[PATCH] D21725: [LoopUnswitch] Unswitch on conditions feeding into guards

[Sanjoy Das via llvm-commits]

sanjoy created this revision.
sanjoy added a reviewer: majnemer.
sanjoy added a subscriber: llvm-commits.
Herald added subscribers: mzolotukhin, mcrosier.

This is a straightforward extension of what LoopUnswitch does to
branches to guards.  That is, we unswitch

```
for (;;) {
  ...
  guard(loop_invariant_cond);
  ...
}
```

into

```
if (loop_invariant_cond) {
  for (;;) {
    ...
    // There is no need to emit guard(true)
    ...
  }
} else {
  for (;;) {
    ...
    guard(false);
    // SimplifyCFG will clean this up by adding an
    // unreachable after the guard(false)
    ...
  }
}
```

http://reviews.llvm.org/D21725

Files:
  lib/Transforms/Scalar/LoopUnswitch.cpp
  test/Transforms/LoopUnswitch/guards.ll

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