[PATCH] D71225: [OpenMP][WIP] atomic update only evaluate once for expression having side effect

[Johannes Doerfert via Phabricator via cfe-commits]

jdoerfert added a comment.

In D71225#1776105 <https://reviews.llvm.org/D71225#1776105>, @cchen wrote:

> Oops, accidentally remove my own comment. I'm not sure why `iarr[foo(), foo(), 0]` violate the rule since it will be evaluated as `iarr[0]`, and the counter `foo()` modified is also in the same location for both left hand side and right hand side.


It's not a violation but it taps into the unspecified behavior, which in combination with side effects seems to be worth a warning.

So my reasoning is:

//x := `iarr[foo(), foo(), 0]`//

which is fine on its own. Using it in the atomic update of the form,

// x = x + 1//

also fine.

However, the standard says the number of times //x// is evaluated is unspecified. 
Thus, transforming the above to

// x += 1 //

should be valid. So should be,

// t = &x; *t = *t + 1 //

and even

// x = (x, x, x, x+1) //


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