libstdc++: Implement std::random_device::entropy() for other sources

Currently this function only returns a non-zero value for /dev/random
and /dev/urandom. When a hardware instruction such as RDRAND is in use
it should (in theory) be perfectly random and produce 32 bits of entropy
in each 32-bit result. Add a helper function to identify the source of
randomness from the _M_func and _M_file data members, and return a
suitable value when RDRAND or RDSEED is being used.

libstdc++-v3/ChangeLog:

	* src/c++11/random.cc (which_source): New helper function.
	(random_device::_M_getentropy()): Use which_source and return
	suitable values for sources other than device files.
	* testsuite/26_numerics/random/random_device/entropy.cc: New test.

libstdc++-v3/src/c++11/random.cc

@@ -192,6 +192,51 @@ namespace std _GLIBCXX_VISIBILITY(default)
       return lcg();
     }
 #endif
+
+    enum Which {
+      rand_s = 1, rdseed = 2, rdrand = 4, device_file = 8, prng = 16,
+      any = 0xffff
+    };
+
+    inline Which
+    which_source(random_device::result_type (*func [[maybe_unused]])(void*),
+		 void* file [[maybe_unused]])
+    {
+#ifdef _GLIBCXX_USE_CRT_RAND_S
+      if (func == &__winxp_rand_s)
+	return rand_s;
+#endif
+
+#ifdef USE_RDSEED
+#ifdef USE_RDRAND
+      if (func == &__x86_rdseed_rdrand)
+	return rdseed;
+#endif
+      if (func == &__x86_rdseed)
+	return rdseed;
+#endif
+
+#ifdef USE_RDRAND
+      if (func == &__x86_rdrand)
+	return rdrand;
+#endif
+
+#ifdef _GLIBCXX_USE_DEV_RANDOM
+      if (file != nullptr)
+	return device_file;
+#endif
+
+#ifdef USE_LCG
+      if (func == &__lcg)
+	return prng;
+#endif
+
+#ifdef USE_MT19937
+      return prng;
+#endif
+
+      return any; // should be unreachable
+    }
   }
 
   void
@@ -209,10 +254,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
 
     const char* fname [[gnu::unused]] = nullptr;
 
-    enum {
-	rand_s = 1, rdseed = 2, rdrand = 4, device_file = 8, prng = 16,
-	any = 0xffff
-    } which;
+    Which which;
 
     if (token == "default")
       {
@@ -449,10 +491,25 @@ namespace std _GLIBCXX_VISIBILITY(default)
   double
   random_device::_M_getentropy() const noexcept
   {
+    const int max = sizeof(result_type) * __CHAR_BIT__;
+
+    switch(which_source(_M_func, _M_file))
+    {
+    case rdrand:
+    case rdseed:
+      return (double) max;
+    case rand_s:
+    case prng:
+      return 0.0;
+    case device_file:
+      // handled below
+      break;
+    default:
+      return 0.0;
+    }
+
 #if defined _GLIBCXX_USE_DEV_RANDOM \
     && defined _GLIBCXX_HAVE_SYS_IOCTL_H && defined RNDGETENTCNT
-    if (!_M_file)
-      return 0.0;
 
 #ifdef USE_POSIX_FILE_IO
     const int fd = _M_fd;
@@ -469,7 +526,6 @@ namespace std _GLIBCXX_VISIBILITY(default)
     if (ent < 0)
       return 0.0;
 
-    const int max = sizeof(result_type) * __CHAR_BIT__;
     if (ent > max)
       ent = max;
 

libstdc++-v3/testsuite/26_numerics/random/random_device/entropy.cc

@@ -0,0 +1,37 @@
+// { dg-do run { target c++11 } }
+
+#include <random>
+#include <testsuite_hooks.h>
+#include <testsuite_random.h>
+
+void
+test01()
+{
+  for (auto token : { "mt19937", "prng", "rand_s" })
+    if (__gnu_test::random_device_available(token))
+      VERIFY( std::random_device(token).entropy() == 0.0 );
+
+  using result_type = std::random_device::result_type;
+  const double max = std::log2(std::numeric_limits<result_type>::max() + 1.0);
+
+  for (auto token : { "/dev/random", "/dev/urandom" })
+    if (__gnu_test::random_device_available(token))
+    {
+      const double entropy = std::random_device(token).entropy();
+      VERIFY( entropy >= 0.0 );
+      VERIFY( entropy <= max );
+    }
+
+  for (auto token : { "rdrand", "rdseed" })
+    if (__gnu_test::random_device_available(token))
+    {
+      const double entropy = std::random_device(token).entropy();
+      VERIFY( entropy == max );
+    }
+}
+
+int
+main()
+{
+  test01();
+}