[VPlan] Create resume phis in scalar preheader early. (NFC)

Create phi recipes for scalar resume value up front in
addInitialSkeleton during initial construction. This will allow moving
the remaining code dealing with resume values to VPlan
transforms/construction.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -1084,7 +1084,7 @@ class LLVM_ABI_FOR_TEST VPInstruction : public VPRecipeWithIRFlags,
   OpcodeTy Opcode;
 
   /// An optional name that can be used for the generated IR instruction.
-  const std::string Name;
+  std::string Name;
 
   /// Returns true if we can generate a scalar for the first lane only if
   /// needed.
@@ -1183,6 +1183,8 @@ class LLVM_ABI_FOR_TEST VPInstruction : public VPRecipeWithIRFlags,
 
   /// Returns the symbolic name assigned to the VPInstruction.
   StringRef getName() const { return Name; }
+
+  void setName(StringRef NewName) { Name = NewName.str(); }
 };
 
 /// A specialization of VPInstruction augmenting it with a dedicated result

llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp

@@ -533,6 +533,15 @@ static void addInitialSkeleton(VPlan &Plan, Type *InductionTy, DebugLoc IVDL,
   Plan.getEntry()->swapSuccessors();
 
   createExtractsForLiveOuts(Plan, MiddleVPBB);
+
+  VPBuilder ScalarPHBuilder(ScalarPH);
+  for (const auto &[PhiR, ScalarPhiR] : zip_equal(
+           drop_begin(HeaderVPBB->phis()), Plan.getScalarHeader()->phis())) {
+    auto *VectorPhiR = cast<VPPhi>(&PhiR);
+    auto *ResumePhiR = ScalarPHBuilder.createScalarPhi(
+        {VectorPhiR, VectorPhiR->getOperand(0)}, VectorPhiR->getDebugLoc());
+    cast<VPIRPhi>(&ScalarPhiR)->addOperand(ResumePhiR);
+  }
 }
 
 std::unique_ptr<VPlan>

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -4459,9 +4459,10 @@ void VPlanTransforms::addBranchWeightToMiddleTerminator(
 /// Create and return a ResumePhi for \p WideIV, unless it is truncated. If the
 /// induction recipe is not canonical, creates a VPDerivedIVRecipe to compute
 /// the end value of the induction.
-static VPInstruction *addResumePhiRecipeForInduction(
-    VPWidenInductionRecipe *WideIV, VPBuilder &VectorPHBuilder,
-    VPBuilder &ScalarPHBuilder, VPTypeAnalysis &TypeInfo, VPValue *VectorTC) {
+static VPValue *addResumePhiRecipeForInduction(VPWidenInductionRecipe *WideIV,
+                                               VPBuilder &VectorPHBuilder,
+                                               VPTypeAnalysis &TypeInfo,
+                                               VPValue *VectorTC) {
   auto *WideIntOrFp = dyn_cast<VPWidenIntOrFpInductionRecipe>(WideIV);
   // Truncated wide inductions resume from the last lane of their vector value
   // in the last vector iteration which is handled elsewhere.
@@ -4487,9 +4488,7 @@ static VPInstruction *addResumePhiRecipeForInduction(
                                                 WideIV->getDebugLoc());
   }
 
-  auto *ResumePhiRecipe = ScalarPHBuilder.createScalarPhi(
-      {EndValue, Start}, WideIV->getDebugLoc(), "bc.resume.val");
-  return ResumePhiRecipe;
+  return EndValue;
 }
 
 void VPlanTransforms::addScalarResumePhis(
@@ -4502,21 +4501,18 @@ void VPlanTransforms::addScalarResumePhis(
   VPBuilder VectorPHBuilder(
       cast<VPBasicBlock>(VectorRegion->getSinglePredecessor()));
   VPBuilder MiddleBuilder(MiddleVPBB, MiddleVPBB->getFirstNonPhi());
-  VPBuilder ScalarPHBuilder(ScalarPH);
-  for (VPRecipeBase &ScalarPhiR : Plan.getScalarHeader()->phis()) {
-    auto *ScalarPhiIRI = cast<VPIRPhi>(&ScalarPhiR);
+  for (VPRecipeBase &PhiR : Plan.getScalarPreheader()->phis()) {
+    auto *ResumePhiR = cast<VPPhi>(&PhiR);
 
     // TODO: Extract final value from induction recipe initially, optimize to
     // pre-computed end value together in optimizeInductionExitUsers.
-    auto *VectorPhiR =
-        cast<VPHeaderPHIRecipe>(Builder.getRecipe(&ScalarPhiIRI->getIRPhi()));
+    auto *VectorPhiR = cast<VPHeaderPHIRecipe>(ResumePhiR->getOperand(0));
     if (auto *WideIVR = dyn_cast<VPWidenInductionRecipe>(VectorPhiR)) {
-      if (VPInstruction *ResumePhi = addResumePhiRecipeForInduction(
-              WideIVR, VectorPHBuilder, ScalarPHBuilder, TypeInfo,
-              &Plan.getVectorTripCount())) {
-        assert(isa<VPPhi>(ResumePhi) && "Expected a phi");
-        IVEndValues[WideIVR] = ResumePhi->getOperand(0);
-        ScalarPhiIRI->addOperand(ResumePhi);
+      if (VPValue *ResumeV = addResumePhiRecipeForInduction(
+              WideIVR, VectorPHBuilder, TypeInfo, &Plan.getVectorTripCount())) {
+        IVEndValues[WideIVR] = ResumeV;
+        ResumePhiR->setOperand(0, ResumeV);
+        ResumePhiR->setName("bc.resume.val");
         continue;
       }
       // TODO: Also handle truncated inductions here. Computing end-values
@@ -4538,10 +4534,8 @@ void VPlanTransforms::addScalarResumePhis(
       ResumeFromVectorLoop = MiddleBuilder.createNaryOp(
           VPInstruction::ExtractLastElement, {ResumeFromVectorLoop}, {},
           "vector.recur.extract");
-    StringRef Name = IsFOR ? "scalar.recur.init" : "bc.merge.rdx";
-    auto *ResumePhiR = ScalarPHBuilder.createScalarPhi(
-        {ResumeFromVectorLoop, VectorPhiR->getStartValue()}, {}, Name);
-    ScalarPhiIRI->addOperand(ResumePhiR);
+    ResumePhiR->setName(IsFOR ? "scalar.recur.init" : "bc.merge.rdx");
+    ResumePhiR->setOperand(0, ResumeFromVectorLoop);
   }
 }
 

llvm/unittests/Transforms/Vectorize/VPlanHCFGTest.cpp

@@ -113,12 +113,13 @@ compound=true
   N0 -> N2 [ label="F"]
   N1 [label =
     "scalar.ph:\l" +
+    "  EMIT-SCALAR vp\<%6\> = phi [ ir\<%indvars.iv\>, middle.block ], [ ir\<0\>, ir-bb\<entry\> ]\l" +
     "Successor(s): ir-bb\<for.body\>\l"
   ]
   N1 -> N3 [ label=""]
   N3 [label =
     "ir-bb\<for.body\>:\l" +
-    "  IR   %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]\l" +
+    "  IR   %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ] (extra operand: vp\<%6\> from scalar.ph)\l" +
     "  IR   %arr.idx = getelementptr inbounds i32, ptr %A, i64 %indvars.iv\l" +
     "  IR   %l1 = load i32, ptr %arr.idx, align 4\l" +
     "  IR   %res = add i32 %l1, 10\l" +
@@ -282,12 +283,13 @@ compound=true
   N0 -> N2 [ label="F"]
   N1 [label =
     "scalar.ph:\l" +
+    "  EMIT-SCALAR vp\<%6\> = phi [ ir\<%iv\>, middle.block ], [ ir\<0\>, ir-bb\<entry\> ]\l" +
     "Successor(s): ir-bb\<loop.header\>\l"
   ]
   N1 -> N3 [ label=""]
   N3 [label =
     "ir-bb\<loop.header\>:\l" +
-    "  IR   %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop.latch ]\l" +
+    "  IR   %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop.latch ] (extra operand: vp\<%6\> from scalar.ph)\l" +
     "  IR   %arr.idx = getelementptr inbounds i32, ptr %A, i64 %iv\l" +
     "  IR   %l1 = load i32, ptr %arr.idx, align 4\l" +
     "  IR   %c = icmp eq i32 %l1, 0\l" +

llvm/unittests/Transforms/Vectorize/VPlanVerifierTest.cpp

@@ -346,6 +346,8 @@ TEST_F(VPIRVerifierTest, testVerifyIRPhiInScalarHeaderVPIRBB) {
   Function *F = M.getFunction("f");
   BasicBlock *LoopHeader = F->getEntryBlock().getSingleSuccessor();
   auto Plan = buildVPlan(LoopHeader);
+  VPValue *Zero = Plan->getConstantInt(32, 0);
+  Plan->getScalarHeader()->front().addOperand(Zero);
 
 #if GTEST_HAS_STREAM_REDIRECTION
   ::testing::internal::CaptureStderr();
@@ -387,8 +389,6 @@ TEST_F(VPIRVerifierTest, testVerifyIRPhiInExitVPIRBB) {
                         {HeaderBlock->front().getVPSingleValue()});
   DefI->insertBefore(Plan->getMiddleBlock()->getTerminator());
   Plan->getExitBlocks()[0]->front().addOperand(DefI);
-  VPValue *Zero = Plan->getConstantInt(32, 0);
-  Plan->getScalarHeader()->front().addOperand(Zero);
 
 #if GTEST_HAS_STREAM_REDIRECTION
   ::testing::internal::CaptureStderr();