Debug info under high optimization is very tricky. A simple example: for (i = 0; i < 4; i++) ... If the compiler unrolls the loop four times the eliminates th

Debug info under high optimization is very tricky. A simple example: for (i = 0; i < 4; i++) ... If the compiler unrolls the loop four times the eliminates the induction variable altogether, then combines the four iteration loop into a single vector instruction, what debug info should it emit for the value of i in the loop body? You can argue that it should emit what i would have had at a given point, but now you have four different values for i all at that one vector instruction. Reasoning about that is quite difficult.