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From: "francesco.zappa.nardelli at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/54900] New: write introduction incorrect wrt the C11 memory model (2)
Date: Thu, 11 Oct 2012 11:07:00 -0000
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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.