Merge blocks in reducer (#2353)

Fixes #2120 

Enhanced the reducer so that it can merge blocks together, leveraging the functionality extracted from the block_merge pass in the optimizer.

source/reduce/CMakeLists.txt

@@ -14,6 +14,8 @@
 set(SPIRV_TOOLS_REDUCE_SOURCES
         change_operand_reduction_opportunity.h
         change_operand_to_undef_reduction_opportunity.h
+        merge_blocks_reduction_opportunity.h
+        merge_blocks_reduction_opportunity_finder.h
         operand_to_const_reduction_opportunity_finder.h
         operand_to_undef_reduction_opportunity_finder.h
         operand_to_dominating_id_reduction_opportunity_finder.h
@@ -30,6 +32,8 @@ set(SPIRV_TOOLS_REDUCE_SOURCES
 
         change_operand_reduction_opportunity.cpp
         change_operand_to_undef_reduction_opportunity.cpp
+        merge_blocks_reduction_opportunity.cpp
+        merge_blocks_reduction_opportunity_finder.cpp
         operand_to_const_reduction_opportunity_finder.cpp
         operand_to_undef_reduction_opportunity_finder.cpp
         operand_to_dominating_id_reduction_opportunity_finder.cpp

source/reduce/merge_blocks_reduction_opportunity.cpp

@@ -0,0 +1,67 @@
+// Copyright (c) 2019 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 "merge_blocks_reduction_opportunity.h"
+#include "source/opt/block_merge_util.h"
+
+#include "source/opt/ir_context.h"
+
+namespace spvtools {
+namespace reduce {
+
+using namespace opt;
+
+MergeBlocksReductionOpportunity::MergeBlocksReductionOpportunity(
+    IRContext* context, Function* function, BasicBlock* block) {
+  // Precondition: the terminator has to be OpBranch.
+  assert(block->terminator()->opcode() == SpvOpBranch);
+  context_ = context;
+  function_ = function;
+  // Get the successor block associated with the OpBranch.
+  successor_block_ =
+      context->cfg()->block(block->terminator()->GetSingleWordInOperand(0));
+}
+
+bool MergeBlocksReductionOpportunity::PreconditionHolds() {
+  // By construction, it is not possible for the merging of A->B to disable the
+  // merging of C->D, even when B and C are the same block.
+  return true;
+}
+
+void MergeBlocksReductionOpportunity::Apply() {
+  // While the original block that targeted the successor may not exist anymore
+  // (it might have been merged with another block), some block must exist that
+  // targets the successor.  Find it.
+
+  const auto predecessors = context_->cfg()->preds(successor_block_->id());
+  assert(1 == predecessors.size() &&
+         "For a successor to be merged into its predecessor, exactly one "
+         "predecessor must be present.");
+  const uint32_t predecessor_id = predecessors[0];
+
+  for (auto bi = function_->begin(); bi != function_->end(); ++bi) {
+    if (bi->id() == predecessor_id) {
+      blockmergeutil::MergeWithSuccessor(context_, function_, bi);
+      // Block merging changes the control flow graph, so invalidate it.
+      context_->InvalidateAnalysesExceptFor(IRContext::Analysis::kAnalysisNone);
+      return;
+    }
+  }
+
+  assert(false &&
+         "Unreachable: we should have found a block with the desired id.");
+}
+
+}  // namespace reduce
+}  // namespace spvtools

source/reduce/merge_blocks_reduction_opportunity.h

@@ -0,0 +1,53 @@
+// Copyright (c) 2019 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_REDUCE_MERGE_BLOCKS_REDUCTION_OPPORTUNITY_H_
+#define SOURCE_REDUCE_MERGE_BLOCKS_REDUCTION_OPPORTUNITY_H_
+
+#include "reduction_opportunity.h"
+#include "source/opt/basic_block.h"
+#include "source/opt/function.h"
+
+namespace spvtools {
+namespace reduce {
+
+// An opportunity to merge two blocks into one.
+class MergeBlocksReductionOpportunity : public ReductionOpportunity {
+ public:
+  // Creates the opportunity to merge |block| with its successor, where |block|
+  // is inside |function|, and |context| is the enclosing IR context.
+  MergeBlocksReductionOpportunity(opt::IRContext* context,
+                                  opt::Function* function,
+                                  opt::BasicBlock* block);
+
+  bool PreconditionHolds() override;
+
+ protected:
+  void Apply() override;
+
+ private:
+  opt::IRContext* context_;
+  opt::Function* function_;
+
+  // Rather than holding on to the block that can be merged with its successor,
+  // we hold on to its successor. This is because the predecessor block might
+  // get merged with *its* predecessor, and so will no longer exist, while the
+  // successor will continue to exist until this opportunity gets applied.
+  opt::BasicBlock* successor_block_;
+};
+
+}  // namespace reduce
+}  // namespace spvtools
+
+#endif  // SOURCE_REDUCE_MERGE_BLOCKS_REDUCTION_OPPORTUNITY_H_

source/reduce/merge_blocks_reduction_opportunity_finder.cpp

@@ -0,0 +1,48 @@
+// Copyright (c) 2019 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/reduce/merge_blocks_reduction_opportunity_finder.h"
+#include "source/opt/block_merge_util.h"
+#include "source/reduce/merge_blocks_reduction_opportunity.h"
+
+namespace spvtools {
+namespace reduce {
+
+using namespace opt;
+
+std::string MergeBlocksReductionOpportunityFinder::GetName() const {
+  return "MergeBlocksReductionOpportunityFinder";
+}
+
+std::vector<std::unique_ptr<ReductionOpportunity>>
+MergeBlocksReductionOpportunityFinder::GetAvailableOpportunities(
+    opt::IRContext* context) const {
+  std::vector<std::unique_ptr<ReductionOpportunity>> result;
+
+  // Consider every block in every function.
+  for (auto& function : *context->module()) {
+    for (auto& block : function) {
+      // See whether it is possible to merge this block with its successor.
+      if (blockmergeutil::CanMergeWithSuccessor(context, &block)) {
+        // It is, so record an opportunity to do this.
+        result.push_back(spvtools::MakeUnique<MergeBlocksReductionOpportunity>(
+            context, &function, &block));
+      }
+    }
+  }
+  return result;
+}
+
+}  // namespace reduce
+}  // namespace spvtools

source/reduce/merge_blocks_reduction_opportunity_finder.h

@@ -0,0 +1,42 @@
+// Copyright (c) 2019 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_REDUCE_MERGE_BLOCKS_REDUCTION_OPPORTUNITY_FINDER_H_
+#define SOURCE_REDUCE_MERGE_BLOCKS_REDUCTION_OPPORTUNITY_FINDER_H_
+
+#include "source/reduce/reduction_opportunity_finder.h"
+
+namespace spvtools {
+namespace reduce {
+
+// A finder of opportunities to merge blocks together.
+class MergeBlocksReductionOpportunityFinder
+    : public ReductionOpportunityFinder {
+ public:
+  MergeBlocksReductionOpportunityFinder() = default;
+
+  ~MergeBlocksReductionOpportunityFinder() override = default;
+
+  std::string GetName() const final;
+
+  std::vector<std::unique_ptr<ReductionOpportunity>> GetAvailableOpportunities(
+      opt::IRContext* context) const final;
+
+ private:
+};
+
+}  // namespace reduce
+}  // namespace spvtools
+
+#endif  // SOURCE_REDUCE_MERGE_BLOCKS_REDUCTION_OPPORTUNITY_FINDER_H_

test/reduce/CMakeLists.txt

@@ -13,7 +13,8 @@
 # limitations under the License.
 
 add_spvtools_unittest(TARGET reduce
-        SRCS operand_to_constant_test.cpp
+        SRCS merge_blocks_test.cpp
+        operand_to_constant_test.cpp
         operand_to_undef_test.cpp
         operand_to_dominating_id_test.cpp
         reduce_test_util.cpp

test/reduce/merge_blocks_test.cpp

@@ -0,0 +1,513 @@
+// Copyright (c) 2019 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 "reduce_test_util.h"
+#include "source/opt/build_module.h"
+#include "source/reduce/merge_blocks_reduction_opportunity_finder.h"
+#include "source/reduce/reduction_opportunity.h"
+
+namespace spvtools {
+namespace reduce {
+namespace {
+
+TEST(MergeBlocksReductionPassTest, BasicCheck) {
+  std::string shader = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %4 "main"
+               OpName %8 "x"
+          %2 = OpTypeVoid
+          %3 = OpTypeFunction %2
+          %6 = OpTypeInt 32 1
+          %7 = OpTypePointer Function %6
+          %9 = OpConstant %6 1
+         %10 = OpConstant %6 2
+         %11 = OpConstant %6 3
+         %12 = OpConstant %6 4
+          %4 = OpFunction %2 None %3
+          %5 = OpLabel
+          %8 = OpVariable %7 Function
+               OpBranch %13
+         %13 = OpLabel
+               OpStore %8 %9
+               OpBranch %14
+         %14 = OpLabel
+               OpStore %8 %10
+               OpBranch %15
+         %15 = OpLabel
+               OpStore %8 %11
+               OpBranch %16
+         %16 = OpLabel
+               OpStore %8 %12
+               OpBranch %17
+         %17 = OpLabel
+               OpReturn
+               OpFunctionEnd
+  )";
+  const auto env = SPV_ENV_UNIVERSAL_1_3;
+  const auto consumer = nullptr;
+  const auto context =
+      BuildModule(env, consumer, shader, kReduceAssembleOption);
+  const auto ops =
+      MergeBlocksReductionOpportunityFinder().GetAvailableOpportunities(
+          context.get());
+  ASSERT_EQ(5, ops.size());
+
+  // Try order 3, 0, 2, 4, 1
+
+  ASSERT_TRUE(ops[3]->PreconditionHolds());
+  ops[3]->TryToApply();
+
+  std::string after_op_3 = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %4 "main"
+               OpName %8 "x"
+          %2 = OpTypeVoid
+          %3 = OpTypeFunction %2
+          %6 = OpTypeInt 32 1
+          %7 = OpTypePointer Function %6
+          %9 = OpConstant %6 1
+         %10 = OpConstant %6 2
+         %11 = OpConstant %6 3
+         %12 = OpConstant %6 4
+          %4 = OpFunction %2 None %3
+          %5 = OpLabel
+          %8 = OpVariable %7 Function
+               OpBranch %13
+         %13 = OpLabel
+               OpStore %8 %9
+               OpBranch %14
+         %14 = OpLabel
+               OpStore %8 %10
+               OpBranch %15
+         %15 = OpLabel
+               OpStore %8 %11
+               OpStore %8 %12
+               OpBranch %17
+         %17 = OpLabel
+               OpReturn
+               OpFunctionEnd
+  )";
+
+  CheckEqual(env, after_op_3, context.get());
+
+  ASSERT_TRUE(ops[0]->PreconditionHolds());
+  ops[0]->TryToApply();
+
+  std::string after_op_0 = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %4 "main"
+               OpName %8 "x"
+          %2 = OpTypeVoid
+          %3 = OpTypeFunction %2
+          %6 = OpTypeInt 32 1
+          %7 = OpTypePointer Function %6
+          %9 = OpConstant %6 1
+         %10 = OpConstant %6 2
+         %11 = OpConstant %6 3
+         %12 = OpConstant %6 4
+          %4 = OpFunction %2 None %3
+          %5 = OpLabel
+          %8 = OpVariable %7 Function
+               OpStore %8 %9
+               OpBranch %14
+         %14 = OpLabel
+               OpStore %8 %10
+               OpBranch %15
+         %15 = OpLabel
+               OpStore %8 %11
+               OpStore %8 %12
+               OpBranch %17
+         %17 = OpLabel
+               OpReturn
+               OpFunctionEnd
+  )";
+
+  CheckEqual(env, after_op_0, context.get());
+
+  ASSERT_TRUE(ops[2]->PreconditionHolds());
+  ops[2]->TryToApply();
+
+  std::string after_op_2 = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %4 "main"
+               OpName %8 "x"
+          %2 = OpTypeVoid
+          %3 = OpTypeFunction %2
+          %6 = OpTypeInt 32 1
+          %7 = OpTypePointer Function %6
+          %9 = OpConstant %6 1
+         %10 = OpConstant %6 2
+         %11 = OpConstant %6 3
+         %12 = OpConstant %6 4
+          %4 = OpFunction %2 None %3
+          %5 = OpLabel
+          %8 = OpVariable %7 Function
+               OpStore %8 %9
+               OpBranch %14
+         %14 = OpLabel
+               OpStore %8 %10
+               OpStore %8 %11
+               OpStore %8 %12
+               OpBranch %17
+         %17 = OpLabel
+               OpReturn
+               OpFunctionEnd
+  )";
+
+  CheckEqual(env, after_op_2, context.get());
+
+  ASSERT_TRUE(ops[4]->PreconditionHolds());
+  ops[4]->TryToApply();
+
+  std::string after_op_4 = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %4 "main"
+               OpName %8 "x"
+          %2 = OpTypeVoid
+          %3 = OpTypeFunction %2
+          %6 = OpTypeInt 32 1
+          %7 = OpTypePointer Function %6
+          %9 = OpConstant %6 1
+         %10 = OpConstant %6 2
+         %11 = OpConstant %6 3
+         %12 = OpConstant %6 4
+          %4 = OpFunction %2 None %3
+          %5 = OpLabel
+          %8 = OpVariable %7 Function
+               OpStore %8 %9
+               OpBranch %14
+         %14 = OpLabel
+               OpStore %8 %10
+               OpStore %8 %11
+               OpStore %8 %12
+               OpReturn
+               OpFunctionEnd
+  )";
+
+  CheckEqual(env, after_op_4, context.get());
+
+  ASSERT_TRUE(ops[1]->PreconditionHolds());
+  ops[1]->TryToApply();
+
+  std::string after_op_1 = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %4 "main"
+               OpName %8 "x"
+          %2 = OpTypeVoid
+          %3 = OpTypeFunction %2
+          %6 = OpTypeInt 32 1
+          %7 = OpTypePointer Function %6
+          %9 = OpConstant %6 1
+         %10 = OpConstant %6 2
+         %11 = OpConstant %6 3
+         %12 = OpConstant %6 4
+          %4 = OpFunction %2 None %3
+          %5 = OpLabel
+          %8 = OpVariable %7 Function
+               OpStore %8 %9
+               OpStore %8 %10
+               OpStore %8 %11
+               OpStore %8 %12
+               OpReturn
+               OpFunctionEnd
+  )";
+
+  CheckEqual(env, after_op_1, context.get());
+}
+
+TEST(MergeBlocksReductionPassTest, Loops) {
+  std::string shader = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %4 "main"
+               OpName %8 "x"
+               OpName %10 "i"
+               OpName %29 "i"
+          %2 = OpTypeVoid
+          %3 = OpTypeFunction %2
+          %6 = OpTypeInt 32 1
+          %7 = OpTypePointer Function %6
+          %9 = OpConstant %6 1
+         %11 = OpConstant %6 0
+         %18 = OpConstant %6 10
+         %19 = OpTypeBool
+          %4 = OpFunction %2 None %3
+          %5 = OpLabel
+          %8 = OpVariable %7 Function
+         %10 = OpVariable %7 Function
+         %29 = OpVariable %7 Function
+               OpStore %8 %9
+               OpBranch %45
+         %45 = OpLabel
+               OpStore %10 %11
+               OpBranch %12
+         %12 = OpLabel
+               OpLoopMerge %14 %15 None
+               OpBranch %16
+         %16 = OpLabel
+         %17 = OpLoad %6 %10
+               OpBranch %46
+         %46 = OpLabel
+         %20 = OpSLessThan %19 %17 %18
+               OpBranchConditional %20 %13 %14
+         %13 = OpLabel
+         %21 = OpLoad %6 %10
+               OpBranch %47
+         %47 = OpLabel
+         %22 = OpLoad %6 %8
+         %23 = OpIAdd %6 %22 %21
+               OpStore %8 %23
+         %24 = OpLoad %6 %10
+         %25 = OpLoad %6 %8
+         %26 = OpIAdd %6 %25 %24
+               OpStore %8 %26
+               OpBranch %48
+         %48 = OpLabel
+               OpBranch %15
+         %15 = OpLabel
+         %27 = OpLoad %6 %10
+         %28 = OpIAdd %6 %27 %9
+               OpStore %10 %28
+               OpBranch %12
+         %14 = OpLabel
+               OpStore %29 %11
+               OpBranch %49
+         %49 = OpLabel
+               OpBranch %30
+         %30 = OpLabel
+               OpLoopMerge %32 %33 None
+               OpBranch %34
+         %34 = OpLabel
+         %35 = OpLoad %6 %29
+         %36 = OpSLessThan %19 %35 %18
+               OpBranch %50
+         %50 = OpLabel
+               OpBranchConditional %36 %31 %32
+         %31 = OpLabel
+         %37 = OpLoad %6 %29
+         %38 = OpLoad %6 %8
+         %39 = OpIAdd %6 %38 %37
+               OpStore %8 %39
+         %40 = OpLoad %6 %29
+         %41 = OpLoad %6 %8
+         %42 = OpIAdd %6 %41 %40
+               OpStore %8 %42
+               OpBranch %33
+         %33 = OpLabel
+         %43 = OpLoad %6 %29
+         %44 = OpIAdd %6 %43 %9
+               OpBranch %51
+         %51 = OpLabel
+               OpStore %29 %44
+               OpBranch %30
+         %32 = OpLabel
+               OpReturn
+               OpFunctionEnd
+  )";
+  const auto env = SPV_ENV_UNIVERSAL_1_3;
+  const auto consumer = nullptr;
+  const auto context =
+      BuildModule(env, consumer, shader, kReduceAssembleOption);
+  const auto ops =
+      MergeBlocksReductionOpportunityFinder().GetAvailableOpportunities(
+          context.get());
+  ASSERT_EQ(11, ops.size());
+
+  for (auto& ri : ops) {
+    ASSERT_TRUE(ri->PreconditionHolds());
+    ri->TryToApply();
+  }
+
+  std::string after = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %4 "main"
+               OpName %8 "x"
+               OpName %10 "i"
+               OpName %29 "i"
+          %2 = OpTypeVoid
+          %3 = OpTypeFunction %2
+          %6 = OpTypeInt 32 1
+          %7 = OpTypePointer Function %6
+          %9 = OpConstant %6 1
+         %11 = OpConstant %6 0
+         %18 = OpConstant %6 10
+         %19 = OpTypeBool
+          %4 = OpFunction %2 None %3
+          %5 = OpLabel
+          %8 = OpVariable %7 Function
+         %10 = OpVariable %7 Function
+         %29 = OpVariable %7 Function
+               OpStore %8 %9
+               OpStore %10 %11
+               OpBranch %12
+         %12 = OpLabel
+         %17 = OpLoad %6 %10
+         %20 = OpSLessThan %19 %17 %18
+               OpLoopMerge %14 %13 None
+               OpBranchConditional %20 %13 %14
+         %13 = OpLabel
+         %21 = OpLoad %6 %10
+         %22 = OpLoad %6 %8
+         %23 = OpIAdd %6 %22 %21
+               OpStore %8 %23
+         %24 = OpLoad %6 %10
+         %25 = OpLoad %6 %8
+         %26 = OpIAdd %6 %25 %24
+               OpStore %8 %26
+         %27 = OpLoad %6 %10
+         %28 = OpIAdd %6 %27 %9
+               OpStore %10 %28
+               OpBranch %12
+         %14 = OpLabel
+               OpStore %29 %11
+               OpBranch %30
+         %30 = OpLabel
+         %35 = OpLoad %6 %29
+         %36 = OpSLessThan %19 %35 %18
+               OpLoopMerge %32 %31 None
+               OpBranchConditional %36 %31 %32
+         %31 = OpLabel
+         %37 = OpLoad %6 %29
+         %38 = OpLoad %6 %8
+         %39 = OpIAdd %6 %38 %37
+               OpStore %8 %39
+         %40 = OpLoad %6 %29
+         %41 = OpLoad %6 %8
+         %42 = OpIAdd %6 %41 %40
+               OpStore %8 %42
+         %43 = OpLoad %6 %29
+         %44 = OpIAdd %6 %43 %9
+               OpStore %29 %44
+               OpBranch %30
+         %32 = OpLabel
+               OpReturn
+               OpFunctionEnd
+  )";
+
+  CheckEqual(env, after, context.get());
+}
+
+TEST(MergeBlocksReductionPassTest, MergeWithOpPhi) {
+  std::string shader = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %4 "main"
+               OpName %8 "x"
+               OpName %10 "y"
+          %2 = OpTypeVoid
+          %3 = OpTypeFunction %2
+          %6 = OpTypeInt 32 1
+          %7 = OpTypePointer Function %6
+          %9 = OpConstant %6 1
+          %4 = OpFunction %2 None %3
+          %5 = OpLabel
+          %8 = OpVariable %7 Function
+         %10 = OpVariable %7 Function
+               OpStore %8 %9
+         %11 = OpLoad %6 %8
+               OpBranch %12
+         %12 = OpLabel
+         %13 = OpPhi %6 %11 %5
+               OpStore %10 %13
+               OpReturn
+               OpFunctionEnd
+  )";
+
+  const auto env = SPV_ENV_UNIVERSAL_1_3;
+  const auto consumer = nullptr;
+  const auto context =
+      BuildModule(env, consumer, shader, kReduceAssembleOption);
+  const auto ops =
+      MergeBlocksReductionOpportunityFinder().GetAvailableOpportunities(
+          context.get());
+  ASSERT_EQ(1, ops.size());
+
+  ASSERT_TRUE(ops[0]->PreconditionHolds());
+  ops[0]->TryToApply();
+
+  std::string after = R"(
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %4 "main"
+               OpExecutionMode %4 OriginUpperLeft
+               OpSource ESSL 310
+               OpName %4 "main"
+               OpName %8 "x"
+               OpName %10 "y"
+          %2 = OpTypeVoid
+          %3 = OpTypeFunction %2
+          %6 = OpTypeInt 32 1
+          %7 = OpTypePointer Function %6
+          %9 = OpConstant %6 1
+          %4 = OpFunction %2 None %3
+          %5 = OpLabel
+          %8 = OpVariable %7 Function
+         %10 = OpVariable %7 Function
+               OpStore %8 %9
+         %11 = OpLoad %6 %8
+               OpStore %10 %11
+               OpReturn
+               OpFunctionEnd
+  )";
+
+  CheckEqual(env, after, context.get());
+}
+
+}  // namespace
+}  // namespace reduce
+}  // namespace spvtools

tools/reduce/reduce.cpp

@@ -20,6 +20,7 @@
 #include "source/opt/build_module.h"
 #include "source/opt/ir_context.h"
 #include "source/opt/log.h"
+#include "source/reduce/merge_blocks_reduction_opportunity_finder.h"
 #include "source/reduce/operand_to_const_reduction_opportunity_finder.h"
 #include "source/reduce/operand_to_dominating_id_reduction_opportunity_finder.h"
 #include "source/reduce/operand_to_undef_reduction_opportunity_finder.h"
@@ -240,6 +241,8 @@ int main(int argc, const char** argv) {
   reducer.AddReductionPass(
       spvtools::MakeUnique<
           StructuredLoopToSelectionReductionOpportunityFinder>());
+  reducer.AddReductionPass(
+      spvtools::MakeUnique<MergeBlocksReductionOpportunityFinder>());
 
   reducer.SetMessageConsumer(spvtools::utils::CLIMessageConsumer);