ChatGPT:

This is a known issue when following the SSA construction method described in Engineering a Compiler (by Cooper and Torczon). The method relies on dominance frontiers to place φ-functions, which can sometimes lead to the insertion of unnecessary φ-functions for variables that are not live at those points. This can cause issues in the renaming phase when a φ-function is encountered, but no reaching definition exists in the current rename stack.
Why This Happens:

Unnecessary φ-Functions: The standard algorithm places φ-functions at dominance frontiers without considering whether the variable is live at that point.

Renaming Phase Assumption: The renaming phase assumes that all φ-functions inserted are necessary and that there is always a valid definition in the rename stack. If a variable is not live at a φ-insertion point, then the rename stack may be empty, leading to errors.

Live-Range Violations: If a φ-function is placed at a block where the variable is not live, there might not be a previous definition, causing renaming to fail.