Question about the source code

[yoyocat]

Hi, I'm reading the source code recently. I cannot figure out in which condition the y can be null. It should always has a value, either a normal value or a Phi value, right? What's the meaning of this variable is dead? Can someone give me a hint?

virgil/aeneas/src/ssa/VstSsaGen.v3

Lines 1378 to 1382 in b01e8f4

	if (y == null && x != null) {
	// this variable is dead on this edge, remove it
	killVar(x, v);
	continue;
	}

[titzer]

A variable can be dead during VST -> SSA translation because locals are "killed" when they reach the end of their lexical scope.

Consider:

def main(a: bool) -> int {
	var r = 12;
	if (a) {
		var x = 0;
		x = 12;
		r = 12;
	} else {
	}
	return r;
}

The variable x is scoped to the true block of the else. However, because it's assigned more than once, it is consider to be a "non SSA" variable that needs to be SSA-renamed, and therefore allocated a slot in the environment, and the environment slot number is global to the function. However, in the else, there is no assignment to x, so when the that reaches the merge at the end of the if, there will be a null for x, so the SSA-renamer will consider the variable dead at the merge.