Here's a little snippet from the Sather FAQ. I don't know if you'll like it, because it talks about why they eliminated undefined order of evaluation, or if you'll hate it, because they say that doing so eliminates implicit parallelization possibilities because of the presense of side-effects.
In many languages, such as C, the order of evaluation of routine arguments is undefined. In Sather, however, arguments are always evaluated left to right. This brings greater determinism across platforms to Sather code; unlike C, there is no place in the Sather specification where we resort to declaring the results of an operation to be undefined.

A frequently cited reason for not specifying an order of evaluation is to allow the compiler to choose an order of evaluation which leads to the most efficient code; for example, simple arguments can be evaluated after complicated ones to relieve register pressure. This can also be done for ordered Sather arguments in the absence of side effects.

While the order is unspecified in C, the evaluations of arguments must appear to occur in _some_ order, not interleaved in execution. (In the extreme this would allow C compilers to fork threads when evaluating arguments, a practice which would break most existing code.) Since a compiler capable of taking advantage of the parallelism made available by unordered arguments must do dependency analysis to make sure the generated instructions appear to evaluate the arguments in some order, such a compiler would of course be able to do the same dependency analysis and instruction reordering on arguments required to be observed evaluating left to right. The generated code would only be different if there were side effects in an argument evaluation which would make the order of evaluation important; and such code would clearly be in error if the argument order was unspecified.

It's really a question about what the language does with erroneous code that depends on the order of evaluation. It would be nice to detect such situations, but this is very hard. By leaving the order unspecified one allows bugs (which usually appear only when changing compilers). Sather chooses to just eliminate the possibility.

In reply to Re^49: Why is EO undefined? (More details!) by Anonymous Monk
in thread Why is the execution order of subexpressions undefined? by BrowserUk

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