[Bug c/65712] New: pthread_self prints wrong result when used with ucontext

["albertnetymk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65712

            Bug ID: 65712
           Summary: pthread_self prints wrong result when used with
                    ucontext
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: albertnetymk at gmail dot com

When using no opitimization, the output is expected:

gcc test.c -pthread && ./a.out

B is 139778852222720
A is 139778843830016
A 139778843830016
B 139778852222720

However, once I turn on the opitmization, the output becomes:

gcc -O1 test.c -pthread && ./a.out

B is 140067829094144
A is 140067820701440
A 140067820701440
B 140067820701440

Inspect the assembly emitted, it seems that the two calls to `pthread_self` is
reduced to be one.

OS:

Linux

Program:

#define _XOPEN_SOURCE 800
#include <ucontext.h>
#include <signal.h>

#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <assert.h>

static int flag = 0;

void swap(ucontext_t *old, ucontext_t *new)
{
    int ret = swapcontext(old, new);
    assert(ret == 0);
}

#define SSIZE MINSIGSTKSZ

static char stack[SSIZE];
static ucontext_t a_ctx[2];
static ucontext_t b_ctx[2];

static void f1 (void)
{
    printf("A %lu\n", pthread_self());
    swap(&a_ctx[1], &a_ctx[0]);
    printf("B %lu\n", pthread_self());
}

void *thread_a(void *arg)
{
    printf("A is %lu\n", pthread_self());

    ucontext_t ctx = a_ctx[1];

    getcontext(&ctx);
    ctx.uc_stack.ss_sp = stack;
    ctx.uc_stack.ss_size = sizeof stack;
    makecontext(&ctx, f1, 0);

    swap(&a_ctx[0], &ctx);
    __atomic_store_n(&flag, 1, __ATOMIC_RELAXED);
    sleep(1);

    return NULL;
}

void *thread_b(void *arg)
{
    printf("B is %lu\n", pthread_self());

    while(__atomic_load_n(&flag, __ATOMIC_RELAXED) == 0) ;
    swap(&b_ctx[0], &a_ctx[1]);

    return NULL;
}

int main(int argc, char **argv)
{
    pthread_t a, b;
    pthread_create(&b, NULL, &thread_b, NULL);
    pthread_create(&a, NULL, &thread_a, NULL);
    pthread_exit(NULL);
}