Force order of execution of C statements?

Question

I have a problem with the MS C compiler reordering certain statements, critical in a multithreading context, at high levels of optimization. I want to know how to force ord

Answer 1

I would use the volatile keyword. It will prevent the compiler from re-ordering the instructions. http://www.barrgroup.com/Embedded-Systems/How-To/C-Volatile-Keyword

Answer 2

So as I understand it the pNext = plog2sizeChunk->pNext publishes the block so that it can be seen by other threads and you have to make sure they see the correct busy flag.

That means you need a uni-directional memory barrier before publishing it (also one before reading it in another thread, although if your code runs on x86 you get those for free) to make sure that threads actually see the change. You also need one before the write to avoid reordering writes after it. No just inserting assembly or using a standard compliant volatile (MSVC volatile gives extra guarantees though that make a difference here) is not enough - yes this stops the compiler from shifting reads and writes around, but the CPU is not bound by it and can do the same reordering internally.

Both MSVC and gcc have intrinsics/macros to create memory barriers (see eg here). MSVC also gives stronger guarantees to volatiles that are good enough for your problem. Finally C++11 atomics would work as well, but I'm not sure if C itself has any portable way to guarantee memory barriers.

Answer 3

See _ReadWriteBarrier. That's a compiler intrinsic dedicated to what you are looking for. Be sure to check the documentation against your precise version od MSVC ("deprecated" on VS2012...). Beware cpu reordering (then see MemoryBarrier

The documentation states that the _ReadBarrier, _WriteBarrier, and _ReadWriteBarrier compiler intrinsics (compiler reordering) and that the MemoryBarrier macro (CPU reordering) are all "deprecated" starting with VS2012. But i think they will continue to work fine for some time...

New code may use the new C++11 facilities (links in the MSDN page)