Re: [boost] Review request: Boost.FSM
This is just wrong. It is perfectly legal
for the compiler to reorder it to be if (!m_fInitialized) { m_fInitialized = true; // Fill m_StatesInfo array for all states const root_type* pRoot = pStates; pStates->__init_states_info(reinterpret_cast< const char*
(pRoot), m_StatesInfo);
} then Thread A if (!m_fInitialized) { m_fInitialized = true; Thread B if (!m_fInitialized) { } //use m_StatesInfo. //die.
I'd rather say, your scenario is possible here but not necessarily it will happen. A compiler _may_ be smart enough to reorder expressions if it may prove that it won't break the code behavior (which is true here) and it will gain something (performance, for example) from it. And even if it does so consider that Thread A in your example may be interleaved by Thread B right between it checks "m_fInitialized" and stores "true" in it. So it is actually possible that more than one thread will execute the "true" branch of the "if" statement concurrently. But we're on the safe side here because only simple operations on POD types are involved in it. I see that it is ok for m_StatesInfo to be initialized more than once. My point was
AMDG Andrey Semashev wrote: that it can be used before it is initialized unless you make everything involved volatile. In Christ, Steven Watanabe
Hello Steven, Sunday, January 7, 2007, 2:02:10 AM, you wrote:
AMDG
This is just wrong. It is perfectly legal
for the compiler to reorder it to be if (!m_fInitialized) { m_fInitialized = true; // Fill m_StatesInfo array for all states const root_type* pRoot = pStates; pStates->__init_states_info(reinterpret_cast< const char*
(pRoot), m_StatesInfo);
} then Thread A if (!m_fInitialized) { m_fInitialized = true; Thread B if (!m_fInitialized) { } //use m_StatesInfo. //die.
I'd rather say, your scenario is possible here but not necessarily it will happen. A compiler _may_ be smart enough to reorder expressions if it may prove that it won't break the code behavior (which is true here) and it will gain something (performance, for example) from it. And even if it does so consider that Thread A in your example may be interleaved by Thread B right between it checks "m_fInitialized" and stores "true" in it. So it is actually possible that more than one thread will execute the "true" branch of the "if" statement concurrently. But we're on the safe side here because only simple operations on POD types are involved in it. I see that it is ok for m_StatesInfo to be initialized more than once. My point was
Andrey Semashev wrote: that it can be used before it is initialized unless you make everything involved volatile.
Oh, you're right. But isn't it enough just to mark "m_fInitialized" as "volatile"? According to 1.9/7 it would enforce the compiler to initialize "m_StatesInfo" prior to setting "m_fInitialized". -- Best regards, Andrey mailto:andysem@mail.ru
participants (2)
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Andrey Semashev -
Steven Watanabe