[gcc r13-5247] libstdc++: Fix std::random_device::entropy() for non-posix targets

[gcc r13-5247] libstdc++: Fix std::random_device::entropy() for non-posix targets

[Jonathan Wakely]

https://gcc.gnu.org/g:26c68b8c31f637cc01f4bf511f9a0ca714231161

commit r13-5247-g26c68b8c31f637cc01f4bf511f9a0ca714231161
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jan 18 13:09:10 2023 +0000

    libstdc++: Fix std::random_device::entropy() for non-posix targets
    
    Since the r12-4515-g58f339fc5eaae7 change std::random_device::entropy()
    returns non-zero for hardware sources such as RDRAND. However, the call
    to the underlying _M_getentropy function is conditionally compiled
    according to #if _GLIBCXX_USE_DEV_RANDOM which means it only happens for
    targets that support /dev/random and /dev/urandom. This means entropy()
    always returns zero for x86 Windows, even though the RDRAND and RDSEED
    sources work there.
    
    The _M_getentropy() function is always compiled into the library, it
    just doesn't get called for targets without /dev/random. We can change
    that just by removing the #if conditional. This is not an ABI change,
    because new code will just start calling the existing _M_getentropy
    function, old code that has inlined entropy() will not call it.
    
    Similarly, the std::random_device destructor doesn't call the underlying
    _M_fini function unless _GLIBCXX_USE_DEV_RANDOM is defined. That's less
    of a problem because it's still true that the only resources that need
    to be freed are when one of /dev/random or /dev/urandom has been opened
    for reading, which is only possible when _GLIBCXX_USE_DEV_RANDOM is
    defined. The _M_fini function does also destroy a random engine object
    if a std::linear_congruential_engine object is used, but that destructor
    is trivial and so no resources are leaked if it's not called. Remove the
    preprocessor condition in the destructor too, so that we always call the
    _M_fini function even if it doesn't have side effects. This makes the
    destructor non-trivial for Windows and bare metal targets, but as the
    class is non-copyable that shouldn't cause any ABI change in practice.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/random.h (random_device) [!_GLIBCXX_USE_DEV_RANDOM]:
            Always call _M_fini and _M_getentropy.

Diff:
---
 libstdc++-v3/include/bits/random.h | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/libstdc++-v3/include/bits/random.h b/libstdc++-v3/include/bits/random.h
index e2b9bdf568c..42f37c1e77e 100644
--- a/libstdc++-v3/include/bits/random.h
+++ b/libstdc++-v3/include/bits/random.h
@@ -1639,10 +1639,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     explicit
     random_device(const std::string& __token) { _M_init(__token); }
 
-#if defined _GLIBCXX_USE_DEV_RANDOM
     ~random_device()
     { _M_fini(); }
-#endif
 
     static constexpr result_type
     min()
@@ -1654,13 +1652,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
     double
     entropy() const noexcept
-    {
-#ifdef _GLIBCXX_USE_DEV_RANDOM
-      return this->_M_getentropy();
-#else
-      return 0.0;
-#endif
-    }
+    { return this->_M_getentropy(); }
 
     result_type
     operator()()