!fixup address latest comments, thanks!

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -3482,19 +3482,18 @@ void InnerLoopVectorizer::fixFixedOrderRecurrence(VPLiveOut *LO,
   Value *ResumeScalarFOR = State.get(VPExtract, UF - 1, true);
 
   // Fix the initial value of the original recurrence in the scalar loop.
-  Builder.SetInsertPoint(LoopScalarPreHeader, LoopScalarPreHeader->begin());
   PHINode *ScalarHeaderPhi = LO->getPhi();
-  auto *NewScalarHeaderPhi =
-      Builder.CreatePHI(ScalarHeaderPhi->getType(), 2, "scalar.recur.init");
   auto *InitScalarFOR =
       ScalarHeaderPhi->getIncomingValueForBlock(LoopScalarPreHeader);
+  Builder.SetInsertPoint(LoopScalarPreHeader, LoopScalarPreHeader->begin());
+  auto *ScalarPreheaderPhi =
+      Builder.CreatePHI(ScalarHeaderPhi->getType(), 2, "scalar.recur.init");
   for (auto *BB : predecessors(LoopScalarPreHeader)) {
     auto *Incoming = BB == LoopMiddleBlock ? ResumeScalarFOR : InitScalarFOR;
-    NewScalarHeaderPhi->addIncoming(Incoming, BB);
+    ScalarPreheaderPhi->addIncoming(Incoming, BB);
   }
-
   ScalarHeaderPhi->setIncomingValueForBlock(LoopScalarPreHeader,
-                                            NewScalarHeaderPhi);
+                                            ScalarPreheaderPhi);
   ScalarHeaderPhi->setName("scalar.recur");
 }
 

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -849,10 +849,13 @@ bool VPlanTransforms::adjustFixedOrderRecurrences(VPlan &Plan,
 
     // This is the second phase of vectorizing first-order recurrences. An
     // overview of the transformation is described below. Suppose we have the
-    // following loop.
+    // following loop with some use after the loop of the last a[i-1],
     //
-    //   for (int i = 0; i < n; ++i)
-    //     b[i] = a[i] - a[i - 1];
+    //   for (int i = 0; i < n; ++i) {
+    //     t = a[i - 1];
+    //     b[i] = a[i] - t;
+    //   }
+    //   use t;
     //
     // There is a first-order recurrence on "a". For this loop, the shorthand
     // scalar IR looks like:
@@ -866,7 +869,10 @@ bool VPlanTransforms::adjustFixedOrderRecurrences(VPlan &Plan,
     //     s1 = phi [s_init, scalar.ph], [s2, scalar.body]
     //     s2 = a[i]
     //     b[i] = s2 - s1
-    //     br cond, scalar.body, ...
+    //     br cond, scalar.body, exit.block
+    //
+    //   exit.block:
+    //     use = lcssa.phi [s1, scalar.body]
     //
     // In this example, s1 is a recurrence because it's value depends on the
     // previous iteration. In the first phase of vectorization, we created a