From e570dee4b03cca54bbf27a7f7a3299c5cdc3d087 Mon Sep 17 00:00:00 2001
From: Nadav Rotem <nrotem@apple.com>
Date: Tue, 4 Dec 2012 22:40:22 +0000
Subject: [PATCH] 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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169324 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Vectorize/LoopVectorize.cpp    | 40 +++++++++++--------
 .../LoopVectorize/if-conversion-reduction.ll  | 38 ++++++++++++++++++
 2 files changed, 61 insertions(+), 17 deletions(-)
 create mode 100644 test/Transforms/LoopVectorize/if-conversion-reduction.ll

diff --git a/lib/Transforms/Vectorize/LoopVectorize.cpp b/lib/Transforms/Vectorize/LoopVectorize.cpp
index 3502e9e3a62..ac62b110adb 100644
--- a/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/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) {
-    // 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 ?
-    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;
 
+    // Any reduction instr must be of one of the allowed kinds.
+    if (!isReductionInstr(Iter, Kind))
+      return false;
+
+    // Did we find a user inside this block ?
+    bool FoundInBlockUser = false;
+    // Did we reach the initial PHI node ?
+    bool FoundStartPHI = 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;
   }
 }
 
diff --git a/test/Transforms/LoopVectorize/if-conversion-reduction.ll b/test/Transforms/LoopVectorize/if-conversion-reduction.ll
new file mode 100644
index 00000000000..bacf9c00d01
--- /dev/null
+++ b/test/Transforms/LoopVectorize/if-conversion-reduction.ll
@@ -0,0 +1,38 @@
+; RUN: opt < %s  -loop-vectorize -force-vector-width=4 -enable-if-conversion -dce -instcombine -licm -S | FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.9.0"
+
+;CHECK: @reduction_func
+;CHECK-NOT: load <4 x i32>
+;CHECK: ret i32
+define i32 @reduction_func(i32* nocapture %A, i32 %n) nounwind uwtable readonly ssp {
+entry:
+  %cmp10 = icmp sgt i32 %n, 0
+  br i1 %cmp10, label %for.body, label %for.end
+
+for.body:                                         ; preds = %entry, %for.inc
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.inc ], [ 0, %entry ]
+  %sum.011 = phi i32 [ %sum.1, %for.inc ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i32* %A, i64 %indvars.iv
+  %0 = load i32* %arrayidx, align 4
+  %cmp1 = icmp sgt i32 %0, 30
+  br i1 %cmp1, label %if.then, label %for.inc
+
+if.then:                                          ; preds = %for.body
+  %add = add i32 %sum.011, 2
+  %add4 = add i32 %add, %0
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body, %if.then
+  %sum.1 = phi i32 [ %add4, %if.then ], [ %sum.011, %for.body ]
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %n
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.inc, %entry
+  %sum.0.lcssa = phi i32 [ 0, %entry ], [ 4, %for.inc ]
+  ret i32 %sum.0.lcssa
+}
+