[hmry]

Why is that missing the point? Loading it twice, possibly with different values, is the intended behavior. It's only undefined because the C spec doesn't specify the order of the loads (unlike most other languages which have a perfectly well-defined order for side effects in a single expression).

[rowanG077]

What you are describing is implementation defined behavior. Using that is perfectly safe and reasonable. Undefined means this programs is malformed.

[hmry]

No I'm just repeating what the original comment said, which is that it's explicitly UB:

"5.1.2.4.1 says any volatile access - including just reading it - is a side effect. 6.5.1.2 says that unsequenced side effects on the same scalar object (in this case, x) are UB. 6.5.3.3.8 tells us that the evaluations of function arguments are indeterminately sequenced w.r.t. each other."

If function arguments were sequenced with respect to each other, it wouldn't be a problem.

But actually, maybe the original comment is wrong. Presumably "indeterminately sequenced" and "unsequenced" mean different things, although I don't have a copy of the standard at hand to check.