diff --git a/source/fuzz/CMakeLists.txt b/source/fuzz/CMakeLists.txt
index ab7e985..905d535 100644
--- a/source/fuzz/CMakeLists.txt
+++ b/source/fuzz/CMakeLists.txt
@@ -59,6 +59,7 @@
         fuzzer_pass_add_local_variables.h
         fuzzer_pass_add_loop_preheaders.h
         fuzzer_pass_add_no_contraction_decorations.h
+        fuzzer_pass_add_opphi_synonyms.h
         fuzzer_pass_add_parameters.h
         fuzzer_pass_add_relaxed_decorations.h
         fuzzer_pass_add_stores.h
@@ -124,6 +125,7 @@
         transformation_add_local_variable.h
         transformation_add_loop_preheader.h
         transformation_add_no_contraction_decoration.h
+        transformation_add_opphi_synonym.h
         transformation_add_parameter.h
         transformation_add_relaxed_decoration.h
         transformation_add_spec_constant_op.h
@@ -210,6 +212,7 @@
         fuzzer_pass_add_local_variables.cpp
         fuzzer_pass_add_loop_preheaders.cpp
         fuzzer_pass_add_no_contraction_decorations.cpp
+        fuzzer_pass_add_opphi_synonyms.cpp
         fuzzer_pass_add_parameters.cpp
         fuzzer_pass_add_relaxed_decorations.cpp
         fuzzer_pass_add_stores.cpp
@@ -274,6 +277,7 @@
         transformation_add_local_variable.cpp
         transformation_add_loop_preheader.cpp
         transformation_add_no_contraction_decoration.cpp
+        transformation_add_opphi_synonym.cpp
         transformation_add_parameter.cpp
         transformation_add_relaxed_decoration.cpp
         transformation_add_spec_constant_op.cpp
diff --git a/source/fuzz/fuzzer.cpp b/source/fuzz/fuzzer.cpp
index fea031c..e35fc8b 100644
--- a/source/fuzz/fuzzer.cpp
+++ b/source/fuzz/fuzzer.cpp
@@ -35,6 +35,7 @@
 #include "source/fuzz/fuzzer_pass_add_local_variables.h"
 #include "source/fuzz/fuzzer_pass_add_loop_preheaders.h"
 #include "source/fuzz/fuzzer_pass_add_no_contraction_decorations.h"
+#include "source/fuzz/fuzzer_pass_add_opphi_synonyms.h"
 #include "source/fuzz/fuzzer_pass_add_parameters.h"
 #include "source/fuzz/fuzzer_pass_add_relaxed_decorations.h"
 #include "source/fuzz/fuzzer_pass_add_stores.h"
@@ -256,6 +257,9 @@
     MaybeAddPass<FuzzerPassAddLoopPreheaders>(
         &passes, ir_context.get(), &transformation_context, &fuzzer_context,
         transformation_sequence_out);
+    MaybeAddPass<FuzzerPassAddOpPhiSynonyms>(
+        &passes, ir_context.get(), &transformation_context, &fuzzer_context,
+        transformation_sequence_out);
     MaybeAddPass<FuzzerPassAddParameters>(
         &passes, ir_context.get(), &transformation_context, &fuzzer_context,
         transformation_sequence_out);
diff --git a/source/fuzz/fuzzer_context.cpp b/source/fuzz/fuzzer_context.cpp
index ee8db86..6f76bbb 100644
--- a/source/fuzz/fuzzer_context.cpp
+++ b/source/fuzz/fuzzer_context.cpp
@@ -43,6 +43,7 @@
 const std::pair<uint32_t, uint32_t> kChanceOfAddingMatrixType = {20, 70};
 const std::pair<uint32_t, uint32_t> kChanceOfAddingNoContractionDecoration = {
     5, 70};
+const std::pair<uint32_t, uint32_t> kChanceOfAddingOpPhiSynonym = {5, 70};
 const std::pair<uint32_t, uint32_t> kChanceOfAddingParameters = {5, 70};
 const std::pair<uint32_t, uint32_t> kChanceOfAddingRelaxedDecoration = {20, 90};
 const std::pair<uint32_t, uint32_t> kChanceOfAddingStore = {5, 50};
@@ -182,6 +183,8 @@
       ChooseBetweenMinAndMax(kChanceOfAddingMatrixType);
   chance_of_adding_no_contraction_decoration_ =
       ChooseBetweenMinAndMax(kChanceOfAddingNoContractionDecoration);
+  chance_of_adding_opphi_synonym_ =
+      ChooseBetweenMinAndMax(kChanceOfAddingOpPhiSynonym);
   chance_of_adding_parameters =
       ChooseBetweenMinAndMax(kChanceOfAddingParameters);
   chance_of_adding_relaxed_decoration_ =
diff --git a/source/fuzz/fuzzer_context.h b/source/fuzz/fuzzer_context.h
index 9f85b13..bc98bc0 100644
--- a/source/fuzz/fuzzer_context.h
+++ b/source/fuzz/fuzzer_context.h
@@ -148,6 +148,9 @@
   uint32_t GetChanceOfAddingNoContractionDecoration() {
     return chance_of_adding_no_contraction_decoration_;
   }
+  uint32_t GetChanceOfAddingOpPhiSynonym() {
+    return chance_of_adding_opphi_synonym_;
+  }
   uint32_t GetChanceOfAddingParameters() { return chance_of_adding_parameters; }
   uint32_t GetChanceOfAddingRelaxedDecoration() {
     return chance_of_adding_relaxed_decoration_;
@@ -367,6 +370,7 @@
   uint32_t chance_of_adding_loop_preheader_;
   uint32_t chance_of_adding_matrix_type_;
   uint32_t chance_of_adding_no_contraction_decoration_;
+  uint32_t chance_of_adding_opphi_synonym_;
   uint32_t chance_of_adding_parameters;
   uint32_t chance_of_adding_relaxed_decoration_;
   uint32_t chance_of_adding_store_;
diff --git a/source/fuzz/fuzzer_pass_add_opphi_synonyms.cpp b/source/fuzz/fuzzer_pass_add_opphi_synonyms.cpp
new file mode 100644
index 0000000..97adfb2
--- /dev/null
+++ b/source/fuzz/fuzzer_pass_add_opphi_synonyms.cpp
@@ -0,0 +1,297 @@
+// Copyright (c) 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "source/fuzz/fuzzer_pass_add_opphi_synonyms.h"
+
+#include "source/fuzz/fuzzer_util.h"
+#include "source/fuzz/transformation_add_opphi_synonym.h"
+
+namespace spvtools {
+namespace fuzz {
+
+FuzzerPassAddOpPhiSynonyms::FuzzerPassAddOpPhiSynonyms(
+    opt::IRContext* ir_context, TransformationContext* transformation_context,
+    FuzzerContext* fuzzer_context,
+    protobufs::TransformationSequence* transformations)
+    : FuzzerPass(ir_context, transformation_context, fuzzer_context,
+                 transformations) {}
+
+FuzzerPassAddOpPhiSynonyms::~FuzzerPassAddOpPhiSynonyms() = default;
+
+void FuzzerPassAddOpPhiSynonyms::Apply() {
+  // Get a list of synonymous ids with the same type that can be used in the
+  // same OpPhi instruction.
+  auto equivalence_classes = GetIdEquivalenceClasses();
+
+  // Make a list of references, to avoid copying sets unnecessarily.
+  std::vector<std::set<uint32_t>*> equivalence_class_pointers;
+  for (auto& set : equivalence_classes) {
+    equivalence_class_pointers.push_back(&set);
+  }
+
+  // Keep a list of transformations to apply at the end.
+  std::vector<TransformationAddOpPhiSynonym> transformations_to_apply;
+
+  for (auto& function : *GetIRContext()->module()) {
+    for (auto& block : function) {
+      // Randomly decide whether to consider this block.
+      if (!GetFuzzerContext()->ChoosePercentage(
+              GetFuzzerContext()->GetChanceOfAddingOpPhiSynonym())) {
+        continue;
+      }
+
+      // The block must have at least one predecessor.
+      size_t num_preds = GetIRContext()->cfg()->preds(block.id()).size();
+      if (num_preds == 0) {
+        continue;
+      }
+
+      std::set<uint32_t>* chosen_equivalence_class = nullptr;
+
+      if (num_preds > 1) {
+        // If the block has more than one predecessor, prioritise sets with at
+        // least 2 ids available at some predecessor.
+        chosen_equivalence_class = MaybeFindSuitableEquivalenceClassRandomly(
+            equivalence_class_pointers, block.id(), 2);
+      }
+
+      // If a set was not already chosen, choose one with at least one available
+      // id.
+      if (!chosen_equivalence_class) {
+        chosen_equivalence_class = MaybeFindSuitableEquivalenceClassRandomly(
+            equivalence_class_pointers, block.id(), 1);
+      }
+
+      // If no suitable set was found, we cannot apply the transformation to
+      // this block.
+      if (!chosen_equivalence_class) {
+        continue;
+      }
+
+      // Initialise the map from predecessor labels to ids.
+      std::map<uint32_t, uint32_t> preds_to_ids;
+
+      // Keep track of the ids used and of the id of a predecessor with at least
+      // two ids to choose from. This is to ensure that, if possible, at least
+      // two distinct ids will be used.
+      std::set<uint32_t> ids_chosen;
+      uint32_t pred_with_alternatives = 0;
+
+      // Choose an id for each predecessor.
+      for (uint32_t pred_id : GetIRContext()->cfg()->preds(block.id())) {
+        auto suitable_ids = GetSuitableIds(*chosen_equivalence_class, pred_id);
+        assert(!suitable_ids.empty() &&
+               "We must be able to find at least one suitable id because the "
+               "equivalence class was chosen among suitable ones.");
+
+        // If this predecessor has more than one id to choose from and it is the
+        // first one of this kind that we found, remember its id.
+        if (suitable_ids.size() > 1 && !pred_with_alternatives) {
+          pred_with_alternatives = pred_id;
+        }
+
+        uint32_t chosen_id =
+            suitable_ids[GetFuzzerContext()->RandomIndex(suitable_ids)];
+
+        // Add this id to the set of ids chosen.
+        ids_chosen.emplace(chosen_id);
+
+        // Add the pair (predecessor, chosen id) to the map.
+        preds_to_ids[pred_id] = chosen_id;
+      }
+
+      // If:
+      // - the block has more than one predecessor
+      // - at least one predecessor has more than one alternative
+      // - the same id has been chosen by all the predecessors
+      // then choose another one for the predecessor with more than one
+      // alternative.
+      if (num_preds > 1 && pred_with_alternatives != 0 &&
+          ids_chosen.size() == 1) {
+        auto suitable_ids =
+            GetSuitableIds(*chosen_equivalence_class, pred_with_alternatives);
+        uint32_t chosen_id =
+            GetFuzzerContext()->RemoveAtRandomIndex(&suitable_ids);
+        if (chosen_id == preds_to_ids[pred_with_alternatives]) {
+          chosen_id = GetFuzzerContext()->RemoveAtRandomIndex(&suitable_ids);
+        }
+
+        preds_to_ids[pred_with_alternatives] = chosen_id;
+      }
+
+      // Add the transformation to the list of transformations to apply.
+      transformations_to_apply.emplace_back(block.id(), preds_to_ids,
+                                            GetFuzzerContext()->GetFreshId());
+    }
+  }
+
+  // Apply the transformations.
+  for (const auto& transformation : transformations_to_apply) {
+    ApplyTransformation(transformation);
+  }
+}
+
+std::vector<std::set<uint32_t>>
+FuzzerPassAddOpPhiSynonyms::GetIdEquivalenceClasses() {
+  std::vector<std::set<uint32_t>> id_equivalence_classes;
+
+  // Keep track of all the ids that have already be assigned to a class.
+  std::set<uint32_t> already_in_a_class;
+
+  for (const auto& pair : GetIRContext()->get_def_use_mgr()->id_to_defs()) {
+    // Exclude ids that have already been assigned to a class.
+    if (already_in_a_class.count(pair.first)) {
+      continue;
+    }
+
+    // Exclude irrelevant ids.
+    if (GetTransformationContext()->GetFactManager()->IdIsIrrelevant(
+            pair.first)) {
+      continue;
+    }
+
+    // Exclude ids having a type that is not allowed by the transformation.
+    if (!TransformationAddOpPhiSynonym::CheckTypeIsAllowed(
+            GetIRContext(), pair.second->type_id())) {
+      continue;
+    }
+
+    // We need a new equivalence class for this id.
+    std::set<uint32_t> new_equivalence_class;
+
+    // Add this id to the class.
+    new_equivalence_class.emplace(pair.first);
+    already_in_a_class.emplace(pair.first);
+
+    // Add all the synonyms with the same type to this class.
+    for (auto synonym :
+         GetTransformationContext()->GetFactManager()->GetSynonymsForId(
+             pair.first)) {
+      // The synonym must be a plain id - it cannot be an indexed access into a
+      // composite.
+      if (synonym->index_size() > 0) {
+        continue;
+      }
+
+      // The synonym must not be irrelevant.
+      if (GetTransformationContext()->GetFactManager()->IdIsIrrelevant(
+              synonym->object())) {
+        continue;
+      }
+
+      auto synonym_def =
+          GetIRContext()->get_def_use_mgr()->GetDef(synonym->object());
+      // The synonym must exist and have the same type as the id we are
+      // considering.
+      if (!synonym_def || synonym_def->type_id() != pair.second->type_id()) {
+        continue;
+      }
+
+      // We can add this synonym to the new equivalence class.
+      new_equivalence_class.emplace(synonym->object());
+      already_in_a_class.emplace(synonym->object());
+    }
+
+    // Add the new equivalence class to the list of equivalence classes.
+    id_equivalence_classes.emplace_back(std::move(new_equivalence_class));
+  }
+
+  return id_equivalence_classes;
+}
+
+bool FuzzerPassAddOpPhiSynonyms::EquivalenceClassIsSuitableForBlock(
+    const std::set<uint32_t>& equivalence_class, uint32_t block_id,
+    uint32_t distinct_ids_required) {
+  bool at_least_one_id_for_each_pred = true;
+
+  // Keep a set of the suitable ids found.
+  std::set<uint32_t> suitable_ids_found;
+
+  // Loop through all the predecessors of the block.
+  for (auto pred_id : GetIRContext()->cfg()->preds(block_id)) {
+    // Find the last instruction in the predecessor block.
+    auto last_instruction =
+        GetIRContext()->get_instr_block(pred_id)->terminator();
+
+    // Initially assume that there is not a suitable id for this predecessor.
+    bool at_least_one_suitable_id_found = false;
+    for (uint32_t id : equivalence_class) {
+      if (fuzzerutil::IdIsAvailableBeforeInstruction(GetIRContext(),
+                                                     last_instruction, id)) {
+        // We have found a suitable id.
+        at_least_one_suitable_id_found = true;
+        suitable_ids_found.emplace(id);
+
+        // If we have already found enough distinct suitable ids, we don't need
+        // to check the remaining ones for this predecessor.
+        if (suitable_ids_found.size() >= distinct_ids_required) {
+          break;
+        }
+      }
+    }
+    // If no suitable id was found for this predecessor, this equivalence class
+    // is not suitable and we don't need to check the other predecessors.
+    if (!at_least_one_suitable_id_found) {
+      at_least_one_id_for_each_pred = false;
+      break;
+    }
+  }
+
+  // The equivalence class is suitable if at least one suitable id was found for
+  // each predecessor and we have found at least |distinct_ids_required|
+  // distinct suitable ids in general.
+  return at_least_one_id_for_each_pred &&
+         suitable_ids_found.size() >= distinct_ids_required;
+}
+
+std::vector<uint32_t> FuzzerPassAddOpPhiSynonyms::GetSuitableIds(
+    const std::set<uint32_t>& ids, uint32_t pred_id) {
+  // Initialise an empty vector of suitable ids.
+  std::vector<uint32_t> suitable_ids;
+
+  // Get the predecessor block.
+  auto predecessor = fuzzerutil::MaybeFindBlock(GetIRContext(), pred_id);
+
+  // Loop through the ids to find the suitable ones.
+  for (uint32_t id : ids) {
+    if (fuzzerutil::IdIsAvailableBeforeInstruction(
+            GetIRContext(), predecessor->terminator(), id)) {
+      suitable_ids.push_back(id);
+    }
+  }
+
+  return suitable_ids;
+}
+
+std::set<uint32_t>*
+FuzzerPassAddOpPhiSynonyms::MaybeFindSuitableEquivalenceClassRandomly(
+    const std::vector<std::set<uint32_t>*>& candidates, uint32_t block_id,
+    uint32_t distinct_ids_required) {
+  auto remaining_candidates = candidates;
+  while (!remaining_candidates.empty()) {
+    // Choose one set randomly and return it if it is suitable.
+    auto chosen =
+        GetFuzzerContext()->RemoveAtRandomIndex(&remaining_candidates);
+    if (EquivalenceClassIsSuitableForBlock(*chosen, block_id,
+                                           distinct_ids_required)) {
+      return chosen;
+    }
+  }
+
+  // No suitable sets were found.
+  return nullptr;
+}
+
+}  // namespace fuzz
+}  // namespace spvtools
diff --git a/source/fuzz/fuzzer_pass_add_opphi_synonyms.h b/source/fuzz/fuzzer_pass_add_opphi_synonyms.h
new file mode 100644
index 0000000..ed61c020
--- /dev/null
+++ b/source/fuzz/fuzzer_pass_add_opphi_synonyms.h
@@ -0,0 +1,73 @@
+// Copyright (c) 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef SOURCE_FUZZ_FUZZER_PASS_ADD_OPPHI_SYNONYMS_
+#define SOURCE_FUZZ_FUZZER_PASS_ADD_OPPHI_SYNONYMS_
+
+#include "source/fuzz/fuzzer_pass.h"
+
+namespace spvtools {
+namespace fuzz {
+
+// A fuzzer pass to add OpPhi instructions which can take the values of ids that
+// have been marked as synonymous. This instruction will itself be marked as
+// synonymous with the others.
+class FuzzerPassAddOpPhiSynonyms : public FuzzerPass {
+ public:
+  FuzzerPassAddOpPhiSynonyms(
+      opt::IRContext* ir_context, TransformationContext* transformation_context,
+      FuzzerContext* fuzzer_context,
+      protobufs::TransformationSequence* transformations);
+
+  ~FuzzerPassAddOpPhiSynonyms() override;
+
+  void Apply() override;
+
+  // Computes the equivalence classes for the non-pointer and non-irrelevant ids
+  // in the module, where two ids are considered equivalent iff they have been
+  // declared synonymous and they have the same type.
+  std::vector<std::set<uint32_t>> GetIdEquivalenceClasses();
+
+  // Returns true iff |equivalence_class| contains at least
+  // |distinct_ids_required| ids so that all of these ids are available at the
+  // end of at least one predecessor of the block with label |block_id|.
+  // Assumes that the block has at least one predecessor.
+  bool EquivalenceClassIsSuitableForBlock(
+      const std::set<uint32_t>& equivalence_class, uint32_t block_id,
+      uint32_t distinct_ids_required);
+
+  // Returns a vector with the ids that are available to use at the end of the
+  // block with id |pred_id|, selected among the given |ids|. Assumes that
+  // |pred_id| is the label of a block and all ids in |ids| exist in the module.
+  std::vector<uint32_t> GetSuitableIds(const std::set<uint32_t>& ids,
+                                       uint32_t pred_id);
+
+ private:
+  // Randomly chooses one of the equivalence classes in |candidates|, so that it
+  // satisfies all of the following conditions:
+  // - For each of the predecessors of the |block_id| block, there is at least
+  //   one id in the chosen equivalence class that is available at the end of
+  //   it.
+  // - There are at least |distinct_ids_required| ids available at the end of
+  //   some predecessor.
+  // Returns nullptr if no equivalence class in |candidates| satisfies the
+  // requirements.
+  std::set<uint32_t>* MaybeFindSuitableEquivalenceClassRandomly(
+      const std::vector<std::set<uint32_t>*>& candidates, uint32_t block_id,
+      uint32_t distinct_ids_required);
+};
+}  // namespace fuzz
+}  // namespace spvtools
+
+#endif  // SOURCE_FUZZ_FUZZER_PASS_ADD_OPPHI_SYNONYMS_
diff --git a/source/fuzz/protobufs/spvtoolsfuzz.proto b/source/fuzz/protobufs/spvtoolsfuzz.proto
index c20fa99..ed3ce0e 100644
--- a/source/fuzz/protobufs/spvtoolsfuzz.proto
+++ b/source/fuzz/protobufs/spvtoolsfuzz.proto
@@ -414,6 +414,7 @@
     TransformationPropagateInstructionUp propagate_instruction_up = 67;
     TransformationCompositeInsert composite_insert = 68;
     TransformationInlineFunction inline_function = 69;
+    TransformationAddOpPhiSynonym add_opphi_synonym = 70;
     // Add additional option using the next available number.
   }
 }
@@ -746,6 +747,24 @@
 
 }
 
+message TransformationAddOpPhiSynonym {
+
+  // Adds an OpPhi instruction at the start of a block with n predecessors (pred_1, pred_2, ..., pred_n)
+  // and n related ids (id_1, id_2, ..., id_n) which are pairwise synonymous.
+  // The instruction will be of the form:
+  //       %fresh_id = OpPhi %type %id_1 %pred_1 %id_2 %pred_2 ... %id_n %pred_n
+  // and fresh_id will be recorded as being synonymous with all the other ids.
+
+  // Label id of the block
+  uint32 block_id = 1;
+
+  // Pairs (pred_i, id_i)
+  repeated UInt32Pair pred_to_id = 2;
+
+  // Fresh id for the new instruction
+  uint32 fresh_id = 3;
+}
+
 message TransformationAddParameter {
 
   // Adds a new parameter into the function.
diff --git a/source/fuzz/transformation.cpp b/source/fuzz/transformation.cpp
index 0df47d5..3cadac9 100644
--- a/source/fuzz/transformation.cpp
+++ b/source/fuzz/transformation.cpp
@@ -33,6 +33,7 @@
 #include "source/fuzz/transformation_add_local_variable.h"
 #include "source/fuzz/transformation_add_loop_preheader.h"
 #include "source/fuzz/transformation_add_no_contraction_decoration.h"
+#include "source/fuzz/transformation_add_opphi_synonym.h"
 #include "source/fuzz/transformation_add_parameter.h"
 #include "source/fuzz/transformation_add_relaxed_decoration.h"
 #include "source/fuzz/transformation_add_spec_constant_op.h"
@@ -141,6 +142,9 @@
         kAddNoContractionDecoration:
       return MakeUnique<TransformationAddNoContractionDecoration>(
           message.add_no_contraction_decoration());
+    case protobufs::Transformation::TransformationCase::kAddOpphiSynonym:
+      return MakeUnique<TransformationAddOpPhiSynonym>(
+          message.add_opphi_synonym());
     case protobufs::Transformation::TransformationCase::kAddParameter:
       return MakeUnique<TransformationAddParameter>(message.add_parameter());
     case protobufs::Transformation::TransformationCase::kAddRelaxedDecoration:
diff --git a/source/fuzz/transformation_add_opphi_synonym.cpp b/source/fuzz/transformation_add_opphi_synonym.cpp
new file mode 100644
index 0000000..3436dd7
--- /dev/null
+++ b/source/fuzz/transformation_add_opphi_synonym.cpp
@@ -0,0 +1,200 @@
+// Copyright (c) 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "source/fuzz/transformation_add_opphi_synonym.h"
+
+#include "source/fuzz/fuzzer_util.h"
+
+namespace spvtools {
+namespace fuzz {
+TransformationAddOpPhiSynonym::TransformationAddOpPhiSynonym(
+    const protobufs::TransformationAddOpPhiSynonym& message)
+    : message_(message) {}
+
+TransformationAddOpPhiSynonym::TransformationAddOpPhiSynonym(
+    uint32_t block_id, const std::map<uint32_t, uint32_t>& preds_to_ids,
+    uint32_t fresh_id) {
+  message_.set_block_id(block_id);
+  *message_.mutable_pred_to_id() =
+      fuzzerutil::MapToRepeatedUInt32Pair(preds_to_ids);
+  message_.set_fresh_id(fresh_id);
+}
+
+bool TransformationAddOpPhiSynonym::IsApplicable(
+    opt::IRContext* ir_context,
+    const TransformationContext& transformation_context) const {
+  // Check that |message_.block_id| is a block label id.
+  auto block = fuzzerutil::MaybeFindBlock(ir_context, message_.block_id());
+  if (!block) {
+    return false;
+  }
+
+  // Check that |message_.fresh_id| is actually fresh.
+  if (!fuzzerutil::IsFreshId(ir_context, message_.fresh_id())) {
+    return false;
+  }
+
+  // Check that |message_.pred_to_id| contains a mapping for all of the block's
+  // predecessors.
+  std::vector<uint32_t> predecessors = ir_context->cfg()->preds(block->id());
+
+  // There must be at least one predecessor.
+  if (predecessors.empty()) {
+    return false;
+  }
+
+  std::map<uint32_t, uint32_t> preds_to_ids =
+      fuzzerutil::RepeatedUInt32PairToMap(message_.pred_to_id());
+
+  // There must not be repeated key values in |message_.pred_to_id|.
+  if (preds_to_ids.size() != static_cast<size_t>(message_.pred_to_id_size())) {
+    return false;
+  }
+
+  // Check that each predecessor has a corresponding mapping and all of the
+  // corresponding ids exist.
+  for (uint32_t pred : predecessors) {
+    if (preds_to_ids.count(pred) == 0) {
+      return false;
+    }
+
+    // Check that the id exists in the module.
+    if (!ir_context->get_def_use_mgr()->GetDef(preds_to_ids[pred])) {
+      return false;
+    }
+  }
+
+  // Get the first id and its type (which should be the same as all the other
+  // ones) and check that the transformation supports this type.
+  uint32_t first_id = preds_to_ids[predecessors[0]];
+  uint32_t type_id = ir_context->get_def_use_mgr()->GetDef(first_id)->type_id();
+  if (!CheckTypeIsAllowed(ir_context, type_id)) {
+    return false;
+  }
+
+  // Check that the ids corresponding to predecessors are all synonymous, have
+  // the same type and are available to use at the end of the predecessor.
+  for (uint32_t pred : predecessors) {
+    auto id = preds_to_ids[pred];
+
+    // Check that the id has the same type as the other ones.
+    if (ir_context->get_def_use_mgr()->GetDef(id)->type_id() != type_id) {
+      return false;
+    }
+
+    // Check that the id is synonymous with the others by checking that it is
+    // synonymous with the first one (or it is the same id).
+    if (id != first_id &&
+        !transformation_context.GetFactManager()->IsSynonymous(
+            MakeDataDescriptor(id, {}), MakeDataDescriptor(first_id, {}))) {
+      return false;
+    }
+
+    // Check that the id is available at the end of the corresponding
+    // predecessor block.
+
+    auto pred_block = ir_context->get_instr_block(pred);
+
+    // We should always be able to find the predecessor block, since it is in
+    // the predecessors list of |block|.
+    assert(pred_block && "Could not find one of the predecessor blocks.");
+
+    // TODO(https://github.com/KhronosGroup/SPIRV-Tools/issues/3722): This
+    //  function always returns false if the block is unreachable, so it may
+    //  need to be refactored.
+    if (!fuzzerutil::IdIsAvailableBeforeInstruction(
+            ir_context, pred_block->terminator(), id)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+void TransformationAddOpPhiSynonym::Apply(
+    opt::IRContext* ir_context,
+    TransformationContext* transformation_context) const {
+  // Get the type id from one of the ids.
+  uint32_t first_id = message_.pred_to_id(0).second();
+  uint32_t type_id = ir_context->get_def_use_mgr()->GetDef(first_id)->type_id();
+
+  // Define the operand list.
+  opt::Instruction::OperandList operand_list;
+
+  // For each predecessor, add the corresponding operands.
+  for (auto& pair : message_.pred_to_id()) {
+    operand_list.emplace_back(
+        opt::Operand{SPV_OPERAND_TYPE_ID, {pair.second()}});
+    operand_list.emplace_back(
+        opt::Operand{SPV_OPERAND_TYPE_ID, {pair.first()}});
+  }
+
+  // Add a new OpPhi instructions at the beginning of the block.
+  ir_context->get_instr_block(message_.block_id())
+      ->begin()
+      .InsertBefore(MakeUnique<opt::Instruction>(ir_context, SpvOpPhi, type_id,
+                                                 message_.fresh_id(),
+                                                 std::move(operand_list)));
+
+  // Update the module id bound.
+  fuzzerutil::UpdateModuleIdBound(ir_context, message_.fresh_id());
+
+  // Invalidate all analyses, since we added an instruction to the module.
+  ir_context->InvalidateAnalysesExceptFor(
+      opt::IRContext::Analysis::kAnalysisNone);
+
+  // Record the fact that the new id is synonym with the other ones by declaring
+  // that it is a synonym of the first one.
+  transformation_context->GetFactManager()->AddFactDataSynonym(
+      MakeDataDescriptor(message_.fresh_id(), {}),
+      MakeDataDescriptor(first_id, {}), ir_context);
+}
+
+protobufs::Transformation TransformationAddOpPhiSynonym::ToMessage() const {
+  protobufs::Transformation result;
+  *result.mutable_add_opphi_synonym() = message_;
+  return result;
+}
+
+bool TransformationAddOpPhiSynonym::CheckTypeIsAllowed(
+    opt::IRContext* ir_context, uint32_t type_id) {
+  auto type = ir_context->get_type_mgr()->GetType(type_id);
+  if (!type) {
+    return false;
+  }
+
+  // We allow the following types: Bool, Integer, Float, Vector, Matrix, Array,
+  // Struct.
+  if (type->AsBool() || type->AsInteger() || type->AsFloat() ||
+      type->AsVector() || type->AsMatrix() || type->AsArray() ||
+      type->AsStruct()) {
+    return true;
+  }
+
+  // We allow pointer types if the VariablePointers capability is enabled and
+  // the pointer has the correct storage class (Workgroup or StorageBuffer).
+  if (type->AsPointer()) {
+    auto storage_class = type->AsPointer()->storage_class();
+    return ir_context->get_feature_mgr()->HasCapability(
+               SpvCapabilityVariablePointers) &&
+           (storage_class == SpvStorageClassWorkgroup ||
+            storage_class == SpvStorageClassStorageBuffer);
+  }
+
+  // We do not allow other types.
+  return false;
+}
+
+}  // namespace fuzz
+}  // namespace spvtools
diff --git a/source/fuzz/transformation_add_opphi_synonym.h b/source/fuzz/transformation_add_opphi_synonym.h
new file mode 100644
index 0000000..dc0e6d9
--- /dev/null
+++ b/source/fuzz/transformation_add_opphi_synonym.h
@@ -0,0 +1,72 @@
+// Copyright (c) 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef SOURCE_FUZZ_TRANSFORMATION_ADD_OPPHI_SYNONYM_H_
+#define SOURCE_FUZZ_TRANSFORMATION_ADD_OPPHI_SYNONYM_H_
+
+#include "source/fuzz/transformation.h"
+
+namespace spvtools {
+namespace fuzz {
+class TransformationAddOpPhiSynonym : public Transformation {
+ public:
+  explicit TransformationAddOpPhiSynonym(
+      const protobufs::TransformationAddOpPhiSynonym& message);
+
+  TransformationAddOpPhiSynonym(
+      uint32_t block_id, const std::map<uint32_t, uint32_t>& preds_to_ids,
+      uint32_t fresh_id);
+
+  // - |message_.block_id| is the label of a block with at least one
+  //   predecessor.
+  // - |message_.pred_to_id| contains a mapping from each of the predecessors of
+  //   the block to an id that is available at the end of the predecessor.
+  // - All the ids corresponding to a predecessor in |message_.pred_to_id|:
+  //    - have been recorded as synonymous and all have the same type.
+  //      TODO(https://github.com/KhronosGroup/SPIRV-Tools/issues/3726): if a
+  //       predecessor is a dead block, any id of the right type could be used,
+  //       even if it is not synonym with the others.
+  //    - have one of the following types: Bool, Integer, Float, Vector, Matrix,
+  //      Array, Struct. Pointer types are also allowed if the VariablePointers
+  //      capability is enabled and the storage class is Workgroup or
+  //      StorageBuffer.
+  // - |message_.fresh_id| is a fresh id.
+  bool IsApplicable(
+      opt::IRContext* ir_context,
+      const TransformationContext& transformation_context) const override;
+
+  // Given a block with n predecessors, with n >= 1, and n corresponding
+  // synonymous ids of the same type, each available to use at the end of the
+  // corresponding predecessor, adds an OpPhi instruction at the beginning of
+  // the block of the form:
+  //   %fresh_id = OpPhi %type %id_1 %pred_1 %id_2 %pred_2 ... %id_n %pred_n
+  // This instruction is then marked as synonymous with the ids.
+  void Apply(opt::IRContext* ir_context,
+             TransformationContext* transformation_context) const override;
+
+  // Returns true if |type_id| is the id of a type in the module, which is one
+  // of the following: Bool, Integer, Float, Vector, Matrix, Array, Struct.
+  // Pointer types are also allowed if the VariablePointers capability is
+  // enabled and the storage class is Workgroup or StorageBuffer.
+  static bool CheckTypeIsAllowed(opt::IRContext* ir_context, uint32_t type_id);
+
+  protobufs::Transformation ToMessage() const override;
+
+ private:
+  protobufs::TransformationAddOpPhiSynonym message_;
+};
+}  // namespace fuzz
+}  // namespace spvtools
+
+#endif  // SOURCE_FUZZ_TRANSFORMATION_ADD_OPPHI_SYNONYM_H_
diff --git a/test/fuzz/CMakeLists.txt b/test/fuzz/CMakeLists.txt
index 7204f69..1322dad 100644
--- a/test/fuzz/CMakeLists.txt
+++ b/test/fuzz/CMakeLists.txt
@@ -21,6 +21,7 @@
           equivalence_relation_test.cpp
           fact_manager_test.cpp
           fuzz_test_util.cpp
+          fuzzer_pass_add_opphi_synonyms_test.cpp
           fuzzer_pass_construct_composites_test.cpp
           fuzzer_pass_donate_modules_test.cpp
           fuzzer_pass_outline_functions_test.cpp
@@ -42,6 +43,7 @@
           transformation_add_local_variable_test.cpp
           transformation_add_loop_preheader_test.cpp
           transformation_add_no_contraction_decoration_test.cpp
+          transformation_add_opphi_synonym_test.cpp
           transformation_add_parameter_test.cpp
           transformation_add_relaxed_decoration_test.cpp
           transformation_add_synonym_test.cpp
diff --git a/test/fuzz/fuzzer_pass_add_opphi_synonyms_test.cpp b/test/fuzz/fuzzer_pass_add_opphi_synonyms_test.cpp
new file mode 100644
index 0000000..9341b56
--- /dev/null
+++ b/test/fuzz/fuzzer_pass_add_opphi_synonyms_test.cpp
@@ -0,0 +1,170 @@
+// Copyright (c) 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "source/fuzz/fuzzer_pass_add_opphi_synonyms.h"
+#include "source/fuzz/pseudo_random_generator.h"
+#include "test/fuzz/fuzz_test_util.h"
+
+namespace spvtools {
+namespace fuzz {
+namespace {
+
+protobufs::Fact MakeSynonymFact(uint32_t first, uint32_t second) {
+  protobufs::FactDataSynonym data_synonym_fact;
+  *data_synonym_fact.mutable_data1() = MakeDataDescriptor(first, {});
+  *data_synonym_fact.mutable_data2() = MakeDataDescriptor(second, {});
+  protobufs::Fact result;
+  *result.mutable_data_synonym_fact() = data_synonym_fact;
+  return result;
+}
+
+// Adds synonym facts to the fact manager.
+void SetUpIdSynonyms(FactManager* fact_manager, opt::IRContext* context) {
+  // Synonyms {9, 11, 15, 16, 21, 22}
+  fact_manager->AddFact(MakeSynonymFact(11, 9), context);
+  fact_manager->AddFact(MakeSynonymFact(15, 9), context);
+  fact_manager->AddFact(MakeSynonymFact(16, 9), context);
+  fact_manager->AddFact(MakeSynonymFact(21, 9), context);
+  fact_manager->AddFact(MakeSynonymFact(22, 9), context);
+
+  // Synonyms {10, 23}
+  fact_manager->AddFact(MakeSynonymFact(10, 23), context);
+
+  // Synonyms {14, 27}
+  fact_manager->AddFact(MakeSynonymFact(14, 27), context);
+
+  // Synonyms {24, 26, 30}
+  fact_manager->AddFact(MakeSynonymFact(26, 24), context);
+  fact_manager->AddFact(MakeSynonymFact(30, 24), context);
+}
+
+// Returns true if the given lists have the same elements, regardless of their
+// order.
+template <typename T>
+bool ListsHaveTheSameElements(const std::vector<T>& list1,
+                              const std::vector<T>& list2) {
+  auto sorted1 = list1;
+  std::sort(sorted1.begin(), sorted1.end());
+
+  auto sorted2 = list2;
+  std::sort(sorted2.begin(), sorted2.end());
+
+  return sorted1 == sorted2;
+}
+
+std::string shader = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %2 "main"
+               OpExecutionMode %2 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %2 "main"
+          %3 = OpTypeVoid
+          %4 = OpTypeFunction %3
+          %5 = OpTypeBool
+          %6 = OpConstantTrue %5
+          %7 = OpTypeInt 32 1
+          %8 = OpTypeInt 32 0
+          %9 = OpConstant %7 1
+         %10 = OpConstant %7 2
+         %11 = OpConstant %8 1
+         %12 = OpTypePointer Function %7
+          %2 = OpFunction %3 None %4
+         %13 = OpLabel
+         %14 = OpVariable %12 Function
+         %15 = OpCopyObject %7 %9
+         %16 = OpCopyObject %8 %11
+               OpBranch %17
+         %17 = OpLabel
+               OpSelectionMerge %18 None
+               OpBranchConditional %6 %19 %20
+         %19 = OpLabel
+         %21 = OpCopyObject %7 %15
+         %22 = OpCopyObject %8 %16
+         %23 = OpCopyObject %7 %10
+         %24 = OpIAdd %7 %9 %10
+               OpBranch %18
+         %20 = OpLabel
+               OpBranch %18
+         %18 = OpLabel
+         %26 = OpIAdd %7 %15 %10
+         %27 = OpCopyObject %12 %14
+               OpSelectionMerge %28 None
+               OpBranchConditional %6 %29 %28
+         %29 = OpLabel
+         %30 = OpCopyObject %7 %26
+               OpBranch %28
+         %28 = OpLabel
+               OpReturn
+               OpFunctionEnd
+)";
+
+TEST(FuzzerPassAddOpPhiSynonymsTest, HelperFunctions) {
+  const auto env = SPV_ENV_UNIVERSAL_1_5;
+  const auto consumer = nullptr;
+  const auto context = BuildModule(env, consumer, shader, kFuzzAssembleOption);
+  ASSERT_TRUE(IsValid(env, context.get()));
+
+  FactManager fact_manager;
+  spvtools::ValidatorOptions validator_options;
+  TransformationContext transformation_context(&fact_manager,
+                                               validator_options);
+
+  PseudoRandomGenerator prng(0);
+  FuzzerContext fuzzer_context(&prng, 100);
+  protobufs::TransformationSequence transformation_sequence;
+
+  FuzzerPassAddOpPhiSynonyms fuzzer_pass(context.get(), &transformation_context,
+                                         &fuzzer_context,
+                                         &transformation_sequence);
+
+  SetUpIdSynonyms(&fact_manager, context.get());
+
+  std::vector<std::set<uint32_t>> expected_equivalence_classes = {
+      {9, 15, 21}, {11, 16, 22}, {10, 23}, {6}, {24, 26, 30}};
+
+  ASSERT_TRUE(ListsHaveTheSameElements<std::set<uint32_t>>(
+      fuzzer_pass.GetIdEquivalenceClasses(), expected_equivalence_classes));
+
+  // The set {24, 26, 30} is not suitable for 18 (none if the ids is available
+  // for predecessor 20).
+  ASSERT_FALSE(
+      fuzzer_pass.EquivalenceClassIsSuitableForBlock({24, 26, 30}, 18, 1));
+
+  // The set {6} is not suitable for 18 if we require at least 2 distinct
+  // available ids.
+  ASSERT_FALSE(fuzzer_pass.EquivalenceClassIsSuitableForBlock({6}, 18, 2));
+
+  // Only id 26 from the set {24, 26, 30} is available to use for the
+  // transformation at block 29, so the set is not suitable if we want at least
+  // 2 available ids.
+  ASSERT_FALSE(
+      fuzzer_pass.EquivalenceClassIsSuitableForBlock({24, 26, 30}, 29, 2));
+
+  ASSERT_TRUE(
+      fuzzer_pass.EquivalenceClassIsSuitableForBlock({24, 26, 30}, 29, 1));
+
+  // %21 is not available at the end of block 20.
+  ASSERT_TRUE(ListsHaveTheSameElements<uint32_t>(
+      fuzzer_pass.GetSuitableIds({9, 15, 21}, 20), {9, 15}));
+
+  // %24 and %30 are not available at the end of block 18.
+  ASSERT_TRUE(ListsHaveTheSameElements<uint32_t>(
+      fuzzer_pass.GetSuitableIds({24, 26, 30}, 18), {26}));
+}
+
+}  // namespace
+}  // namespace fuzz
+}  // namespace spvtools
diff --git a/test/fuzz/transformation_add_opphi_synonym_test.cpp b/test/fuzz/transformation_add_opphi_synonym_test.cpp
new file mode 100644
index 0000000..2dad010
--- /dev/null
+++ b/test/fuzz/transformation_add_opphi_synonym_test.cpp
@@ -0,0 +1,423 @@
+// Copyright (c) 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "source/fuzz/transformation_add_opphi_synonym.h"
+
+#include "test/fuzz/fuzz_test_util.h"
+
+namespace spvtools {
+namespace fuzz {
+namespace {
+
+protobufs::Fact MakeSynonymFact(uint32_t first, uint32_t second) {
+  protobufs::FactDataSynonym data_synonym_fact;
+  *data_synonym_fact.mutable_data1() = MakeDataDescriptor(first, {});
+  *data_synonym_fact.mutable_data2() = MakeDataDescriptor(second, {});
+  protobufs::Fact result;
+  *result.mutable_data_synonym_fact() = data_synonym_fact;
+  return result;
+}
+
+// Adds synonym facts to the fact manager.
+void SetUpIdSynonyms(FactManager* fact_manager, opt::IRContext* context) {
+  fact_manager->AddFact(MakeSynonymFact(11, 9), context);
+  fact_manager->AddFact(MakeSynonymFact(13, 9), context);
+  fact_manager->AddFact(MakeSynonymFact(14, 9), context);
+  fact_manager->AddFact(MakeSynonymFact(19, 9), context);
+  fact_manager->AddFact(MakeSynonymFact(20, 9), context);
+  fact_manager->AddFact(MakeSynonymFact(10, 21), context);
+}
+
+TEST(TransformationAddOpPhiSynonymTest, Inapplicable) {
+  std::string shader = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %2 "main"
+               OpExecutionMode %2 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %2 "main"
+          %3 = OpTypeVoid
+          %4 = OpTypeFunction %3
+          %5 = OpTypeBool
+          %6 = OpConstantTrue %5
+          %7 = OpTypeInt 32 1
+          %8 = OpTypeInt 32 0
+         %22 = OpTypePointer Function %7
+          %9 = OpConstant %7 1
+         %10 = OpConstant %7 2
+         %11 = OpConstant %8 1
+          %2 = OpFunction %3 None %4
+         %12 = OpLabel
+         %23 = OpVariable %22 Function
+         %13 = OpCopyObject %7 %9
+         %14 = OpCopyObject %8 %11
+               OpBranch %15
+         %15 = OpLabel
+               OpSelectionMerge %16 None
+               OpBranchConditional %6 %17 %18
+         %17 = OpLabel
+         %19 = OpCopyObject %7 %13
+         %20 = OpCopyObject %8 %14
+         %21 = OpCopyObject %7 %10
+               OpBranch %16
+         %18 = OpLabel
+         %24 = OpCopyObject %22 %23
+               OpBranch %16
+         %16 = OpLabel
+               OpReturn
+               OpFunctionEnd
+)";
+
+  const auto env = SPV_ENV_UNIVERSAL_1_5;
+  const auto consumer = nullptr;
+  const auto context = BuildModule(env, consumer, shader, kFuzzAssembleOption);
+  ASSERT_TRUE(IsValid(env, context.get()));
+
+  FactManager fact_manager;
+  spvtools::ValidatorOptions validator_options;
+  TransformationContext transformation_context(&fact_manager,
+                                               validator_options);
+
+  SetUpIdSynonyms(&fact_manager, context.get());
+  fact_manager.AddFact(MakeSynonymFact(23, 24), context.get());
+
+  // %13 is not a block label.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(13, {{}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // Block %12 does not have a predecessor.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(12, {{}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // Not all predecessors of %16 (%17 and %18) are considered in the map.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(16, {{{17, 19}}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // %30 does not exist in the module.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(16, {{{30, 19}}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // %20 is not a block label.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(16, {{{20, 19}}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // %15 is not the id of one of the predecessors of the block.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(16, {{{15, 19}}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // %30 does not exist in the module.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(16, {{{17, 30}, {18, 13}}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // %19 and %10 are not synonymous.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(16, {{{17, 19}, {18, 10}}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // %19 and %14 do not have the same type.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(16, {{{17, 19}, {18, 14}}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // %19 is not available at the end of %18.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(16, {{{17, 9}, {18, 19}}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // %21 is not a fresh id.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(16, {{{17, 9}, {18, 9}}}, 21)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // %23 and %24 have pointer id.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(16, {{{17, 23}, {18, 24}}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+}
+
+TEST(TransformationAddOpPhiSynonymTest, Apply) {
+  std::string shader = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %2 "main"
+               OpExecutionMode %2 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %2 "main"
+          %3 = OpTypeVoid
+          %4 = OpTypeFunction %3
+          %5 = OpTypeBool
+          %6 = OpConstantTrue %5
+          %7 = OpTypeInt 32 1
+          %8 = OpTypeInt 32 0
+          %9 = OpConstant %7 1
+         %10 = OpConstant %7 2
+         %11 = OpConstant %8 1
+          %2 = OpFunction %3 None %4
+         %12 = OpLabel
+         %13 = OpCopyObject %7 %9
+         %14 = OpCopyObject %8 %11
+               OpBranch %15
+         %15 = OpLabel
+               OpSelectionMerge %16 None
+               OpBranchConditional %6 %17 %18
+         %17 = OpLabel
+         %19 = OpCopyObject %7 %13
+         %20 = OpCopyObject %8 %14
+         %21 = OpCopyObject %7 %10
+               OpBranch %16
+         %18 = OpLabel
+               OpBranch %16
+         %16 = OpLabel
+               OpBranch %22
+         %22 = OpLabel
+               OpLoopMerge %23 %24 None
+               OpBranchConditional %6 %25 %23
+         %25 = OpLabel
+               OpSelectionMerge %26 None
+               OpBranchConditional %6 %27 %26
+         %27 = OpLabel
+         %28 = OpCopyObject %7 %13
+               OpBranch %23
+         %26 = OpLabel
+               OpSelectionMerge %29 None
+               OpBranchConditional %6 %29 %24
+         %29 = OpLabel
+         %30 = OpCopyObject %7 %13
+               OpBranch %23
+         %24 = OpLabel
+               OpBranch %22
+         %23 = OpLabel
+               OpSelectionMerge %31 None
+               OpBranchConditional %6 %31 %31
+         %31 = OpLabel
+               OpReturn
+               OpFunctionEnd
+)";
+
+  const auto env = SPV_ENV_UNIVERSAL_1_5;
+  const auto consumer = nullptr;
+  const auto context = BuildModule(env, consumer, shader, kFuzzAssembleOption);
+  ASSERT_TRUE(IsValid(env, context.get()));
+
+  FactManager fact_manager;
+  spvtools::ValidatorOptions validator_options;
+  TransformationContext transformation_context(&fact_manager,
+                                               validator_options);
+
+  SetUpIdSynonyms(&fact_manager, context.get());
+
+  // Add some further synonym facts.
+  fact_manager.AddFact(MakeSynonymFact(28, 9), context.get());
+  fact_manager.AddFact(MakeSynonymFact(30, 9), context.get());
+
+  auto transformation1 = TransformationAddOpPhiSynonym(17, {{{15, 13}}}, 100);
+  ASSERT_TRUE(
+      transformation1.IsApplicable(context.get(), transformation_context));
+  transformation1.Apply(context.get(), &transformation_context);
+  ASSERT_TRUE(fact_manager.IsSynonymous(MakeDataDescriptor(100, {}),
+                                        MakeDataDescriptor(9, {})));
+
+  auto transformation2 =
+      TransformationAddOpPhiSynonym(16, {{{17, 19}, {18, 13}}}, 101);
+  ASSERT_TRUE(
+      transformation2.IsApplicable(context.get(), transformation_context));
+  transformation2.Apply(context.get(), &transformation_context);
+  ASSERT_TRUE(fact_manager.IsSynonymous(MakeDataDescriptor(101, {}),
+                                        MakeDataDescriptor(9, {})));
+
+  auto transformation3 =
+      TransformationAddOpPhiSynonym(23, {{{22, 13}, {27, 28}, {29, 30}}}, 102);
+  ASSERT_TRUE(
+      transformation3.IsApplicable(context.get(), transformation_context));
+  transformation3.Apply(context.get(), &transformation_context);
+  ASSERT_TRUE(fact_manager.IsSynonymous(MakeDataDescriptor(102, {}),
+                                        MakeDataDescriptor(9, {})));
+
+  auto transformation4 = TransformationAddOpPhiSynonym(31, {{{23, 13}}}, 103);
+  ASSERT_TRUE(
+      transformation4.IsApplicable(context.get(), transformation_context));
+  transformation4.Apply(context.get(), &transformation_context);
+  ASSERT_TRUE(fact_manager.IsSynonymous(MakeDataDescriptor(103, {}),
+                                        MakeDataDescriptor(9, {})));
+
+  ASSERT_TRUE(IsValid(env, context.get()));
+
+  std::string after_transformations = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %2 "main"
+               OpExecutionMode %2 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %2 "main"
+          %3 = OpTypeVoid
+          %4 = OpTypeFunction %3
+          %5 = OpTypeBool
+          %6 = OpConstantTrue %5
+          %7 = OpTypeInt 32 1
+          %8 = OpTypeInt 32 0
+          %9 = OpConstant %7 1
+         %10 = OpConstant %7 2
+         %11 = OpConstant %8 1
+          %2 = OpFunction %3 None %4
+         %12 = OpLabel
+         %13 = OpCopyObject %7 %9
+         %14 = OpCopyObject %8 %11
+               OpBranch %15
+         %15 = OpLabel
+               OpSelectionMerge %16 None
+               OpBranchConditional %6 %17 %18
+         %17 = OpLabel
+        %100 = OpPhi %7 %13 %15
+         %19 = OpCopyObject %7 %13
+         %20 = OpCopyObject %8 %14
+         %21 = OpCopyObject %7 %10
+               OpBranch %16
+         %18 = OpLabel
+               OpBranch %16
+         %16 = OpLabel
+        %101 = OpPhi %7 %19 %17 %13 %18
+               OpBranch %22
+         %22 = OpLabel
+               OpLoopMerge %23 %24 None
+               OpBranchConditional %6 %25 %23
+         %25 = OpLabel
+               OpSelectionMerge %26 None
+               OpBranchConditional %6 %27 %26
+         %27 = OpLabel
+         %28 = OpCopyObject %7 %13
+               OpBranch %23
+         %26 = OpLabel
+               OpSelectionMerge %29 None
+               OpBranchConditional %6 %29 %24
+         %29 = OpLabel
+         %30 = OpCopyObject %7 %13
+               OpBranch %23
+         %24 = OpLabel
+               OpBranch %22
+         %23 = OpLabel
+        %102 = OpPhi %7 %13 %22 %28 %27 %30 %29
+               OpSelectionMerge %31 None
+               OpBranchConditional %6 %31 %31
+         %31 = OpLabel
+        %103 = OpPhi %7 %13 %23
+               OpReturn
+               OpFunctionEnd
+)";
+
+  ASSERT_TRUE(IsEqual(env, after_transformations, context.get()));
+}
+
+TEST(TransformationAddOpPhiSynonymTest, VariablePointers) {
+  std::string shader = R"(
+               OpCapability Shader
+               OpCapability VariablePointers
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %2 "main" %3
+               OpExecutionMode %2 OriginUpperLeft
+               OpSource ESSL 310
+          %4 = OpTypeVoid
+          %5 = OpTypeFunction %4
+          %6 = OpTypeBool
+          %7 = OpConstantTrue %6
+          %8 = OpTypeInt 32 1
+          %9 = OpTypePointer Function %8
+         %10 = OpTypePointer Workgroup %8
+          %3 = OpVariable %10 Workgroup
+          %2 = OpFunction %4 None %5
+         %11 = OpLabel
+         %12 = OpVariable %9 Function
+               OpSelectionMerge %13 None
+               OpBranchConditional %7 %14 %13
+         %14 = OpLabel
+         %15 = OpCopyObject %10 %3
+         %16 = OpCopyObject %9 %12
+               OpBranch %13
+         %13 = OpLabel
+               OpReturn
+               OpFunctionEnd
+)";
+
+  const auto env = SPV_ENV_UNIVERSAL_1_5;
+  const auto consumer = nullptr;
+  const auto context = BuildModule(env, consumer, shader, kFuzzAssembleOption);
+  ASSERT_TRUE(IsValid(env, context.get()));
+
+  FactManager fact_manager;
+  spvtools::ValidatorOptions validator_options;
+  TransformationContext transformation_context(&fact_manager,
+                                               validator_options);
+
+  // Declare synonyms
+  fact_manager.AddFact(MakeSynonymFact(3, 15), context.get());
+  fact_manager.AddFact(MakeSynonymFact(12, 16), context.get());
+
+  // Remove the VariablePointers capability.
+  context.get()->get_feature_mgr()->RemoveCapability(
+      SpvCapabilityVariablePointers);
+
+  // The VariablePointers capability is required to add an OpPhi instruction of
+  // pointer type.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(13, {{{11, 3}, {14, 15}}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  // Add the VariablePointers capability back.
+  context.get()->get_feature_mgr()->AddCapability(
+      SpvCapabilityVariablePointers);
+
+  // If the ids have pointer type, the storage class must be Workgroup or
+  // StorageBuffer, but it is Function in this case.
+  ASSERT_FALSE(TransformationAddOpPhiSynonym(13, {{{11, 12}, {14, 16}}}, 100)
+                   .IsApplicable(context.get(), transformation_context));
+
+  auto transformation =
+      TransformationAddOpPhiSynonym(13, {{{11, 3}, {14, 15}}}, 100);
+  ASSERT_TRUE(
+      transformation.IsApplicable(context.get(), transformation_context));
+  transformation.Apply(context.get(), &transformation_context);
+
+  std::string after_transformation = R"(
+               OpCapability Shader
+               OpCapability VariablePointers
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %2 "main" %3
+               OpExecutionMode %2 OriginUpperLeft
+               OpSource ESSL 310
+          %4 = OpTypeVoid
+          %5 = OpTypeFunction %4
+          %6 = OpTypeBool
+          %7 = OpConstantTrue %6
+          %8 = OpTypeInt 32 1
+          %9 = OpTypePointer Function %8
+         %10 = OpTypePointer Workgroup %8
+          %3 = OpVariable %10 Workgroup
+          %2 = OpFunction %4 None %5
+         %11 = OpLabel
+         %12 = OpVariable %9 Function
+               OpSelectionMerge %13 None
+               OpBranchConditional %7 %14 %13
+         %14 = OpLabel
+         %15 = OpCopyObject %10 %3
+         %16 = OpCopyObject %9 %12
+               OpBranch %13
+         %13 = OpLabel
+        %100 = OpPhi %10 %3 %11 %15 %14
+               OpReturn
+               OpFunctionEnd
+)";
+
+  ASSERT_TRUE(IsEqual(env, after_transformation, context.get()));
+}
+}  // namespace
+}  // namespace fuzz
+}  // namespace spvtools