https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85494
--- Comment #19 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:5f070ba29803c99a5fe94ed7632d7b8c55593df3 commit r11-7867-g5f070ba29803c99a5fe94ed7632d7b8c55593df3 Author: Jonathan Wakely <jwak...@redhat.com> Date: Fri Mar 26 18:39:49 2021 +0000 libstdc++: Add PRNG fallback to std::random_device This makes std::random_device usable on VxWorks when running on older x86 hardware. Since the r10-728 fix for PR libstdc++/85494 the library will use the new code unconditionally on x86, but the cpuid checks for RDSEED and RDRAND can fail at runtime, depending on the hardware where the code is executing. If the OS does not provide /dev/urandom then this means the std::random_device constructor always fails. In previous releases if /dev/urandom is unavailable then std::mt19937 was used unconditionally. This patch adds a fallback for the case where the runtime cpuid checks for x86 hardware instructions fail, and no /dev/urandom is available. When this happens a std::linear_congruential_engine object will be used, with a seed based on hashing the engine's address and the current time. Distinct std::random_device objects will use different seeds, unless an object is created and destroyed and a new object created at the same memory location within the clock tick. This is not great, but is better than always throwing from the constructor, and better than always using std::mt19937 with the same seed (as GCC 9 and earlier do). libstdc++-v3/ChangeLog: * src/c++11/random.cc (USE_LCG): Define when a pseudo-random fallback is needed. [USE_LCG] (bad_seed, construct_lcg_at, destroy_lcg_at, __lcg): New helper functions and callback. (random_device::_M_init): Add 'prng' and 'all' enumerators. Replace switch with fallthrough with a series of 'if' statements. [USE_LCG]: Construct an lcg_type engine and use __lcg when cpuid checks fail. (random_device::_M_init_pretr1) [USE_MT19937]: Accept "prng" token. (random_device::_M_getval): Check for callback unconditionally and always pass _M_file pointer. * testsuite/26_numerics/random/random_device/85494.cc: Remove effective-target check. Use new random_device_available helper. * testsuite/26_numerics/random/random_device/94087.cc: Likewise. * testsuite/26_numerics/random/random_device/cons/default-cow.cc: Remove effective-target check. * testsuite/26_numerics/random/random_device/cons/default.cc: Likewise. * testsuite/26_numerics/random/random_device/cons/token.cc: Use new random_device_available helper. Test "prng" token. * testsuite/util/testsuite_random.h (random_device_available): New helper function.
