From 93fa5ef9572d2c5f80080ea44a192d8a1b5d5365 Mon Sep 17 00:00:00 2001
From: Nadav Rotem <nrotem@apple.com>
Date: Tue, 4 Dec 2012 22:40:22 +0000
Subject: Fix a bug in vectorization of if-converted reduction variables. If
 the reduction variable is not used outside the loop then we ran into an
 endless loop. This change checks if we found the original PHI.

llvm-svn: 169324
---
 llvm/lib/Transforms/Vectorize/LoopVectorize.cpp | 34 +++++++++++++++----------
 1 file changed, 20 insertions(+), 14 deletions(-)

(limited to 'llvm/lib/Transforms/Vectorize/LoopVectorize.cpp')

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 3502e9e3a62..ac62b110adb 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -1985,20 +1985,20 @@ bool LoopVectorizationLegality::AddReductionVar(PHINode *Phi,
   // Also, we can't have multiple block-local users.
   Instruction *Iter = Phi;
   while (true) {
+    // If the instruction has no users then this is a broken
+    // chain and can't be a reduction variable.
+    if (Iter->use_empty())
+      return false;
+
     // Any reduction instr must be of one of the allowed kinds.
     if (!isReductionInstr(Iter, Kind))
       return false;
 
-    // Did we found a user inside this block ?
+    // Did we find a user inside this block ?
     bool FoundInBlockUser = false;
     // Did we reach the initial PHI node ?
     bool FoundStartPHI = false;
 
-    // If the instruction has no users then this is a broken
-    // chain and can't be a reduction variable.
-    if (Iter->use_empty())
-      return false;
-
     // For each of the *users* of iter.
     for (Value::use_iterator it = Iter->use_begin(), e = Iter->use_end();
          it != e; ++it) {
@@ -2009,21 +2009,22 @@ bool LoopVectorizationLegality::AddReductionVar(PHINode *Phi,
         continue;
       }
 
-      // We allow in-loop PHINodes which are not the original reduction PHI
-      // node. If this PHI is the only user of Iter (happens in IF w/ no ELSE
-      // structure) then don't skip this PHI.
-      if (isa<PHINode>(U) && U->getParent() != TheLoop->getHeader() &&
-          TheLoop->contains(U->getParent()) && Iter->getNumUses() > 1)
-        continue;
-
       // Check if we found the exit user.
       BasicBlock *Parent = U->getParent();
       if (!TheLoop->contains(Parent)) {
-        // We must have a single exit instruction.
+        // Exit if you find multiple outside users.
         if (ExitInstruction != 0)
           return false;
         ExitInstruction = Iter;
       }
+
+      // We allow in-loop PHINodes which are not the original reduction PHI
+      // node. If this PHI is the only user of Iter (happens in IF w/ no ELSE
+      // structure) then don't skip this PHI.
+      if (isa<PHINode>(U) && U->getParent() != TheLoop->getHeader() &&
+          TheLoop->contains(U) && Iter->getNumUses() > 1)
+        continue;
+
       // We can't have multiple inside users.
       if (FoundInBlockUser)
         return false;
@@ -2043,6 +2044,11 @@ bool LoopVectorizationLegality::AddReductionVar(PHINode *Phi,
      Reductions[Phi] = RD;
      return true;
    }
+
+    // If we've reached the start PHI but did not find an outside user then
+    // this is dead code. Abort.
+    if (FoundStartPHI)
+      return false;
   }
 }
 
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