Question

It is a general question but:

In a multithreaded program, is it safe for the compiler to use registers to temporarily store global variables?

I think its not, since storing global variables in registers may change saved values
for other threads.

And how about using registers to store local variables defined within a function?

I think it is ok,since no other thread will be able to get these variables.

Please correct me if im wrong.
Thank you!

Answer 1

Things are much more complicated than you think they are.

Even if the compiler stores a value to memory, the CPU generally does not immediately push the data out to RAM. It stores it in a cache (and some systems have 2 or 3 levels of caches between the processor and the memory).

To make things worse, the order of instructions that the compiler decides, may not be what actually gets executed as many processors can reorder instructions (and even sub-parts of instructions) in their own pipelines.

In general, in a multithreaded environment you should personally take care to never access (either read or write) the same memory from two separate threads unless one of the following is true:

• you are using one of several special atomic operations that ensure proper synchronization.
• you have used one of several synchronization operations to “reserve” access to shared data and then to “relinquish” it. These do include the required memory barriers that also guarantee the data is what it’s supposed to be.

You may want to read http://en.wikipedia.org/wiki/Memory_ordering#Memory_barrier_types and http://en.wikipedia.org/wiki/Memory_barrier

If you are ready for a little headache and want to see how complicated things can actually get, here is your evening lecture Memory Barriers: a Hardware View for Software Hackers.