[PATCH] D33787: [IR] BasicBlock updatePHIs function for updating predecessors

[Marcin Słowik via Phabricator via llvm-commits]

Marandil added inline comments.


================
Comment at: lib/IR/BasicBlock.cpp:466
+      if (oldPredecessor) {
+        // Cannot just change the incoming when IncomingOld is still valid,
+        // need to insert another value with the same incoming value.
----------------
dberlin wrote:
> Can you explain why?
> This seems like  a bug?
Consider this:

  switch (a) {
    default: BB0;
    case 0: BB0;
    case 1: BB1;
    case 2: BB2;
    case 19: BB0;
    case 20: BB1;
    case 21: BB2;
  }

changed into:

  if (0 <= a <=2) {
    switch (a) {
      default: unreachable;
      case 0: BB0;
      case 1: BB1;
      case 2: BB2;
    }
  } else {
    switch (a-19) {
      default: BB0;
      case 0: BB0;
      case 1: BB1;
      case 2: BB2;
    }
  }

For BB0 a single predecessor changed into 2 different possible predecessors, that should share the same incoming value.


https://reviews.llvm.org/D33787