[llvm] [VPlan] Skip disconnected exit blocks in hasEarlyExit. (PR #151718)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/151718

Currently hasEarlyExit returns true, if there are multiple exit blocks. ExitBlocks contains the wrapped original IR exit blocks. Without checking the predecessors we incorrectly return true for loops with multiple countable exits, that have been vectorized by requiring a scalar epilogue. In that case, the exit blocks will get disconnected.

Fix this by filtering out disconnected exit blocks.

Currently this should only impact the 'early-exit vectorized' statistic.

>From 91559265d1c73d43fe1e603983241de6c55e39b3 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 1 Aug 2025 16:50:39 +0100
Subject: [PATCH] [VPlan] Skip disconnected exit blocks in hasEarlyExit.

Currently hasEarlyExit returns true, if there are multiple exit blocks.
ExitBlocks contains the wrapped original IR exit blocks. Without
checking the predecessors we incorrectly return true for loops with
multiple countable exits, that have been vectorized by requiring a
scalar epilogue. In that case, the exit blocks will get disconnected.

Fix this by filtering out disconnected exit blocks.
---
 llvm/lib/Transforms/Vectorize/VPlan.h            | 5 ++++-
 llvm/test/Transforms/LoopVectorize/vect.stats.ll | 2 +-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/VPlan.h b/llvm/lib/Transforms/Vectorize/VPlan.h
index a5de5933d5ff1..758aea48eb930 100644
--- a/llvm/lib/Transforms/Vectorize/VPlan.h
+++ b/llvm/lib/Transforms/Vectorize/VPlan.h
@@ -4229,7 +4229,10 @@ class VPlan {
   /// block with multiple predecessors (one for the exit via the latch and one
   /// via the other early exit).
   bool hasEarlyExit() const {
-    return ExitBlocks.size() > 1 ||
+    return count_if(ExitBlocks,
+                    [](VPIRBasicBlock *EB) {
+                      return EB->getNumPredecessors() != 0;
+                    }) > 1 ||
            (ExitBlocks.size() == 1 && ExitBlocks[0]->getNumPredecessors() > 1);
   }
 
diff --git a/llvm/test/Transforms/LoopVectorize/vect.stats.ll b/llvm/test/Transforms/LoopVectorize/vect.stats.ll
index 033907e079a7d..e3240c8181519 100644
--- a/llvm/test/Transforms/LoopVectorize/vect.stats.ll
+++ b/llvm/test/Transforms/LoopVectorize/vect.stats.ll
@@ -5,7 +5,7 @@
 ; vectorized) and the third one is not.
 
 ; CHECK: 4 loop-vectorize               - Number of loops analyzed for vectorization
-; CHECK: 2 loop-vectorize               - Number of early exit loops vectorized
+; CHECK: 1 loop-vectorize               - Number of early exit loops vectorized
 ; CHECK: 3 loop-vectorize               - Number of loops vectorized
 
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