[Bug tree-optimization/108552] Linux i386 kernel 5.14 memory corruption for pre_compound_page() when gcov is enabled

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108552

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |jakub at gcc dot gnu.org

--- Comment #21 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I'd say using the (default unless -pthread is used) -fprofile-update=single is
wrong for the kernel, it can't work correctly in multi-threaded case which is
the case of kernel.
In the -fprofile-update=single (as opposed to -fprofile-update=atomic) the
updates to the counters aren't atomic and the arrays aren't marked volatile or
something similar, it is really meant for single threaded coverage.

Anyway, before ivopts we have:
  pretmp_93 = __gcov0.prep_compound_page[7];

  <bb 9> [local count: 955630225]:
  # i_66 = PHI <i_17(26), 1(8)>
  # prephitmp_92 = PHI <PROF_edge_counter_46(26), pretmp_93(8)>
  i.144_1 = (unsigned int) i_66;
  _2 = i.144_1 * 40;
  p_15 = page_12(D) + _2;
  p_15->D.13727.D.13672.mapping = 1024B;
  MEM[(volatile long unsigned int *)p_15 + 4B] ={v} _159;
  i_17 = i_66 + 1;
  PROF_edge_counter_46 = prephitmp_92 + 1;
  __gcov0.prep_compound_page[7] = PROF_edge_counter_46;
  if (nr_pages_11 > i_17)
    goto <bb 26>; [89.00%]
  else
    goto <bb 7>; [11.00%]

  <bb 26> [local count: 850510901]:
  goto <bb 9>; [100.00%]
which given the non-volatile non-atomically updated arrays is to be expected,
instead of re-reading __gcov0.prep_compound_page[7] in every iteration it just
reads it once and stores in each iteration, which is possible because another
thread changing it concurrently would mean a data race anyway.