[gcc r12-8592] libstdc++: Simplify test by not using std::log2

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:145ef5e2be28183f2b92b1f81b4d75210b524503

commit r12-8592-g145ef5e2be28183f2b92b1f81b4d75210b524503
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jun 23 11:58:16 2022 +0100

    libstdc++: Simplify test by not using std::log2
    
    This test uses std::log2 without including <cmath>, but it doesn't need
    to use it at all. Just get the number of digits from numeric_limits
    instead.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/26_numerics/random/random_device/entropy.cc: Use
            numeric_limits<unsigned>::digits.
    
    (cherry picked from commit 78fd15fd4a641cfac28ce60bf43a3e1ca309537b)

Diff:
---
 libstdc++-v3/testsuite/26_numerics/random/random_device/entropy.cc | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/testsuite/26_numerics/random/random_device/entropy.cc b/libstdc++-v3/testsuite/26_numerics/random/random_device/entropy.cc
index 63b7043bf9b..9f529f5d814 100644
--- a/libstdc++-v3/testsuite/26_numerics/random/random_device/entropy.cc
+++ b/libstdc++-v3/testsuite/26_numerics/random/random_device/entropy.cc
@@ -1,6 +1,7 @@
 // { dg-do run { target c++11 } }
 
 #include <random>
+#include <limits>
 #include <testsuite_hooks.h>
 #include <testsuite_random.h>
 
@@ -12,7 +13,7 @@ test01()
       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);
+  const double max = std::numeric_limits<result_type>::digits;
 
   for (auto token : { "/dev/random", "/dev/urandom" })
     if (__gnu_test::random_device_available(token))