[Bug tree-optimization/54900] New: write introduction incorrect wrt the C11 memory model (2)

[Bug tree-optimization/54900] New: write introduction incorrect wrt the C11 memory model (2)

[francesco.zappa.nardelli at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54900

             Bug #: 54900
           Summary: write introduction incorrect wrt the C11 memory model
                    (2)
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: francesco.zappa.nardelli@gmail.com


This program:

#include <stdio.h>
#include <pthread.h>

int g_8 = 1;
int g_140;
int *g_139 = &g_140;
int **g_138 = &g_139;
int g_182;

void func_2 (p1) {
  **g_138 = 0;
}

int func_11 (int p1, int p2, int p3, int p4) {
  if (g_8)
    return 0;
  ++g_182;
  return 0;
}

void *context (void *ptr) {
  g_182 = 1;
  printf ("%d\n",g_182);
}

void main () {
  pthread_t thread1;
  int  iret1;
  iret1 = pthread_create( &thread1, NULL, context, (void*) 0);

  func_2 (func_11 (0, 0, 0, 0) );

  pthread_join( thread1, NULL);
}

is miscompiled by gcc --param allow-store-data-races=0 -O2 (or -O3) on x86_64.

[ gcc version 4.8.0 20121011 (experimental) (GCC) ]

The program has no data-races because the ++g_182 instruction in func_11 is
never executed by the main thread, and the context thread is expected to always
print 1.

The -O2 and -O3 optimisers (invoked with --param allow-store-data-races=0)
compile main as:

main:
        subq    $24, %rsp
        xorl    %ecx, %ecx
        xorl    %esi, %esi
        leaq    8(%rsp), %rdi
        movl    $context, %edx
        call    pthread_create

        xorl    %eax, %eax
        cmpl    $1, g_8(%rip)
        movq    8(%rsp), %rdi
        setb    %al
(**)    addl    %eax, g_182(%rip)
        movq    g_138(%rip), %rax

        xorl    %esi, %esi
        movq    (%rax), %rax
        movl    $0, (%rax)
        call    pthread_join
        addq    $24, %rsp
        ret

The problem is in the (**) instruction:

      addl    %eax, g_182(%rip)

which inserts a write of the value 0 in the run-time trace of the main thread,
possibly resulting in the context thread printing 0.

[Bug tree-optimization/54900] write introduction incorrect wrt the C11 memory model (2)

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54900

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

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

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-10-11 11:59:14 UTC ---
This is ifcvt.c in action.
This is the if (!set_b && MEM_P (orig_x)) case where we already do some checks:
if (noce_mem_write_may_trap_or_fault_p (orig_x)) return FALSE; and
if (!noce_can_store_speculate_p (test_bb, orig_x)) return FALSE;
I'd say noce_can_store_speculate_p is buggy, it uses
          if (memory_modified_in_insn_p (mem, insn))
            return true;
but memory_modified_in_insn_p is pessimistic, it doesn't tell whether mem is
surely set, but whether it might be set.  I guess it would need to use
note_stores that would just do rtx_equal_p on the addresses or similarly prove
it is surely (and unconditionally) written.  So even note_stores might not be
the right thing, perhaps just looking at single_set SET_DEST.  And avoiding
inline asm, that doesn't have to store unconditionally.

[Bug tree-optimization/54900] write introduction incorrect wrt the C11 memory model (2)

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54900

--- Comment #2 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-10-14 12:51:15 UTC ---
Created attachment 28444
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28444
reduced testcase

[Bug tree-optimization/54900] write introduction incorrect wrt the C11 memory model (2)

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54900

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-10-14
     Ever Confirmed|0                           |1

[Bug rtl-optimization/54900] write introduction incorrect wrt the C11 memory model (2)

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54900

--- Comment #3 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-10-17 20:59:43 UTC ---
Author: aldyh
Date: Wed Oct 17 20:59:40 2012
New Revision: 192548

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=192548
Log:
    PR rtl-optimization/54900
    * ifcvt.c (noce_can_store_speculate_p): Call
    memory_must_be_modified_in_insn_p.
    * alias.c (memory_must_be_modified_in_insn_p): New.
    (set_dest_equal_p): New.
    * rtl.h (memory_must_be_modified_in_p): Protoize.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/alias.c
    trunk/gcc/ifcvt.c
    trunk/gcc/rtl.h

[Bug rtl-optimization/54900] write introduction incorrect wrt the C11 memory model (2)

[francesco.zappa.nardelli at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54900

--- Comment #4 from Francesco Zappa Nardelli <francesco.zappa.nardelli at gmail dot com> 2012-10-18 13:39:30 UTC ---
gcc version 4.8.0 20121018 (experimental) - which includes revision 192548 -
compiles this example correctly.  

It also fixes http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54906 .

Great, thanks.

[Bug rtl-optimization/54900] write introduction incorrect wrt the C11 memory model (2)

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54900

--- Comment #5 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-10-18 23:46:04 UTC ---
I am leaving this PR open while I address the corner case presented by Jakub
somewhere in this thread:

http://gcc.gnu.org/ml/gcc-patches/2012-10/msg01763.html

...though technically the testcase in this PR has been fixed :).

[Bug rtl-optimization/54900] write introduction incorrect wrt the C11 memory model (2)

[steven at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54900

--- Comment #6 from Steven Bosscher <steven at gcc dot gnu.org> ---
(In reply to Aldy Hernandez from comment #5)
> I am leaving this PR open while I address the corner case presented by Jakub
> somewhere in this thread:
> 
> http://gcc.gnu.org/ml/gcc-patches/2012-10/msg01763.html
> 
> ...though technically the testcase in this PR has been fixed :).

Maybe open a new PR for those corner cases, and put some test cases in
it?  Leaving this open without further reference to an actual problem
is confusing...