llvm-project

8557e5b4 - [LoopInterchange] Reject interchange when a freeze would move or be cloned

[LoopInterchange] Reject interchange when a freeze would move or be cloned Loop interchange moves four blocks to a different loop depth: the outer loop header and latch, and the inner loop preheader and exit block. It also splits the inner loop latch and clones the instructions that compute the latch branch condition and induction variable updates into the new latch block. LangRef guarantees that all uses of the value returned by one execution of a `freeze` observe that same value. The guarantee does not extend across executions, so one `freeze` may yield a different value each time it runs. Different `freeze` instructions may also yield different values for the same `undef` or poison operand. Both steps above can therefore change which value a use observes. Moving a `freeze` to a different loop depth changes which loop iterations share one dynamic result. Uses that observed a single frozen value in the original nest can observe values from separate executions after interchange. Cloning a `freeze` creates a second, independent instruction, so the original and clone can yield different values for the same operand. Reject the interchange when a `freeze` appears in one of the four moved blocks or among the instructions cloned into the new latch. A `freeze` elsewhere still reaches the same uses from each execution, so it remains allowed. That includes a `freeze` in the outer loop preheader and one in the inner loop body outside the cloned computations. The check is conservative and does not try to prove that the operand of a `freeze` is never `undef` or poison. Assisted-by: Claude Opus 5, GPT-5.6 Sol.