[LV] Fix code gen for conditionally executed loads and stores

Fix a latent bug in loop vectorizer which generates incorrect code for
memory accesses that are executed conditionally. As pointed in review,
this bug definitely affects uniform loads and may affect conditional
stores that should have turned into scatters as well).

The code gen for conditionally executed uniform loads on architectures
that support masked gather instructions is broken.

Without this patch, we were unconditionally executing the *conditional*
load in the vectorized version.

This patch does the following:
1. Uniform conditional loads on architectures with gather support will
   have correct code generated. In particular, the cost model
   (setCostBasedWideningDecision) is fixed.
2. For the recipes which are handled after the widening decision is set,
   we use the isScalarWithPredication(I, VF) form which is added in the
   patch.

3. Fix the vectorization cost model for scalarization
   (getMemInstScalarizationCost): implement and use isPredicatedInst to
   identify *all* predicated instructions, not just scalar+predicated. So,
   now the cost for scalarization will be increased for maskedloads/stores
   and gather/scatter operations. In short, we should be choosing the
   gather/scatter in place of scalarization on archs where it is
   profitable.
4. We needed to weaken the assert in useEmulatedMaskMemRefHack.

Reviewers: Ayal, hsaito, mkuper

Differential Revision: https://reviews.llvm.org/D51313

llvm-svn: 341673

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1420,7 +1420,22 @@ class LoopVectorizationCostModel {
   /// Returns true if \p I is an instruction that will be scalarized with
   /// predication. Such instructions include conditional stores and
   /// instructions that may divide by zero.
-  bool isScalarWithPredication(Instruction *I);
+  /// If a non-zero VF has been calculated, we check if I will be scalarized
+  /// predication for that VF.
+  bool isScalarWithPredication(Instruction *I, unsigned VF = 1);
+
+  // Returns true if \p I is an instruction that will be predicated either
+  // through scalar predication or masked load/store or masked gather/scatter.
+  // Superset of instructions that return true for isScalarWithPredication.
+  bool isPredicatedInst(Instruction *I) {
+    if (!Legal->blockNeedsPredication(I->getParent()))
+      return false;
+    // Loads and stores that need some form of masked operation are predicated
+    // instructions.
+    if (isa<LoadInst>(I) || isa<StoreInst>(I))
+      return Legal->isMaskRequired(I);
+    return isScalarWithPredication(I);
+  }
 
   /// Returns true if \p I is a memory instruction with consecutive memory
   /// access that can be widened.
@@ -4367,7 +4382,7 @@ void LoopVectorizationCostModel::collectLoopScalars(unsigned VF) {
   Scalars[VF].insert(Worklist.begin(), Worklist.end());
 }
 
-bool LoopVectorizationCostModel::isScalarWithPredication(Instruction *I) {
+bool LoopVectorizationCostModel::isScalarWithPredication(Instruction *I, unsigned VF) {
   if (!Legal->blockNeedsPredication(I->getParent()))
     return false;
   switch(I->getOpcode()) {
@@ -4379,6 +4394,14 @@ bool LoopVectorizationCostModel::isScalarWithPredication(Instruction *I) {
       return false;
     auto *Ptr = getLoadStorePointerOperand(I);
     auto *Ty = getMemInstValueType(I);
+    // We have already decided how to vectorize this instruction, get that
+    // result.
+    if (VF > 1) {
+      InstWidening WideningDecision = getWideningDecision(I, VF);
+      assert(WideningDecision != CM_Unknown &&
+             "Widening decision should be ready at this moment");
+      return WideningDecision == CM_Scalarize;
+    }
     return isa<LoadInst>(I) ?
         !(isLegalMaskedLoad(Ty, Ptr)  || isLegalMaskedGather(Ty))
       : !(isLegalMaskedStore(Ty, Ptr) || isLegalMaskedScatter(Ty));
@@ -5472,8 +5495,7 @@ bool LoopVectorizationCostModel::useEmulatedMaskMemRefHack(Instruction *I){
   // from moving "masked load/store" check from legality to cost model.
   // Masked Load/Gather emulation was previously never allowed.
   // Limited number of Masked Store/Scatter emulation was allowed.
-  assert(isScalarWithPredication(I) &&
-         "Expecting a scalar emulated instruction");
+  assert(isPredicatedInst(I) && "Expecting a scalar emulated instruction");
   return isa<LoadInst>(I) ||
          (isa<StoreInst>(I) &&
           NumPredStores > NumberOfStoresToPredicate);
@@ -5734,7 +5756,7 @@ unsigned LoopVectorizationCostModel::getMemInstScalarizationCost(Instruction *I,
   // If we have a predicated store, it may not be executed for each vector
   // lane. Scale the cost by the probability of executing the predicated
   // block.
-  if (isScalarWithPredication(I)) {
+  if (isPredicatedInst(I)) {
     Cost /= getReciprocalPredBlockProb();
 
     if (useEmulatedMaskMemRefHack(I))
@@ -5880,7 +5902,12 @@ void LoopVectorizationCostModel::setCostBasedWideningDecision(unsigned VF) {
 
       if (isa<StoreInst>(&I) && isScalarWithPredication(&I))
         NumPredStores++;
-      if (isa<LoadInst>(&I) && Legal->isUniform(Ptr)) {
+
+      if (isa<LoadInst>(&I) && Legal->isUniform(Ptr) &&
+          // Conditional loads should be scalarized and predicated.
+          // isScalarWithPredication cannot be used here since masked
+          // gather/scatters are not considered scalar with predication.
+          !Legal->blockNeedsPredication(I.getParent())) {
         // Scalar load + broadcast
         unsigned Cost = getUniformMemOpCost(&I, VF);
         setWideningDecision(&I, VF, CM_Scalarize, Cost);
@@ -6720,7 +6747,11 @@ VPBlendRecipe *VPRecipeBuilder::tryToBlend(Instruction *I, VPlanPtr &Plan) {
 
 bool VPRecipeBuilder::tryToWiden(Instruction *I, VPBasicBlock *VPBB,
                                  VFRange &Range) {
-  if (CM.isScalarWithPredication(I))
+
+  bool IsPredicated = LoopVectorizationPlanner::getDecisionAndClampRange(
+      [&](unsigned VF) { return CM.isScalarWithPredication(I, VF); }, Range);
+
+  if (IsPredicated)
     return false;
 
   auto IsVectorizableOpcode = [](unsigned Opcode) {
@@ -6827,7 +6858,9 @@ VPBasicBlock *VPRecipeBuilder::handleReplication(
       [&](unsigned VF) { return CM.isUniformAfterVectorization(I, VF); },
       Range);
 
-  bool IsPredicated = CM.isScalarWithPredication(I);
+  bool IsPredicated = LoopVectorizationPlanner::getDecisionAndClampRange(
+      [&](unsigned VF) { return CM.isScalarWithPredication(I, VF); }, Range);
+
   auto *Recipe = new VPReplicateRecipe(I, IsUniform, IsPredicated);
 
   // Find if I uses a predicated instruction. If so, it will use its scalar