http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59448
--- Comment #6 from algrant at acm dot org ---
Here is a complete C++11 test case. The code generated for the two loads
in thread B doesn't maintain the required ordering:
...
ldrb w1, [x0]
uxtb w1, w1
adrp x0, .LANCHOR0
ldr w2, [x3]
...
According to the architecture specification, to achieve the ordering it's
sufficient to use the result of the first load in the calculation of the
address of the second, even if it's done in such a way as to have no
dependence on the value.
/* Reproducer for 59448: compile with -std=c++11 -lpthread */
#include <atomic>
#include <thread>
#include <stdio.h>
#include <assert.h>
static std::atomic<bool> flag(0);
static int data;
/*
Thread A tries to release a data value 1 to thread B via a
release/consume sequence.
In the demonstration, this is tried repeatedly. The iterations of
the two threads are synchronized via a release/acquire sequence
from B to A. This is not relevant to the ordering problem.
*/
void threadA(void)
{
for (;;) {
data = 1; // A.A
flag.store(1, std::memory_order_release); // A.B
/* By 1.9#14, A.A is sequenced before A.B */
/* Now wait for thread B to accept the data. */
while (flag.load(std::memory_order_acquire) == 1);
assert(data == 0);
}
}
void threadB(void)
{
for (;;) {
int f, d;
do {
f = flag.load(std::memory_order_consume); // B.A
d = *(&data + f - f); // B.B
/* By 1.10#9, B.A carries a dependency to B.B */
/* By 1.10#10, A.B is dependency-ordered before B.B */
/* By 1.10#11, A.A intra-thread happens before B.B */
/* By 1.10#12, A.A happens before B.B */
/* By 1.10#13, A.A is a visible side-effect with respect to B.B
and the value determined by B.B shall be the
value (1) stored by A.A. */
} while (!f);
assert(d == 1);
/* Release thread A to start another iteration. */
data = 0;
flag.store(0, std::memory_order_release);
}
}
int main(void)
{
std::thread a(&threadA);
std::thread b(&threadB);
a.join();
b.join();
return 0;
}
