LoopVectorize: External uses must use the last value in a reduction cycle

Otherwise, we don't perform operations that would have been performed on
the scalar version.

Fixes PR17498.

llvm-svn: 192133

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index a71df08aba0..415f86e8938 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -4015,6 +4015,12 @@ bool LoopVectorizationLegality::AddReductionVar(PHINode *Phi,
         if (ExitInstruction != 0 || Cur == Phi)
           return false;
 
+        // The instruction used by an outside user must be the last instruction
+        // before we feed back to the reduction phi. Otherwise, we loose VF-1
+        // operations on the value.
+        if (std::find(Phi->op_begin(), Phi->op_end(), Cur) == Phi->op_end())
+         return false;
+
         ExitInstruction = Cur;
         continue;
       }