🚀 go-pugleaf

Certain contexts, the left operand of the comma operator,
expression statements, and the first and third expressions of a
for statement, are designated as void expressions. (I'd assume
that the operand of a cast to void is also.) A void expression is
defined as one where the value of the expression is discarded. The
key is the word "discarded". This implies that the value is
computed but then ignored.

The problem is then what to do with statements like a; and a=b;
when a is a volatile object. The first expression accesses a but
then discards the value, which is fine. The other does NOT do
what you'd expect, though. The reason is that the assignment has a
value which is the value of a after the assignment, which can only
be obtained by accessing a. Thus a=b results in a sequence like:

	load b
	store a
	load a

rather than the intended load and store.

I personally would prefer the other interpretation, in which a
void expression is, as the standard _also_ says, evaluated only
for its side effects. Otherwise, one would have serious problems
using volatile for device registers. (This would have the effect
that a; doesn't actually reference a because no value is computed
but I think that that is a legitimate action. (int)a; would force
the access anyway.)

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Bill                            { uunet | decwrl | telesci }!twwells!bill
bill@twwells.com