[llvm] [LoopVectorize] Add support for vectorisation of simple early exit loops (PR #88385)

[Graham Hunter via llvm-commits]

================
@@ -2292,6 +2352,73 @@ void MemoryDepChecker::Dependence::print(
   OS.indent(Depth + 2) << *Instrs[Destination] << "\n";
 }
 
+bool LoopAccessInfo::isAnalyzableEarlyExitLoop() {
+  // At least one of the exiting blocks must be the latch.
+  BasicBlock *LatchBB = TheLoop->getLoopLatch();
+  if (!LatchBB)
+    return false;
+
+  SmallVector<BasicBlock *, 8> ExitingBlocks;
+  TheLoop->getExitingBlocks(ExitingBlocks);
+
+  // This is definitely not an early exit loop.
+  if (ExitingBlocks.size() < 2)
+    return false;
+
+  SmallVector<BasicBlock *, 4> ExactExitingBlocks;
+  PSE->getSE()->getExactExitingBlocks(TheLoop, &ExactExitingBlocks);
+
+  // We only support one speculative early exit.
+  if ((ExitingBlocks.size() - ExactExitingBlocks.size()) > 1)
+    return false;
+
+  // There could be multiple exiting blocks with an exact exit-not-taken
+  // count. Find the speculative early exit block, i.e. the one with an
+  // unknown count.
+  BasicBlock *TmpBB = nullptr;
+  for (BasicBlock *BB1 : ExitingBlocks) {
+    bool Found = false;
----------------
huntergr-arm wrote:

I think this can be made a bit simpler, e.g.
```
if (!is_contained(ExactExitingBlocks, BB1)) {
  TmpBB = BB1;
  break;
}
```

Or you could potentially have a getUncountedExitBlocks() instead.

https://github.com/llvm/llvm-project/pull/88385