_cast<void*>(&__x86_rdrand));
+    }
+#endif
 #endif
 
 #ifdef _GLIBCXX_USE_CRT_RAND_S
@@ -205,6 +218,15 @@ namespace std _GLIBCXX_VISIBILITY(default)
 	    __cpuid_count(7, 0, eax, ebx, ecx, edx);
 	    if (ebx & bit_RDSEED)
 	      {
+#ifdef USE_RDRAND
+		// CPUID.01H:ECX.RDRAND[bit 30]
+		__cpuid(1, eax, ebx, ecx, edx);
+		if (ecx & bit_RDRND)
+		  {
+		    _M_func = &__x86_rdseed_rdrand;
+		    return;
+		  }
+#endif
 		_M_func = &__x86_rdseed;
 		return;
 	      }