Introduction to Concurrency in Programming Languages

90 Introduction to Concurrency in Programming Languages

Trivial as it may seem to start with expressions, even this most fundamen-

tal of language abstractions has consequences in the context of concurrent

programming. Expressions are not necessarily atomic. In the example above,

we see that the single expression is actually implemented as a sequence of

simpler subexpressions that are generated by the compiler. Clearly if atomic-

ity of an expression is important, then we must take care to protect complex

expressions just as carefully as we would a large critical region. If one writes

an expression such as

x = a[1] + (a[0] * (b[1] / a[1]))

which is ultimately compiled to a sequence of operations, what happens if the

subexpression for b[1] / a[1] executes, ...