1 files changed, 79 insertions, 0 deletions
diff --git a/lib/Transforms/InstCombine/InstructionCombining.cpp b/lib/Transforms/InstCombine/InstructionCombining.cpp
index 4c36887f62..80eec1b311 100644
--- a/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1220,6 +1220,85 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
     if (MadeChange) return &GEP;
   }
 
+  // Check to see if the inputs to the PHI node are getelementptr instructions.
+  if (PHINode *PN = dyn_cast<PHINode>(PtrOp)) {
+    GetElementPtrInst *Op1 = dyn_cast<GetElementPtrInst>(PN->getOperand(0));
+    if (!Op1)
+      return nullptr;
+
+    signed DI = -1;
+
+    for (auto I = PN->op_begin()+1, E = PN->op_end(); I !=E; ++I) {
+      GetElementPtrInst *Op2 = dyn_cast<GetElementPtrInst>(*I);
+      if (!Op2 || Op1->getNumOperands() != Op2->getNumOperands())
+        return nullptr;
+
+      for (unsigned J = 0, F = Op1->getNumOperands(); J != F; ++J) {
+        if (Op1->getOperand(J)->getType() != Op2->getOperand(J)->getType())
+          return nullptr;
+
+        if (Op1->getOperand(J) != Op2->getOperand(J)) {
+          if (DI == -1) {
+            // We have not seen any differences yet in the GEPs feeding the
+            // PHI yet, so we record this one if it is allowed to be a
+            // variable.
+
+            // The first two arguments can vary for any GEP, the rest have to be
+            // static for struct slots
+            if (J > 1) {
+              SmallVector<Value*, 8> Idxs(GEP.idx_begin(), GEP.idx_begin()+J);
+              Type *Ty =
+                GetElementPtrInst::getIndexedType(Op1->getOperand(0)->getType(),
+                                                  Idxs);
+              if (Ty->isStructTy())
+                return nullptr;
+            }
+
+            DI = J;
+          } else {
+            // The GEP is different by more than one input. While this could be
+            // extended to support GEPs that vary by more than one variable it
+            // doesn't make sense since it greatly increases the complexity and
+            // would result in an R+R+R addressing mode which no backend
+            // directly supports and would need to be broken into several
+            // simpler instructions anyway.
+            return nullptr;
+          }
+        }
+      }
+    }
+
+    GetElementPtrInst *NewGEP = cast<GetElementPtrInst>(Op1->clone());
+
+    if (DI == -1) {
+      // All the GEPs feeding the PHI are identical. Clone one down into our
+      // BB so that it can be merged with the current GEP.
+      GEP.getParent()->getInstList().insert(GEP.getParent()->getFirstNonPHI(),
+                                            NewGEP);
+    } else {
+      // All the GEPs feeding the PHI differ at a single offset. Clone a GEP
+      // into the current block so it can be merged, and create a new PHI to
+      // set that index.
+      Instruction *InsertPt = Builder->GetInsertPoint();
+      Builder->SetInsertPoint(PN);
+      PHINode *NewPN = Builder->CreatePHI(Op1->getOperand(DI)->getType(),
+                                          PN->getNumOperands());
+      Builder->SetInsertPoint(InsertPt);
+
+      for (auto &I : PN->operands())
+        NewPN->addIncoming(cast<GEPOperator>(I)->getOperand(DI),
+                           PN->getIncomingBlock(I));
+
+      NewGEP->setOperand(DI, NewPN);
+      GEP.getParent()->getInstList().insert(GEP.getParent()->getFirstNonPHI(),
+                                            NewGEP);
+      NewGEP->setOperand(DI, NewPN);
+    }
+
+    GEP.setOperand(0, NewGEP);
+    PtrOp = NewGEP;
+  }
+
   // Combine Indices - If the source pointer to this getelementptr instruction
   // is a getelementptr instruction, combine the indices of the two
   // getelementptr instructions into a single instruction.