[LoadStoreVectorizer] Postprocess and merge equivalence classes (llvm#114501)

This patch introduces a new method:
void Vectorizer::mergeEquivalenceClasses(EquivalenceClassMap &EQClasses)
const

The method is called at the end of
Vectorizer::collectEquivalenceClasses() and is needed to merge
equivalence classes that differ only by their underlying objects (UO1
and UO2), where UO1 is 1-level-indirection underlying base for UO2. This
situation arises due to the limited lookup depth used during the search
of underlying bases with llvm::getUnderlyingObject(ptr).

Using any fixed lookup depth can result into creation of multiple
equivalence classes that only differ by 1-level indirection bases.

The new approach merges equivalence classes if they have adjacent bases
(1-level indirection). If a series of equivalence classes form ladder
formed of 1-step/level indirections, they are all merged into a single
equivalence class. This provides more opportunities for the load-store
vectorizer to generate better vectors.

---------

Signed-off-by: Klochkov, Vyacheslav N <[email protected]>

Author: v-klochkov

llvm/lib/Transforms/Vectorize/LoadStoreVectorizer.cpp

@@ -324,6 +324,11 @@ class Vectorizer {
       Instruction *ChainElem, Instruction *ChainBegin,
       const DenseMap<Instruction *, APInt /*OffsetFromLeader*/> &ChainOffsets);
 
+  /// Merges the equivalence classes if they have underlying objects that differ
+  /// by one level of indirection (i.e., one is a getelementptr and the other is
+  /// the base pointer in that getelementptr).
+  void mergeEquivalenceClasses(EquivalenceClassMap &EQClasses) const;
+
   /// Collects loads and stores grouped by "equivalence class", where:
   ///   - all elements in an eq class are a load or all are a store,
   ///   - they all load/store the same element size (it's OK to have e.g. i8 and
@@ -1305,6 +1310,123 @@ std::optional<APInt> Vectorizer::getConstantOffsetSelects(
   return std::nullopt;
 }
 
+void Vectorizer::mergeEquivalenceClasses(EquivalenceClassMap &EQClasses) const {
+  if (EQClasses.size() < 2) // There is nothing to merge.
+    return;
+
+  // The reduced key has all elements of the ECClassKey except the underlying
+  // object. Check that EqClassKey has 4 elements and define the reduced key.
+  static_assert(std::tuple_size_v<EqClassKey> == 4,
+                "EqClassKey has changed - EqClassReducedKey needs changes too");
+  using EqClassReducedKey =
+      std::tuple<std::tuple_element_t<1, EqClassKey> /* AddrSpace */,
+                 std::tuple_element_t<2, EqClassKey> /* Element size */,
+                 std::tuple_element_t<3, EqClassKey> /* IsLoad; */>;
+  using ECReducedKeyToUnderlyingObjectMap =
+      MapVector<EqClassReducedKey,
+                SmallPtrSet<std::tuple_element_t<0, EqClassKey>, 4>>;
+
+  // Form a map from the reduced key (without the underlying object) to the
+  // underlying objects: 1 reduced key to many underlying objects, to form
+  // groups of potentially merge-able equivalence classes.
+  ECReducedKeyToUnderlyingObjectMap RedKeyToUOMap;
+  bool FoundPotentiallyOptimizableEC = false;
+  for (const auto &EC : EQClasses) {
+    const auto &Key = EC.first;
+    EqClassReducedKey RedKey{std::get<1>(Key), std::get<2>(Key),
+                             std::get<3>(Key)};
+    RedKeyToUOMap[RedKey].insert(std::get<0>(Key));
+    if (RedKeyToUOMap[RedKey].size() > 1)
+      FoundPotentiallyOptimizableEC = true;
+  }
+  if (!FoundPotentiallyOptimizableEC)
+    return;
+
+  LLVM_DEBUG({
+    dbgs() << "LSV: mergeEquivalenceClasses: before merging:\n";
+    for (const auto &EC : EQClasses) {
+      dbgs() << "    Key: ([" << std::get<0>(EC.first)
+             << "]: " << *std::get<0>(EC.first) << ", " << std::get<1>(EC.first)
+             << ", " << std::get<2>(EC.first) << ", "
+             << static_cast<int>(std::get<3>(EC.first)) << ")\n";
+      for (const auto &Inst : EC.second)
+        dbgs() << "\tInst: " << *Inst << '\n';
+    }
+  });
+  LLVM_DEBUG({
+    dbgs() << "LSV: mergeEquivalenceClasses: RedKeyToUOMap:\n";
+    for (const auto &RedKeyToUO : RedKeyToUOMap) {
+      dbgs() << "    Reduced key: (" << std::get<0>(RedKeyToUO.first) << ", "
+             << std::get<1>(RedKeyToUO.first) << ", "
+             << static_cast<int>(std::get<2>(RedKeyToUO.first)) << ") --> "
+             << RedKeyToUO.second.size() << " underlying objects:\n";
+      for (auto UObject : RedKeyToUO.second)
+        dbgs() << "    [" << UObject << "]: " << *UObject << '\n';
+    }
+  });
+
+  using UObjectToUObjectMap = DenseMap<const Value *, const Value *>;
+
+  // Compute the ultimate targets for a set of underlying objects.
+  auto GetUltimateTargets =
+      [](SmallPtrSetImpl<const Value *> &UObjects) -> UObjectToUObjectMap {
+    UObjectToUObjectMap IndirectionMap;
+    for (const auto *UObject : UObjects) {
+      const unsigned MaxLookupDepth = 1; // look for 1-level indirections only
+      const auto *UltimateTarget = getUnderlyingObject(UObject, MaxLookupDepth);
+      if (UltimateTarget != UObject)
+        IndirectionMap[UObject] = UltimateTarget;
+    }
+    UObjectToUObjectMap UltimateTargetsMap;
+    for (const auto *UObject : UObjects) {
+      auto Target = UObject;
+      auto It = IndirectionMap.find(Target);
+      for (; It != IndirectionMap.end(); It = IndirectionMap.find(Target))
+        Target = It->second;
+      UltimateTargetsMap[UObject] = Target;
+    }
+    return UltimateTargetsMap;
+  };
+
+  // For each item in RedKeyToUOMap, if it has more than one underlying object,
+  // try to merge the equivalence classes.
+  for (auto &[RedKey, UObjects] : RedKeyToUOMap) {
+    if (UObjects.size() < 2)
+      continue;
+    auto UTMap = GetUltimateTargets(UObjects);
+    for (const auto &[UObject, UltimateTarget] : UTMap) {
+      if (UObject == UltimateTarget)
+        continue;
+
+      EqClassKey KeyFrom{UObject, std::get<0>(RedKey), std::get<1>(RedKey),
+                         std::get<2>(RedKey)};
+      EqClassKey KeyTo{UltimateTarget, std::get<0>(RedKey), std::get<1>(RedKey),
+                       std::get<2>(RedKey)};
+      const auto &VecFrom = EQClasses[KeyFrom];
+      const auto &VecTo = EQClasses[KeyTo];
+      SmallVector<Instruction *, 8> MergedVec;
+      std::merge(VecFrom.begin(), VecFrom.end(), VecTo.begin(), VecTo.end(),
+                 std::back_inserter(MergedVec),
+                 [](Instruction *A, Instruction *B) {
+                   return A && B && A->comesBefore(B);
+                 });
+      EQClasses[KeyTo] = std::move(MergedVec);
+      EQClasses.erase(KeyFrom);
+    }
+  }
+  LLVM_DEBUG({
+    dbgs() << "LSV: mergeEquivalenceClasses: after merging:\n";
+    for (const auto &EC : EQClasses) {
+      dbgs() << "    Key: ([" << std::get<0>(EC.first)
+             << "]: " << *std::get<0>(EC.first) << ", " << std::get<1>(EC.first)
+             << ", " << std::get<2>(EC.first) << ", "
+             << static_cast<int>(std::get<3>(EC.first)) << ")\n";
+      for (const auto &Inst : EC.second)
+        dbgs() << "\tInst: " << *Inst << '\n';
+    }
+  });
+}
+
 EquivalenceClassMap
 Vectorizer::collectEquivalenceClasses(BasicBlock::iterator Begin,
                                       BasicBlock::iterator End) {
@@ -1377,6 +1499,7 @@ Vectorizer::collectEquivalenceClasses(BasicBlock::iterator Begin,
         .emplace_back(&I);
   }
 
+  mergeEquivalenceClasses(Ret);
   return Ret;
 }
 

llvm/test/Transforms/LoadStoreVectorizer/X86/massive_indirection.ll

@@ -0,0 +1,142 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt %s -mtriple=x86_64-unknown-linux-gnu -passes=load-store-vectorizer -mcpu=skx -S -o - | FileCheck %s
+
+; This test verifies that the vectorizer can handle an extended sequence of
+; getelementptr instructions and generate longer vectors. With special handling,
+; some elements can still be vectorized even if they require looking up the
+; common underlying object deeper than 6 levels from the original pointer.
+
+; The test below is the simplified version of actual performance oriented
+; workload; the offsets in getelementptr instructions are similar or same for
+; the test simplicity.
+
+define void @v1_v2_v4_v1_to_v8_levels_6_7_8_8(i32 %arg0, ptr align 16 %arg1) {
+; CHECK-LABEL: define void @v1_v2_v4_v1_to_v8_levels_6_7_8_8(
+; CHECK-SAME: i32 [[ARG0:%.*]], ptr align 16 [[ARG1:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:    [[LEVEL1:%.*]] = getelementptr i8, ptr [[ARG1]], i32 917504
+; CHECK-NEXT:    [[LEVEL2:%.*]] = getelementptr i8, ptr [[LEVEL1]], i32 [[ARG0]]
+; CHECK-NEXT:    [[LEVEL3:%.*]] = getelementptr i8, ptr [[LEVEL2]], i32 32768
+; CHECK-NEXT:    [[LEVEL4:%.*]] = getelementptr i8, ptr [[LEVEL3]], i32 [[ARG0]]
+; CHECK-NEXT:    [[LEVEL5:%.*]] = getelementptr i8, ptr [[LEVEL4]], i32 [[ARG0]]
+; CHECK-NEXT:    [[A6:%.*]] = getelementptr i8, ptr [[LEVEL5]], i32 [[ARG0]]
+; CHECK-NEXT:    store <8 x half> zeroinitializer, ptr [[A6]], align 16
+; CHECK-NEXT:    ret void
+;
+
+  %level1 = getelementptr i8, ptr %arg1, i32 917504
+  %level2 = getelementptr i8, ptr %level1, i32 %arg0
+  %level3 = getelementptr i8, ptr %level2, i32 32768
+  %level4 = getelementptr i8, ptr %level3, i32 %arg0
+  %level5 = getelementptr i8, ptr %level4, i32 %arg0
+
+  %a6 = getelementptr i8, ptr %level5, i32 %arg0
+  %b7 = getelementptr i8, ptr %a6, i32 2
+  %c8 = getelementptr i8, ptr %b7, i32 8
+  %d8 = getelementptr i8, ptr %b7, i32 12
+
+  store half 0xH0000, ptr %a6, align 16
+  store <4 x half> zeroinitializer, ptr %b7, align 2
+  store <2 x half> zeroinitializer, ptr %c8, align 2
+  store half 0xH0000, ptr %d8, align 2
+  ret void
+}
+
+define void @v1x8_levels_6_7_8_9_10_11_12_13(i32 %arg0, ptr align 16 %arg1) {
+; CHECK-LABEL: define void @v1x8_levels_6_7_8_9_10_11_12_13(
+; CHECK-SAME: i32 [[ARG0:%.*]], ptr align 16 [[ARG1:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    [[LEVEL1:%.*]] = getelementptr i8, ptr [[ARG1]], i32 917504
+; CHECK-NEXT:    [[LEVEL2:%.*]] = getelementptr i8, ptr [[LEVEL1]], i32 [[ARG0]]
+; CHECK-NEXT:    [[LEVEL3:%.*]] = getelementptr i8, ptr [[LEVEL2]], i32 32768
+; CHECK-NEXT:    [[LEVEL4:%.*]] = getelementptr i8, ptr [[LEVEL3]], i32 [[ARG0]]
+; CHECK-NEXT:    [[LEVEL5:%.*]] = getelementptr i8, ptr [[LEVEL4]], i32 [[ARG0]]
+; CHECK-NEXT:    [[A6:%.*]] = getelementptr i8, ptr [[LEVEL5]], i32 [[ARG0]]
+; CHECK-NEXT:    store <8 x half> zeroinitializer, ptr [[A6]], align 16
+; CHECK-NEXT:    ret void
+;
+
+  %level1 = getelementptr i8, ptr %arg1, i32 917504
+  %level2 = getelementptr i8, ptr %level1, i32 %arg0
+  %level3 = getelementptr i8, ptr %level2, i32 32768
+  %level4 = getelementptr i8, ptr %level3, i32 %arg0
+  %level5 = getelementptr i8, ptr %level4, i32 %arg0
+
+  %a6 = getelementptr i8, ptr %level5, i32 %arg0
+  %b7 = getelementptr i8, ptr %a6, i32 2
+  %c8 = getelementptr i8, ptr %b7, i32 2
+  %d9 = getelementptr i8, ptr %c8, i32 2
+  %e10 = getelementptr i8, ptr %d9, i32 2
+  %f11 = getelementptr i8, ptr %e10, i32 2
+  %g12 = getelementptr i8, ptr %f11, i32 2
+  %h13 = getelementptr i8, ptr %g12, i32 2
+
+  store half 0xH0000, ptr %a6, align 16
+  store half 0xH0000, ptr %b7, align 2
+  store half 0xH0000, ptr %c8, align 2
+  store half 0xH0000, ptr %d9, align 2
+  store half 0xH0000, ptr %e10, align 8
+  store half 0xH0000, ptr %f11, align 2
+  store half 0xH0000, ptr %g12, align 2
+  store half 0xH0000, ptr %h13, align 2
+  ret void
+}
+
+define void @v1_4_4_4_2_1_to_v8_8_levels_6_7(i32 %arg0, ptr addrspace(3) align 16 %arg1_ptr, i32 %arg2, i32 %arg3, i32 %arg4, i32 %arg5, half %arg6_half, half %arg7_half, <2 x half> %arg8_2xhalf) {
+; CHECK-LABEL: define void @v1_4_4_4_2_1_to_v8_8_levels_6_7(
+; CHECK-SAME: i32 [[ARG0:%.*]], ptr addrspace(3) align 16 [[ARG1_PTR:%.*]], i32 [[ARG2:%.*]], i32 [[ARG3:%.*]], i32 [[ARG4:%.*]], i32 [[ARG5:%.*]], half [[ARG6_HALF:%.*]], half [[ARG7_HALF:%.*]], <2 x half> [[ARG8_2XHALF:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i8, ptr addrspace(3) [[ARG1_PTR]], i32 458752
+; CHECK-NEXT:    br [[DOTPREHEADER11_PREHEADER:label %.*]]
+; CHECK:       [[_PREHEADER11_PREHEADER:.*:]]
+; CHECK-NEXT:    [[TMP2:%.*]] = shl nuw nsw i32 [[ARG0]], 6
+; CHECK-NEXT:    [[TMP3:%.*]] = getelementptr inbounds i8, ptr addrspace(3) [[TMP1]], i32 [[TMP2]]
+; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr inbounds i8, ptr addrspace(3) [[TMP3]], i32 [[ARG2]]
+; CHECK-NEXT:    [[TMP5:%.*]] = getelementptr inbounds i8, ptr addrspace(3) [[TMP4]], i32 [[ARG3]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i32 [[ARG0]], 2
+; CHECK-NEXT:    br i1 [[CMP]], [[DOTLR_PH:label %.*]], [[DOTEXIT_POINT:label %.*]]
+; CHECK:       [[_LR_PH:.*:]]
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, ptr addrspace(3) [[TMP5]], i32 [[ARG4]]
+; CHECK-NEXT:    [[TMP6:%.*]] = getelementptr inbounds i8, ptr addrspace(3) [[GEP]], i32 [[ARG5]]
+; CHECK-NEXT:    [[TMP7:%.*]] = insertelement <8 x half> poison, half [[ARG6_HALF]], i32 0
+; CHECK-NEXT:    [[TMP8:%.*]] = insertelement <8 x half> [[TMP7]], half 0xH0000, i32 1
+; CHECK-NEXT:    [[TMP9:%.*]] = insertelement <8 x half> [[TMP8]], half 0xH0000, i32 2
+; CHECK-NEXT:    [[TMP10:%.*]] = insertelement <8 x half> [[TMP9]], half 0xH0000, i32 3
+; CHECK-NEXT:    [[TMP11:%.*]] = insertelement <8 x half> [[TMP10]], half 0xH0000, i32 4
+; CHECK-NEXT:    [[TMP12:%.*]] = extractelement <2 x half> [[ARG8_2XHALF]], i32 0
+; CHECK-NEXT:    [[TMP13:%.*]] = insertelement <8 x half> [[TMP11]], half [[TMP12]], i32 5
+; CHECK-NEXT:    [[TMP14:%.*]] = extractelement <2 x half> [[ARG8_2XHALF]], i32 1
+; CHECK-NEXT:    [[TMP15:%.*]] = insertelement <8 x half> [[TMP13]], half [[TMP14]], i32 6
+; CHECK-NEXT:    [[TMP16:%.*]] = insertelement <8 x half> [[TMP15]], half [[ARG7_HALF]], i32 7
+; CHECK-NEXT:    store <8 x half> [[TMP16]], ptr addrspace(3) [[TMP6]], align 2
+; CHECK-NEXT:    br [[DOTEXIT_POINT]]
+; CHECK:       [[_EXIT_POINT:.*:]]
+; CHECK-NEXT:    ret void
+;
+  %base1 = getelementptr inbounds i8, ptr addrspace(3) %arg1_ptr, i32 458752
+  br label %.preheader11.preheader
+
+.preheader11.preheader:
+  %base2 = shl nuw nsw i32 %arg0, 6
+  %base3 = getelementptr inbounds i8, ptr addrspace(3) %base1, i32 %base2
+
+  %base4 = getelementptr inbounds i8, ptr addrspace(3) %base3, i32 %arg2
+  %base5 = getelementptr inbounds i8, ptr addrspace(3) %base4, i32 %arg3
+
+  %cmp = icmp sgt i32 %arg0, 2
+  br i1 %cmp, label %.lr.ph, label %.exit_point
+
+.lr.ph:
+  %gep = getelementptr inbounds i8, ptr addrspace(3) %base5, i32 %arg4
+
+  %dst = getelementptr inbounds i8, ptr addrspace(3) %gep, i32 %arg5
+  %dst_off2 = getelementptr inbounds i8, ptr addrspace(3) %dst, i32 2
+  %dst_off10 = getelementptr inbounds i8, ptr addrspace(3) %dst, i32 10
+  %dst_off14 = getelementptr inbounds i8, ptr addrspace(3) %dst, i32 14
+
+  store half %arg6_half, ptr addrspace(3) %dst, align 2
+  store <4 x half> zeroinitializer, ptr addrspace(3) %dst_off2, align 2
+  store <2 x half> %arg8_2xhalf, ptr addrspace(3) %dst_off10, align 2
+  store half %arg7_half, ptr addrspace(3) %dst_off14, align 2
+  br label %.exit_point
+
+.exit_point:
+  ret void
+}