Observe memory reordering
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
#include <pthread.h> | |
#include <stdatomic.h> | |
#include <stdio.h> | |
#ifndef N | |
# define N 10000000 | |
#endif | |
// Spin-lock barrier for two threads. Initialize to zero. | |
static void | |
barrier(_Atomic unsigned *barrier) | |
{ | |
unsigned v = ++*barrier; | |
if (v & 1) { | |
for (v &= 2; (*barrier&2) == v;); | |
} | |
} | |
struct vars { | |
_Atomic unsigned barrier; | |
int w0, w1; | |
int r0, r1; | |
}; | |
static int | |
experiment(int *w0, int *w1) | |
{ | |
int r1; | |
#if __amd64__ | |
__asm volatile ( | |
"movl $1, %1\n" | |
"movl %2, %0\n" | |
: "=r"(r1), "=m"(*w0) | |
: "m"(*w1) | |
); | |
#elif __aarch64__ | |
r1 = 1; | |
__asm volatile ( | |
"str %w0, %1\n" | |
"ldr %w0, %2\n" | |
: "+r"(r1), "=m"(*w0) | |
: "m"(*w1) | |
); | |
#else | |
atomic_store_explicit((atomic_int *)w0, 1, memory_order_relaxed); | |
r1 = atomic_load_explicit((atomic_int *)w1, memory_order_relaxed); | |
#endif | |
return r1; | |
} | |
static void * | |
thread(void *arg) | |
{ | |
struct vars *t = arg; | |
for (int i = 0; i < N; i++) { | |
barrier(&t->barrier); | |
t->r0 = experiment(&t->w1, &t->w0); | |
barrier(&t->barrier); | |
} | |
return 0; | |
} | |
int | |
main(void) | |
{ | |
int count = 0; | |
struct vars t[1] = {{0}}; | |
pthread_t thr; | |
pthread_create(&thr, 0, thread, t); | |
for (int i = 0; i < N; i++) { | |
t->w0 = t->w1 = 0; | |
barrier(&t->barrier); | |
t->r1 = experiment(&t->w0, &t->w1); | |
barrier(&t->barrier); | |
count += !t->r0 && !t->r1; | |
} | |
pthread_join(thr, 0); | |
printf("%d / %d (%.9g%%)\n", count, N, 100.0*count/N); | |
} |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment