[Bug target/81935] New: [8 Regression] r251260 miscompiles libstdc++ (on ppc64le)

[trippels at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81935

            Bug ID: 81935
           Summary: [8 Regression] r251260 miscompiles libstdc++ (on
                    ppc64le)
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: trippels at gcc dot gnu.org
                CC: aldyh at gcc dot gnu.org
  Target Milestone: ---
            Target: ppc64le

Since r251260 several programs spin in libstdc++.so.6.0.25.

For example cmake:

-   99.99%     1.59%  cmake    libstdc++.so.6.0.25  [.] __cxa_guard_acquire
   - 98.40% __cxa_guard_acquire
      + 10.87% futex_wait_setup
      + 6.86% gup_pte_range
      + 6.86% get_futex_key
        6.63% fast_exception_return
      + 6.10% drop_futex_key_refs.isra.11
      + 6.09% put_page
      + 4.97% syscall
      + 4.90% _raw_spin_lock
      + 4.86% unlock_page
      + 3.08% __get_user_pages_fast
      + 3.02% system_call_relon_pSeries
      + 2.77% system_call_common
      + 2.57% system_call
      + 2.46% __smp_mb__after_atomic
      + 2.44% syscall_exit_work
      + 2.28% __audit_syscall_exit
      + 2.16% do_futex
      + 2.01% __audit_syscall_entry
      + 1.94% do_syscall_trace_leave
      + 1.42% kfree
      + 1.37% dput
        1.17% ret_from_except_lite
      + 1.10% do_syscall_trace_enter
      + 1.04% mntput
      + 1.01% futex_wait
        0.94% current_kernel_time
      + 0.88% __cxa_guard_acquire
      + 0.87% sys_futex
        0.76% __syscall_error
      + 0.74% hash_futex
      + 0.73% memset
        0.61% path_put
        0.59% get_user_pages_fast
   + 1.59% _dl_start_user
+   99.99%     0.00%  cmake    libstdc++.so.6.0.25  [.]
_GLOBAL__sub_I_compatibility_thread_c__0x.cc
+   99.99%     0.00%  cmake    ld-2.17.so           [.] _dl_init_internal
+   99.99%     0.00%  cmake    ld-2.17.so           [.] _dl_start_user
+   87.27%     0.00%  cmake    libstdc++.so.6.0.25  [.]
std::future_error::future_error
+   23.29%    10.87%  cmake    [kernel.kallsyms]    [k] futex_wait_setup
+   21.54%     2.57%  cmake    [kernel.kallsyms]    [k] system_call
+   18.08%     6.86%  cmake    [kernel.kallsyms]    [k] get_futex_key
+   16.45%     0.87%  cmake    [kernel.kallsyms]    [k] sys_futex
+   15.82%     2.16%  cmake    [kernel.kallsyms]    [k] do_futex

ranlib also spins in the same place.
Reverting the commit "fixes" the problem.