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Consider this deceivingly simple program statement
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int * a ( * a ) ++ j
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(Forgive the C++-isms for those managed programmers reading this ( * a ) ++ is used instead of a ++, just to make it obvious that a points to some shared memory location) When translated into machine code by the compiler this seemingly simple, high-level, single-line statement involves multiple machine instructions:
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MOV EAX , [ a ] INC EAX MOV [ a ] , EAX
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Notice that, as a first step, the machine code dereferences a to get some virtual memory address and copies 4 bytes' worth of memory starting at that address into the processor local EAX register The code then incre ments the value of its private copy in EAX, and, lastly, makes yet another copy of the value, this time to copy the incremented value held in its private register back to the shared memory location referred to by a The multiple steps and copies involved i n the + + operator weren' t apparent in the source file at all If you were manipulating multiple vari ables explicitly, the fact that there are multiple steps would be a little more apparent In fact, it's as though we had written:
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int * a j int tmp *aj tmp++ j *a = tmp j
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= =
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Any software operation that requires multiple hardware instructions is nonatomic And thus we've now established that ++ is nonatomic (as is - - ), meaning we will have to take extra steps to ensure concurrency safety There are some other nonobvious sources of nonatomic operations Modern proces sors guarantee that single reads from and writes to memory in increments of the natural word size of the machine will be carried out atomically covering 32-bit values on 32-bit machines and 64-bit values on 64-bit machines
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Conversely, reading or writing data with a size larger than the addressable unit of memory on your CPU is nonatomic For instance, if you wrote a 64-bit value on a 32-bit machine, it will entail two move instructions from processor private to shared memory, each to copy a 4-byte segment Similarly, reading from or writing to unaligned addresses (Le, address ranges that span an addressable unit of memory) also require multiple memory operations in addition to some bit masking and shifting, even if the size of the value is less than or equal to the machine's addressable memory size Alignment is a tricky subject and is discussed in much more detail in 1 0, Memory Models and Lock Freedom So why is all of this a problem An increment statement is meant to monotonically increase the value held in some memory location by a delta of 1 If three increments were made to a counter with an original value 0, you'd expect the final result to be 3 It should never be possible (overflow aside) for the value of the counter to decrease from one read to the next; therefore, if a thread executes two ( * a ) ++ operations, one after the other, you would expect that the sec ond update always yields a higher value than the first These are some very basic correctness conditions for our simple ( * a ) ++ program (Note: You shouldn' t be expecting that the two values will differ by precisely 1 , how ever, since another thread might have snuck in and run between them) There's a problem While the actual loads and stores execute atomically by themselves, the three operation sequence of load, increment, and store is nonatomic, as we've already established Imagine three threads, tl , t2, and t3, are running the compiled program instructions simultaneously
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t1 t l ( 0 ) : MOV EAX , [ a ] t 1 ( 1 ) : INC , EAX t l ( 2 ) : MOV [ a ] , EAX t2 t 2 ( 0 ) : MOV EAX , [ a ] t 2 ( 1 ) : INC , EAX t 2 ( 2 ) : MOV [ a ] , EAX t3 t 3 ( 0 ) : MOV EAX , [ a ] t 3 ( 1 ) : INC , EAX t 3 ( 2 ) : MOV [ a ] , EAX
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Each thread is running on a separate processor Of course, this means that each processor has its own private EAX register, but all threads see the same value in a and therefore access the same shared memory This is where time becomes a very useful tool for explaining the behavior of our concurrent programs Each of these steps won't really happen "simultane ously" Although separate processors can certainly execute instructions
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