[llvm] [LoopVectorize] Teach LoopVectorizationLegality about more early exits (PR #107004)

[David Sherwood via llvm-commits]

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@@ -1442,6 +1486,95 @@ bool LoopVectorizationLegality::canVectorizeLoopNestCFG(
   return Result;
 }
 
+bool LoopVectorizationLegality::isVectorizableEarlyExitLoop() {
+  // At least one of the exiting blocks must be the latch.
+  BasicBlock *LatchBB = TheLoop->getLoopLatch();
+  if (!LatchBB) {
+    reportVectorizationFailure("Loop does not have a latch",
+                               "Cannot vectorize early exit loop",
+                               "NoLatchEarlyExit", ORE, TheLoop);
+    return false;
+  }
+
+  SmallVector<BasicBlock *, 8> ExitingBlocks;
+  TheLoop->getExitingBlocks(ExitingBlocks);
+
+  // Keep a record of all the exiting blocks with exact exit counts, as well as
+  // those with inexact counts.
+  SmallVector<const SCEVPredicate *, 4> Predicates;
+  for (BasicBlock *BB1 : ExitingBlocks) {
+    const SCEV *EC =
+        PSE.getSE()->getPredicatedExitCount(TheLoop, BB1, &Predicates);
+    if (isa<SCEVCouldNotCompute>(EC)) {
+      UncountableExitingBlocks.push_back(BB1);
+
+      unsigned NumExitBlocks = 0;
+      for (BasicBlock *BB2 : successors(BB1)) {
+        if (!TheLoop->contains(BB2)) {
+          UncountableExitBlocks.push_back(BB2);
+          NumExitBlocks++;
+        }
+      }
+      if (NumExitBlocks > 1) {
----------------
david-arm wrote:

You may be right at the moment that the only way this can happen is with a switch statement, but I can't 100% convince myself of that. :) That's why I wrote this in a more paranoid way by checking the number of blocks manually. What I could do is bail out early if the number of successors > 2, then add the single successor outside of the loop.

https://github.com/llvm/llvm-project/pull/107004