[Bug c++/82803] New: Wildly excessive calls to __tls_get_addr with optimizations enabled.

["amohr at amohr dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82803

            Bug ID: 82803
           Summary: Wildly excessive calls to __tls_get_addr with
                    optimizations enabled.
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amohr at amohr dot org
  Target Milestone: ---

gcc emits a call to __tls_get_addr for every iteration of the while loop below
at -O1 or above.  It does not happen at -O0.

static thread_local int x;
int g();
int f(int c) {
  int *px = &x;
  while (c--)
    *px =+ g();
  return *px;
}

Compile with: -O2 -std=c++11 -fPIC -Wall -Wextra

I tried it with several versions back to 4.8.2 with the same results.  At -O0
exactly one call is emitted.  Seems like the optimizer is doing something silly
with thread_locals.

https://godbolt.org/g/S9hvHQ