[llvm] [VPlan] Introduce CSE pass (PR #151872)

[Ramkumar Ramachandra via llvm-commits]

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@@ -1765,6 +1765,113 @@ void VPlanTransforms::clearReductionWrapFlags(VPlan &Plan) {
   }
 }
 
+namespace {
+struct VPCSEDenseMapInfo : public DenseMapInfo<VPSingleDefRecipe *> {
+  static bool isSentinel(const VPSingleDefRecipe *Def) {
+    return Def == getEmptyKey() || Def == getTombstoneKey();
+  }
+
+  /// Get any instruction opcode or intrinsic ID data embedded in recipe \p R.
+  /// Returns an optional pair, where the first element indicates whether it is
+  /// an intrinsic ID.
+  static std::optional<std::pair<bool, unsigned>>
+  getOpcodeOrIntrinsicID(const VPSingleDefRecipe *R) {
+    return TypeSwitch<const VPSingleDefRecipe *,
+                      std::optional<std::pair<bool, unsigned>>>(R)
+        .Case<VPInstruction, VPWidenRecipe, VPWidenCastRecipe,
+              VPWidenSelectRecipe, VPHistogramRecipe, VPPartialReductionRecipe,
+              VPReplicateRecipe>(
+            [](auto *I) { return std::make_pair(false, I->getOpcode()); })
+        .Case<VPWidenIntrinsicRecipe>([](auto *I) {
+          return std::make_pair(true, I->getVectorIntrinsicID());
+        })
+        .Default([](auto *) { return std::nullopt; });
+  }
+
+  /// During CSE, we can only handle certain recipes that don't read from
+  /// memory: if they read from memory, there could be an intervening write to
+  /// memory before the next instance is CSE'd, leading to an incorrect result.
+  /// We can extend the list of handled recipes in the future, provided we
+  /// account for the data embedded in them while checking for equality or
+  /// hashing.
+  static bool canHandle(const VPSingleDefRecipe *Def) {
+    return isa<VPInstruction, VPWidenRecipe, VPWidenCastRecipe,
+               VPWidenSelectRecipe, VPHistogramRecipe, VPReplicateRecipe,
+               VPWidenIntrinsicRecipe>(Def) &&
+           !Def->mayReadFromMemory();
+  }
+
+  /// Hash the underlying data of \p Def.
+  static unsigned getHashValue(const VPSingleDefRecipe *Def) {
+    const VPlan *Plan = Def->getParent()->getPlan();
+    VPTypeAnalysis TypeInfo(*Plan);
+    hash_code Result = hash_combine(
+        Def->getVPDefID(), getOpcodeOrIntrinsicID(Def),
+        TypeInfo.inferScalarType(Def), vputils::isSingleScalar(Def),
+        hash_combine_range(Def->operands()));
+    if (isa<VPReplicateRecipe>(Def))
+      return hash_combine(Result, Def->getUnderlyingInstr());
+    if (auto *RFlags = dyn_cast<VPRecipeWithIRFlags>(Def))
+      return hash_combine(Result, RFlags->getPredicate());
+    return Result;
+  }
+
+  /// Check equality of underlying data of \p L and \p R.
+  static bool isEqual(const VPSingleDefRecipe *L, const VPSingleDefRecipe *R) {
+    if (isSentinel(L) || isSentinel(R))
+      return L == R;
+    const VPlan *Plan = L->getParent()->getPlan();
+    VPTypeAnalysis TypeInfo(*Plan);
+    bool Result = L->getVPDefID() == R->getVPDefID() &&
+                  getOpcodeOrIntrinsicID(L) == getOpcodeOrIntrinsicID(R) &&
+                  TypeInfo.inferScalarType(L) == TypeInfo.inferScalarType(R) &&
+                  vputils::isSingleScalar(L) == vputils::isSingleScalar(R) &&
+                  equal(L->operands(), R->operands());
+    if (Result && isa<VPReplicateRecipe>(L))
+      Result = L->getUnderlyingInstr() == R->getUnderlyingInstr();
+    if (Result && isa<VPRecipeWithIRFlags>(L))
+      Result = cast<VPRecipeWithIRFlags>(L)->getPredicate() ==
+               cast<VPRecipeWithIRFlags>(R)->getPredicate();
+    assert((!Result || getHashValue(L) == getHashValue(R)) &&
+           "Divergent hashes of equal values");
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artagnon wrote:

This duplication is the convention used in LLVM's CSE, and several other places, presumably because isEqual is slightly cheaper than getHashValue.

https://github.com/llvm/llvm-project/pull/151872