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

[Paul Walker 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) {
+        reportVectorizationFailure(
+            "Early exiting block has more than one successor outside of loop",
+            "Too many successors from early exiting block",
+            "EarlyExitTooManySuccessors", ORE, TheLoop);
+        return false;
+      }
+    } else
+      CountableExitingBlocks.push_back(BB1);
+  }
+  Predicates.clear();
+
+  // We only support one uncountable early exit.
+  if (getUncountableExitingBlocks().size() != 1) {
+    reportVectorizationFailure(
+        "Loop has too many uncountable exits",
+        "Cannot vectorize early exit loop with more than one early exit",
+        "TooManyUncountableEarlyExits", ORE, TheLoop);
+    return false;
+  }
+
+  // The only supported early exit loops so far are ones where the early
+  // exiting block is a unique predecessor of the latch block.
----------------
paulwalker-arm wrote:

This answers my earlier question but that means the code in `canVectorizeMemory` is fragile.  Can an assert be added to `canVectorizeMemory` to ensure this is the case?

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