[PATCH] D52002: Switch optimization for known maximum switch values

[Hans Wennborg via Phabricator via llvm-commits]

hans added a comment.

In https://reviews.llvm.org/D52002#1237955, @ayonam wrote:

> If the default basic block is unreachable and we completely omit the conditional branch in all such cases, then if the switch value at runtime turns out to be not one of the cases given in the switch block, the execution would reach the vectored branch to the jump table which would fail to reach anywhere.


If the default basic block is unreachable, that implies that one of the other cases must always be taken. Otherwise, the default wouldn't be unreachable after all.

For example:

  int func(int x) {
      switch (x) {
      case 0: return f();
      case 1: return g();
      case 2: return h();
      case 3: return f();
      case 4: return h();
      }
  }

The default is unreachable here because it would "fall off" the end of the function without returning, which is undefined behaviour. So the switch lowering should be able to remove the range check for the jump table.


Repository:
  rL LLVM

https://reviews.llvm.org/D52002