[gpderetta]

I do believe that by a strict reading of the standard GCC is non conforming here. This reading of the standard is not agreed by the GCC developers though: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104800

If the first div happens before the first printf, then it can be CSE out of the loop as any trap would have happened before the printf anyway, so no reordering, and if it didn't trap the first time it wouldn't have trapped later either. In this case CSE is fine and there are no reordering.

If the div happens after printf, then reordering is prohibited not only to preserve side effects before UB (which we have seen GCC doesn't necessarily respects), but because for the most part printf is treated as an opaque function: it could legitimately exit, or longjump out of the function or never return, so on the abstract machine the UB might not happen at all. So it is not safe to hoist trapping instruction like div above opaque functions (but it is safe to sink them).

Still the modification I showed for volatile can be applied as well: peel the first iteration out of the loop so that the first printf can be done before computing the div to be CSEd out of the loop. But GCC doesn't do it although it seems desirable.