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: unassig...@gcc.gnu.org
ReportedBy: francesco.zappa.narde...@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.
