Dead constant elimination

A pass to remove dead constants, including both front-end constants and spec
constants.

This pass does not handle dead variables and types.

source/opt/module.cpp

@@ -39,6 +39,15 @@ std::vector<Instruction*> Module::types() {
   return insts;
 };
 
+std::vector<Instruction*> Module::GetConstants() {
+  std::vector<Instruction*> insts;
+  for (uint32_t i = 0; i < types_values_.size(); ++i) {
+    if (IsConstantInst(types_values_[i].opcode()))
+      insts.push_back(&types_values_[i]);
+  }
+  return insts;
+};
+
 void Module::ForEachInst(const std::function<void(Instruction*)>& f) {
   for (auto& i : capabilities_) f(&i);
   for (auto& i : extensions_) f(&i);

source/opt/module.h

@@ -94,6 +94,8 @@ class Module {
   // Returns a vector of pointers to type-declaration instructions in this
   // module.
   std::vector<Instruction*> types();
+  // Returns the constant-defining instructions.
+  std::vector<Instruction*> GetConstants();
   const std::vector<Instruction>& debugs() const { return debugs_; }
   std::vector<Instruction>& debugs() { return debugs_; }
   const std::vector<Instruction>& annotations() const { return annotations_; }

source/opt/passes.cpp

@@ -26,6 +26,14 @@
 
 #include "passes.h"
 
+#include <algorithm>
+#include <iostream>
+#include <unordered_map>
+#include <unordered_set>
+
+#include "def_use_manager.h"
+#include "reflect.h"
+
 namespace spvtools {
 namespace opt {
 
@@ -71,5 +79,93 @@ bool FreezeSpecConstantValuePass::Process(ir::Module* module) {
   return modified;
 }
 
+bool EliminateDeadConstantPass::Process(ir::Module* module) {
+  analysis::DefUseManager def_use;
+  def_use.AnalyzeDefUse(module);
+  std::unordered_set<ir::Instruction*> working_list;
+  // Traverse all the instructions to get the initial set of dead constants as
+  // working list and count number of real uses for constants. Uses in
+  // annotation instructions do not count.
+  std::unordered_map<ir::Instruction*, size_t> use_counts;
+  std::vector<ir::Instruction*> constants = module->GetConstants();
+  for (auto* c : constants) {
+    uint32_t const_id = c->result_id();
+    size_t count = 0;
+    if (analysis::UseList* uses = def_use.GetUses(const_id)) {
+      count =
+          std::count_if(uses->begin(), uses->end(), [](const analysis::Use& u) {
+            return !(ir::IsAnnotationInst(u.inst->opcode()) ||
+                     ir::IsDebugInst(u.inst->opcode()));
+          });
+    }
+    use_counts[c] = count;
+    if (!count) {
+      working_list.insert(c);
+    }
+  }
+
+  // Start from the constants with 0 uses, back trace through the def-use chain
+  // to find all dead constants.
+  std::unordered_set<ir::Instruction*> dead_consts;
+  while (!working_list.empty()) {
+    ir::Instruction* inst = *working_list.begin();
+    // Back propagate if the instruction contains IDs in its operands.
+    switch (inst->opcode()) {
+      case SpvOp::SpvOpConstantComposite:
+      case SpvOp::SpvOpSpecConstantComposite:
+      case SpvOp::SpvOpSpecConstantOp:
+        for (uint32_t i = 0; i < inst->NumInOperands(); i++) {
+          // SpecConstantOp instruction contains 'opcode' as its operand. Need
+          // to exclude such operands when decreasing uses.
+          if (inst->GetInOperand(i).type != SPV_OPERAND_TYPE_ID) {
+            continue;
+          }
+          uint32_t operand_id = inst->GetSingleWordInOperand(i);
+          ir::Instruction* def_inst = def_use.GetDef(operand_id);
+          // If the use_count does not have any count for the def_inst,
+          // def_inst must not be a constant, and should be ignored here.
+          if (!use_counts.count(def_inst)) {
+            continue;
+          }
+          // The number of uses should never be less then 0, so it can not be
+          // less than 1 before it decreases.
+          assert(use_counts[def_inst] > 0);
+          --use_counts[def_inst];
+          if (!use_counts[def_inst]) {
+            working_list.insert(def_inst);
+          }
+        }
+        break;
+      default:
+        break;
+    }
+    dead_consts.insert(inst);
+    working_list.erase(inst);
+  }
+
+  // Find all annotation and debug instructions that are referencing dead
+  // constants.
+  std::unordered_set<ir::Instruction*> dead_others;
+  for (auto* dc : dead_consts) {
+    if (analysis::UseList* uses = def_use.GetUses(dc->result_id())) {
+      for (const auto& u : *uses) {
+        if (ir::IsAnnotationInst(u.inst->opcode()) ||
+            ir::IsDebugInst(u.inst->opcode())) {
+          dead_others.insert(u.inst);
+        }
+      }
+    }
+  }
+
+  // Turn all dead instructions and uses of them to nop
+  for (auto* dc : dead_consts) {
+    def_use.KillDef(dc->result_id());
+  }
+  for (auto* da : dead_others) {
+    da->ToNop();
+  }
+  return !dead_consts.empty();
+}
+
 }  // namespace opt
 }  // namespace spvtools

source/opt/passes.h

@@ -72,6 +72,17 @@ class FreezeSpecConstantValuePass : public Pass {
   bool Process(ir::Module*) override;
 };
 
+// The optimization pass to remove dead constants, including front-end
+// contants: defined by OpConstant, OpConstantComposite, OpConstantTrue and
+// OpConstantFalse; and spec constants: defined by OpSpecConstant,
+// OpSpecConstantComposite, OpSpecConstantTrue, OpSpecConstantFalse and
+// OpSpecConstantOp.
+class EliminateDeadConstantPass : public Pass {
+  public:
+    const char* name() const override { return "eliminate-dead-const"; }
+    bool Process(ir::Module*) override;
+};
+
 }  // namespace opt
 }  // namespace spvtools
 

test/opt/CMakeLists.txt

@@ -44,6 +44,11 @@ add_spvtools_unittest(TARGET pass_freeze_spec_const
   LIBS SPIRV-Tools-opt ${SPIRV_TOOLS}
 )
 
+add_spvtools_unittest(TARGET pass_eliminate_dead_const
+  SRCS test_eliminate_dead_const.cpp pass_utils.cpp
+  LIBS SPIRV-Tools-opt ${SPIRV_TOOLS}
+)
+
 add_spvtools_unittest(TARGET pass_utils
   SRCS test_utils.cpp pass_utils.cpp
   LIBS SPIRV-Tools-opt ${SPIRV_TOOLS}

test/opt/test_eliminate_dead_const.cpp

@@ -0,0 +1,852 @@
+// Copyright (c) 2016 Google Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and/or associated documentation files (the
+// "Materials"), to deal in the Materials without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Materials, and to
+// permit persons to whom the Materials are furnished to do so, subject to
+// the following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Materials.
+//
+// MODIFICATIONS TO THIS FILE MAY MEAN IT NO LONGER ACCURATELY REFLECTS
+// KHRONOS STANDARDS. THE UNMODIFIED, NORMATIVE VERSIONS OF KHRONOS
+// SPECIFICATIONS AND HEADER INFORMATION ARE LOCATED AT
+//    https://www.khronos.org/registry/
+//
+// THE MATERIALS ARE PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+// IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+// CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+// TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+// MATERIALS OR THE USE OR OTHER DEALINGS IN THE MATERIALS.
+
+#include "assembly_builder.h"
+#include "pass_fixture.h"
+#include "pass_utils.h"
+
+#include <algorithm>
+#include <cstdarg>
+#include <iostream>
+#include <sstream>
+#include <unordered_set>
+
+namespace {
+
+using namespace spvtools;
+
+using EliminateDeadConstantBasicTest = PassTest<::testing::Test>;
+
+TEST_F(EliminateDeadConstantBasicTest, BasicAllDeadConstants) {
+  const std::vector<const char*> text = {
+      // clang-format off
+               "OpCapability Shader",
+               "OpCapability Float64",
+          "%1 = OpExtInstImport \"GLSL.std.450\"",
+               "OpMemoryModel Logical GLSL450",
+               "OpEntryPoint Vertex %main \"main\"",
+               "OpName %main \"main\"",
+       "%void = OpTypeVoid",
+          "%4 = OpTypeFunction %void",
+       "%bool = OpTypeBool",
+          "%6 = OpConstantTrue %bool",
+          "%7 = OpConstantFalse %bool",
+        "%int = OpTypeInt 32 1",
+          "%9 = OpConstant %int 1",
+       "%uint = OpTypeInt 32 0",
+         "%11 = OpConstant %uint 2",
+      "%float = OpTypeFloat 32",
+         "%13 = OpConstant %float 3.14",
+     "%double = OpTypeFloat 64",
+         "%15 = OpConstant %double 3.14159265358979",
+       "%main = OpFunction %void None %4",
+         "%16 = OpLabel",
+               "OpReturn",
+               "OpFunctionEnd",
+      // clang-format on
+  };
+  // None of the above constants is ever used, so all of them should be
+  // eliminated.
+  const char* const_decl_opcodes[] = {
+      " OpConstantTrue ", " OpConstantFalse ", " OpConstant ",
+  };
+  // Skip lines that have any one of const_decl_opcodes.
+  const std::string expected_disassembly =
+      SelectiveJoin(text, [&const_decl_opcodes](const char* line) {
+        return std::any_of(
+            std::begin(const_decl_opcodes), std::end(const_decl_opcodes),
+            [&line](const char* const_decl_op) {
+              return std::string(line).find(const_decl_op) != std::string::npos;
+            });
+      });
+
+  SinglePassRunAndCheck<opt::EliminateDeadConstantPass>(
+      JoinAllInsts(text), expected_disassembly, /* skip_nop = */ true);
+}
+
+TEST_F(EliminateDeadConstantBasicTest, BasicNoneDeadConstants) {
+  const std::vector<const char*> text = {
+      // clang-format off
+                "OpCapability Shader",
+                "OpCapability Float64",
+           "%1 = OpExtInstImport \"GLSL.std.450\"",
+                "OpMemoryModel Logical GLSL450",
+                "OpEntryPoint Vertex %main \"main\"",
+                "OpName %main \"main\"",
+                "OpName %btv \"btv\"",
+                "OpName %bfv \"bfv\"",
+                "OpName %iv \"iv\"",
+                "OpName %uv \"uv\"",
+                "OpName %fv \"fv\"",
+                "OpName %dv \"dv\"",
+        "%void = OpTypeVoid",
+          "%10 = OpTypeFunction %void",
+        "%bool = OpTypeBool",
+ "%_ptr_Function_bool = OpTypePointer Function %bool",
+          "%13 = OpConstantTrue %bool",
+          "%14 = OpConstantFalse %bool",
+         "%int = OpTypeInt 32 1",
+ "%_ptr_Function_int = OpTypePointer Function %int",
+          "%17 = OpConstant %int 1",
+        "%uint = OpTypeInt 32 0",
+ "%_ptr_Function_uint = OpTypePointer Function %uint",
+          "%20 = OpConstant %uint 2",
+       "%float = OpTypeFloat 32",
+ "%_ptr_Function_float = OpTypePointer Function %float",
+          "%23 = OpConstant %float 3.14",
+      "%double = OpTypeFloat 64",
+ "%_ptr_Function_double = OpTypePointer Function %double",
+          "%26 = OpConstant %double 3.14159265358979",
+        "%main = OpFunction %void None %10",
+          "%27 = OpLabel",
+         "%btv = OpVariable %_ptr_Function_bool Function",
+         "%bfv = OpVariable %_ptr_Function_bool Function",
+          "%iv = OpVariable %_ptr_Function_int Function",
+          "%uv = OpVariable %_ptr_Function_uint Function",
+          "%fv = OpVariable %_ptr_Function_float Function",
+          "%dv = OpVariable %_ptr_Function_double Function",
+                "OpStore %btv %13",
+                "OpStore %bfv %14",
+                "OpStore %iv %17",
+                "OpStore %uv %20",
+                "OpStore %fv %23",
+                "OpStore %dv %26",
+                "OpReturn",
+                "OpFunctionEnd",
+      // clang-format on
+  };
+  // All constants are used, so none of them should be eliminated.
+  SinglePassRunAndCheck<opt::EliminateDeadConstantPass>(
+      JoinAllInsts(text), JoinAllInsts(text), /* skip_nop = */ true);
+}
+
+struct EliminateDeadConstantTestCase {
+  // Type declarations and constants that should be kept.
+  std::vector<std::string> used_consts;
+  // Instructions that refer to constants, this is added to create uses for
+  // some constants so they won't be treated as dead constants.
+  std::vector<std::string> main_insts;
+  // Dead constants that should be removed.
+  std::vector<std::string> dead_consts;
+};
+
+// All types that are potentially required in EliminateDeadConstantTest.
+const std::vector<std::string> CommonTypes = {
+    // clang-format off
+    // scalar types
+    "%bool = OpTypeBool",
+    "%uint = OpTypeInt 32 0",
+    "%int = OpTypeInt 32 1",
+    "%float = OpTypeFloat 32",
+    "%double = OpTypeFloat 64",
+    // vector types
+    "%v2bool = OpTypeVector %bool 2",
+    "%v2uint = OpTypeVector %uint 2",
+    "%v2int = OpTypeVector %int 2",
+    "%v3int = OpTypeVector %int 3",
+    "%v4int = OpTypeVector %int 4",
+    "%v2float = OpTypeVector %float 2",
+    "%v3float = OpTypeVector %float 3",
+    "%v2double = OpTypeVector %double 2",
+    // variable pointer types
+    "%_pf_bool = OpTypePointer Function %bool",
+    "%_pf_uint = OpTypePointer Function %uint",
+    "%_pf_int = OpTypePointer Function %int",
+    "%_pf_float = OpTypePointer Function %float",
+    "%_pf_double = OpTypePointer Function %double",
+    "%_pf_v2int = OpTypePointer Function %v2int",
+    "%_pf_v3int = OpTypePointer Function %v3int",
+    "%_pf_v2float = OpTypePointer Function %v2float",
+    "%_pf_v3float = OpTypePointer Function %v3float",
+    "%_pf_v2double = OpTypePointer Function %v2double",
+    // struct types
+    "%inner_struct = OpTypeStruct %bool %int %float %double",
+    "%outer_struct = OpTypeStruct %inner_struct %int %double",
+    "%flat_struct = OpTypeStruct %bool %int %float %double",
+    // clang-format on
+};
+
+using EliminateDeadConstantTest =
+    PassTest<::testing::TestWithParam<EliminateDeadConstantTestCase>>;
+
+TEST_P(EliminateDeadConstantTest, Custom) {
+  auto& tc = GetParam();
+  AssemblyBuilder builder;
+  builder.AppendTypesConstantsGlobals(CommonTypes)
+      .AppendTypesConstantsGlobals(tc.used_consts)
+      .AppendInMain(tc.main_insts);
+  const std::string expected = builder.GetCode();
+  builder.AppendTypesConstantsGlobals(tc.dead_consts);
+  const std::string assembly_with_dead_const = builder.GetCode();
+  SinglePassRunAndCheck<opt::EliminateDeadConstantPass>(
+      assembly_with_dead_const, expected, /*  skip_nop = */ true);
+}
+
+INSTANTIATE_TEST_CASE_P(
+    ScalarTypeConstants, EliminateDeadConstantTest,
+    ::testing::ValuesIn(std::vector<EliminateDeadConstantTestCase>({
+        // clang-format off
+        // Scalar type constants, one dead constant and one used constant.
+        {
+            /* .used_consts = */
+            {
+                "%used_const_int = OpConstant %int 1",
+            },
+            /* .main_insts = */
+            {
+                "%int_var = OpVariable %_pf_int Function",
+                "OpStore %int_var %used_const_int",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_const_int = OpConstant %int 1",
+            },
+        },
+        {
+            /* .used_consts = */
+            {
+                "%used_const_uint = OpConstant %uint 1",
+            },
+            /* .main_insts = */
+            {
+                "%uint_var = OpVariable %_pf_uint Function",
+                "OpStore %uint_var %used_const_uint",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_const_uint = OpConstant %uint 1",
+            },
+        },
+        {
+            /* .used_consts = */
+            {
+                "%used_const_float = OpConstant %float 3.14",
+            },
+            /* .main_insts = */
+            {
+                "%float_var = OpVariable %_pf_float Function",
+                "OpStore %float_var %used_const_float",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_const_float = OpConstant %float 3.14",
+            },
+        },
+        {
+            /* .used_consts = */
+            {
+                "%used_const_double = OpConstant %double 3.14",
+            },
+            /* .main_insts = */
+            {
+                "%double_var = OpVariable %_pf_double Function",
+                "OpStore %double_var %used_const_double",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_const_double = OpConstant %double 3.14",
+            },
+        },
+        // clang-format on
+    })));
+
+INSTANTIATE_TEST_CASE_P(
+    VectorTypeConstants, EliminateDeadConstantTest,
+    ::testing::ValuesIn(std::vector<EliminateDeadConstantTestCase>({
+        // clang-format off
+        // Tests eliminating dead constant type ivec2. One dead constant vector
+        // and one used constant vector, each built from its own group of
+        // scalar constants.
+        {
+            /* .used_consts = */
+            {
+                "%used_int_x = OpConstant %int 1",
+                "%used_int_y = OpConstant %int 2",
+                "%used_v2int = OpConstantComposite %v2int %used_int_x %used_int_y",
+            },
+            /* .main_insts = */
+            {
+                "%v2int_var = OpVariable %_pf_v2int Function",
+                "OpStore %v2int_var %used_v2int",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_int_x = OpConstant %int 1",
+                "%dead_int_y = OpConstant %int 2",
+                "%dead_v2int = OpConstantComposite %v2int %dead_int_x %dead_int_y",
+            },
+        },
+        // Tests eliminating dead constant ivec2. One dead constant vector and
+        // one used constant vector. But both built from a same group of
+        // scalar constants.
+        {
+            /* .used_consts = */
+            {
+                "%used_int_x = OpConstant %int 1",
+                "%used_int_y = OpConstant %int 2",
+                "%used_int_z = OpConstant %int 3",
+                "%used_v3int = OpConstantComposite %v3int %used_int_x %used_int_y %used_int_z",
+            },
+            /* .main_insts = */
+            {
+                "%v3int_var = OpVariable %_pf_v3int Function",
+                "OpStore %v3int_var %used_v3int",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_v3int = OpConstantComposite %v3int %used_int_x %used_int_y %used_int_z",
+            },
+        },
+        // Tests eliminating dead cosntant vec2. One dead constant vector and
+        // one used constant vector. Each built from its own group of scalar
+        // constants.
+        {
+            /* .used_consts = */
+            {
+                "%used_float_x = OpConstant %float 3.14",
+                "%used_float_y = OpConstant %float 4.13",
+                "%used_v2float = OpConstantComposite %v2float %used_float_x %used_float_y",
+            },
+            /* .main_insts = */
+            {
+                "%v2float_var = OpVariable %_pf_v2float Function",
+                "OpStore %v2float_var %used_v2float",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_float_x = OpConstant %float 3.14",
+                "%dead_float_y = OpConstant %float 4.13",
+                "%dead_v2float = OpConstantComposite %v2float %dead_float_x %dead_float_y",
+            },
+        },
+        // Tests eliminating dead cosntant vec2. One dead constant vector and
+        // one used constant vector. Both built from a same group of scalar
+        // constants.
+        {
+            /* .used_consts = */
+            {
+                "%used_float_x = OpConstant %float 3.14",
+                "%used_float_y = OpConstant %float 4.13",
+                "%used_float_z = OpConstant %float 4.31",
+                "%used_v3float = OpConstantComposite %v3float %used_float_x %used_float_y %used_float_z",
+            },
+            /* .main_insts = */
+            {
+                "%v3float_var = OpVariable %_pf_v3float Function",
+                "OpStore %v3float_var %used_v3float",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_v3float = OpConstantComposite %v3float %used_float_x %used_float_y %used_float_z",
+            },
+        },
+        // clang-format on
+    })));
+
+INSTANTIATE_TEST_CASE_P(
+    StructTypeConstants, EliminateDeadConstantTest,
+    ::testing::ValuesIn(std::vector<EliminateDeadConstantTestCase>({
+        // clang-format off
+        // A plain struct type dead constants. All of its components are dead
+        // constants too.
+        {
+            /* .used_consts = */ {},
+            /* .main_insts = */ {},
+            /* .dead_consts = */
+            {
+                "%dead_bool = OpConstantTrue %bool",
+                "%dead_int = OpConstant %int 1",
+                "%dead_float = OpConstant %float 2.5",
+                "%dead_double = OpConstant %double 3.14159265358979",
+                "%dead_struct = OpConstantComposite %flat_struct %dead_bool %dead_int %dead_float %dead_double",
+            },
+        },
+        // A plain struct type dead constants. Some of its components are dead
+        // constants while others are not.
+        {
+            /* .used_consts = */
+            {
+                "%used_int = OpConstant %int 1",
+                "%used_double = OpConstant %double 3.14159265358979",
+            },
+            /* .main_insts = */
+            {
+                "%int_var = OpVariable %_pf_int Function",
+                "OpStore %int_var %used_int",
+                "%double_var = OpVariable %_pf_double Function",
+                "OpStore %double_var %used_double",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_bool = OpConstantTrue %bool",
+                "%dead_float = OpConstant %float 2.5",
+                "%dead_struct = OpConstantComposite %flat_struct %dead_bool %used_int %dead_float %used_double",
+            },
+        },
+        // A nesting struct type dead constants. All components of both outer
+        // and inner structs are dead and should be removed after dead constant
+        // elimination.
+        {
+            /* .used_consts = */ {},
+            /* .main_insts = */ {},
+            /* .dead_consts = */
+            {
+                "%dead_bool = OpConstantTrue %bool",
+                "%dead_int = OpConstant %int 1",
+                "%dead_float = OpConstant %float 2.5",
+                "%dead_double = OpConstant %double 3.1415926535",
+                "%dead_inner_struct = OpConstantComposite %inner_struct %dead_bool %dead_int %dead_float %dead_double",
+                "%dead_int2 = OpConstant %int 2",
+                "%dead_double2 = OpConstant %double 1.428571428514",
+                "%dead_outer_struct = OpConstantComposite %outer_struct %dead_inner_struct %dead_int2 %dead_double2",
+            },
+        },
+        // A nesting struct type dead constants. Some of its components are
+        // dead constants while others are not.
+        {
+            /* .used_consts = */
+            {
+                "%used_int = OpConstant %int 1",
+                "%used_double = OpConstant %double 3.14159265358979",
+            },
+            /* .main_insts = */
+            {
+                "%int_var = OpVariable %_pf_int Function",
+                "OpStore %int_var %used_int",
+                "%double_var = OpVariable %_pf_double Function",
+                "OpStore %double_var %used_double",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_bool = OpConstantTrue %bool",
+                "%dead_float = OpConstant %float 2.5",
+                "%dead_inner_struct = OpConstantComposite %inner_struct %dead_bool %used_int %dead_float %used_double",
+                "%dead_int = OpConstant %int 2",
+                "%dead_outer_struct = OpConstantComposite %outer_struct %dead_inner_struct %dead_int %used_double",
+            },
+        },
+        // A nesting struct case. The inner struct is used while the outer struct is not
+        {
+          /* .used_const = */
+          {
+            "%used_bool = OpConstantTrue %bool",
+            "%used_int = OpConstant %int 1",
+            "%used_float = OpConstant %float 1.23",
+            "%used_double = OpConstant %double 1.2345678901234",
+            "%used_inner_struct = OpConstantComposite %inner_struct %used_bool %used_int %used_float %used_double",
+          },
+          /* .main_insts = */
+          {
+            "%bool_var = OpVariable %_pf_bool Function",
+            "%bool_from_inner_struct = OpCompositeExtract %bool %used_inner_struct 0",
+            "OpStore %bool_var %bool_from_inner_struct",
+          },
+          /* .dead_consts = */
+          {
+            "%dead_int = OpConstant %int 2",
+            "%dead_outer_struct = OpConstantComposite %outer_struct %used_inner_struct %dead_int %used_double"
+          },
+        },
+        // A nesting struct case. The outer struct is used, so the inner struct should not
+        // be removed even though it is not used anywhere.
+        {
+          /* .used_const = */
+          {
+            "%used_bool = OpConstantTrue %bool",
+            "%used_int = OpConstant %int 1",
+            "%used_float = OpConstant %float 1.23",
+            "%used_double = OpConstant %double 1.2345678901234",
+            "%used_inner_struct = OpConstantComposite %inner_struct %used_bool %used_int %used_float %used_double",
+            "%used_outer_struct = OpConstantComposite %outer_struct %used_inner_struct %used_int %used_double"
+          },
+          /* .main_insts = */
+          {
+            "%int_var = OpVariable %_pf_int Function",
+            "%int_from_outer_struct = OpCompositeExtract %int %used_outer_struct 1",
+            "OpStore %int_var %int_from_outer_struct",
+          },
+          /* .dead_consts = */ {},
+        },
+        // clang-format on
+    })));
+
+INSTANTIATE_TEST_CASE_P(
+    ScalarTypeSpecConstants, EliminateDeadConstantTest,
+    ::testing::ValuesIn(std::vector<EliminateDeadConstantTestCase>({
+        // clang-format off
+        // All scalar type spec constants.
+        {
+            /* .used_consts = */
+            {
+                "%used_bool = OpSpecConstantTrue %bool",
+                "%used_uint = OpSpecConstant %uint 2",
+                "%used_int = OpSpecConstant %int 2",
+                "%used_float = OpSpecConstant %float 2.5",
+                "%used_double = OpSpecConstant %double 1.428571428514",
+            },
+            /* .main_insts = */
+            {
+                "%bool_var = OpVariable %_pf_bool Function",
+                "%uint_var = OpVariable %_pf_uint Function",
+                "%int_var = OpVariable %_pf_int Function",
+                "%float_var = OpVariable %_pf_float Function",
+                "%double_var = OpVariable %_pf_double Function",
+                "OpStore %bool_var %used_bool", "OpStore %uint_var %used_uint",
+                "OpStore %int_var %used_int", "OpStore %float_var %used_float",
+                "OpStore %double_var %used_double",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_bool = OpSpecConstantTrue %bool",
+                "%dead_uint = OpSpecConstant %uint 2",
+                "%dead_int = OpSpecConstant %int 2",
+                "%dead_float = OpSpecConstant %float 2.5",
+                "%dead_double = OpSpecConstant %double 1.428571428514",
+            },
+        },
+        // clang-format on
+    })));
+
+INSTANTIATE_TEST_CASE_P(
+    VectorTypeSpecConstants, EliminateDeadConstantTest,
+    ::testing::ValuesIn(std::vector<EliminateDeadConstantTestCase>({
+        // clang-format off
+        // Bool vector type spec constants. One vector has all component dead,
+        // another vector has one dead boolean and one used boolean.
+        {
+            /* .used_consts = */
+            {
+                "%used_bool = OpSpecConstantTrue %bool",
+            },
+            /* .main_insts = */
+            {
+                "%bool_var = OpVariable %_pf_bool Function",
+                "OpStore %bool_var %used_bool",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_bool = OpSpecConstantFalse %bool",
+                "%dead_bool_vec1 = OpSpecConstantComposite %v2bool %dead_bool %dead_bool",
+                "%dead_bool_vec2 = OpSpecConstantComposite %v2bool %dead_bool %used_bool",
+            },
+        },
+
+        // Uint vector type spec constants. One vector has all component dead,
+        // another vector has one dead unsigend integer and one used unsigned
+        // integer.
+        {
+            /* .used_consts = */
+            {
+                "%used_uint = OpSpecConstant %uint 3",
+            },
+            /* .main_insts = */
+            {
+                "%uint_var = OpVariable %_pf_uint Function",
+                "OpStore %uint_var %used_uint",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_uint = OpSpecConstant %uint 1",
+                "%dead_uint_vec1 = OpSpecConstantComposite %v2uint %dead_uint %dead_uint",
+                "%dead_uint_vec2 = OpSpecConstantComposite %v2uint %dead_uint %used_uint",
+            },
+        },
+
+        // Int vector type spec constants. One vector has all component dead,
+        // another vector has one dead integer and one used integer.
+        {
+            /* .used_consts = */
+            {
+                "%used_int = OpSpecConstant %int 3",
+            },
+            /* .main_insts = */
+            {
+                "%int_var = OpVariable %_pf_int Function",
+                "OpStore %int_var %used_int",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_int = OpSpecConstant %int 1",
+                "%dead_int_vec1 = OpSpecConstantComposite %v2int %dead_int %dead_int",
+                "%dead_int_vec2 = OpSpecConstantComposite %v2int %dead_int %used_int",
+            },
+        },
+
+        // Int vector type spec constants built with both spec constants and
+        // front-end constants.
+        {
+            /* .used_consts = */
+            {
+                "%used_spec_int = OpSpecConstant %int 3",
+                "%used_front_end_int = OpConstant %int 3",
+            },
+            /* .main_insts = */
+            {
+                "%int_var1 = OpVariable %_pf_int Function",
+                "OpStore %int_var1 %used_spec_int",
+                "%int_var2 = OpVariable %_pf_int Function",
+                "OpStore %int_var2 %used_front_end_int",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_spec_int = OpSpecConstant %int 1",
+                "%dead_front_end_int = OpConstant %int 1",
+                // Dead front-end and dead spec constants
+                "%dead_int_vec1 = OpSpecConstantComposite %v2int %dead_spec_int %dead_front_end_int",
+                // Used front-end and dead spec constants
+                "%dead_int_vec2 = OpSpecConstantComposite %v2int %dead_spec_int %used_front_end_int",
+                // Dead front-end and used spec constants
+                "%dead_int_vec3 = OpSpecConstantComposite %v2int %dead_front_end_int %used_spec_int",
+            },
+        },
+        // clang-format on
+    })));
+
+INSTANTIATE_TEST_CASE_P(
+    SpecConstantOp, EliminateDeadConstantTest,
+    ::testing::ValuesIn(std::vector<EliminateDeadConstantTestCase>({
+        // clang-format off
+        // Cast operations: uint <-> int <-> bool
+        {
+            /* .used_consts = */ {},
+            /* .main_insts = */ {},
+            /* .dead_consts = */
+            {
+                // Assistant constants, only used in dead spec constant
+                // operations.
+                "%signed_zero = OpConstant %int 0",
+                "%signed_zero_vec = OpConstantComposite %v2int %signed_zero %signed_zero",
+                "%unsigned_zero = OpConstant %uint 0",
+                "%unsigned_zero_vec = OpConstantComposite %v2uint %unsigned_zero %unsigned_zero",
+                "%signed_one = OpConstant %int 1",
+                "%signed_one_vec = OpConstantComposite %v2int %signed_one %signed_one",
+                "%unsigned_one = OpConstant %uint 1",
+                "%unsigned_one_vec = OpConstantComposite %v2uint %unsigned_one %unsigned_one",
+
+                // Spec constants that support casting to each other.
+                "%dead_bool = OpSpecConstantTrue %bool",
+                "%dead_uint = OpSpecConstant %uint 1",
+                "%dead_int = OpSpecConstant %int 2",
+                "%dead_bool_vec = OpSpecConstantComposite %v2bool %dead_bool %dead_bool",
+                "%dead_uint_vec = OpSpecConstantComposite %v2uint %dead_uint %dead_uint",
+                "%dead_int_vec = OpSpecConstantComposite %v2int %dead_int %dead_int",
+
+                // Scalar cast to boolean spec constant.
+                "%int_to_bool = OpSpecConstantOp %bool INotEqual %dead_int %signed_zero",
+                "%uint_to_bool = OpSpecConstantOp %bool INotEqual %dead_uint %unsigned_zero",
+
+                // Vector cast to boolean spec constant.
+                "%int_to_bool_vec = OpSpecConstantOp %v2bool INotEqual %dead_int_vec %signed_zero_vec",
+                "%uint_to_bool_vec = OpSpecConstantOp %v2bool INotEqual %dead_uint_vec %unsigned_zero_vec",
+
+                // Scalar cast to int spec constant.
+                "%bool_to_int = OpSpecConstantOp %int Select %dead_bool %signed_one %signed_zero",
+                "%uint_to_int = OpSpecConstantOp %uint IAdd %dead_uint %unsigned_zero",
+
+                // Vector cast to int spec constant.
+                "%bool_to_int_vec = OpSpecConstantOp %v2int Select %dead_bool_vec %signed_one_vec %signed_zero_vec",
+                "%uint_to_int_vec = OpSpecConstantOp %v2uint IAdd %dead_uint_vec %unsigned_zero_vec",
+
+                // Scalar cast to uint spec constant.
+                "%bool_to_uint = OpSpecConstantOp %uint Select %dead_bool %unsigned_one %unsigned_zero",
+                "%int_to_uint_vec = OpSpecConstantOp %uint IAdd %dead_int %signed_zero",
+
+                // Vector cast to uint spec constant.
+                "%bool_to_uint_vec = OpSpecConstantOp %v2uint Select %dead_bool_vec %unsigned_one_vec %unsigned_zero_vec",
+                "%int_to_uint = OpSpecConstantOp %v2uint IAdd %dead_int_vec %signed_zero_vec",
+            },
+        },
+
+        // Add, sub, mul, div, rem.
+        {
+            /* .used_consts = */ {},
+            /* .main_insts = */ {},
+            /* .dead_consts = */
+            {
+                "%dead_spec_int_a = OpSpecConstant %int 1",
+                "%dead_spec_int_a_vec = OpSpecConstantComposite %v2int %dead_spec_int_a %dead_spec_int_a",
+
+                "%dead_spec_int_b = OpSpecConstant %int 2",
+                "%dead_spec_int_b_vec = OpSpecConstantComposite %v2int %dead_spec_int_b %dead_spec_int_b",
+
+                "%dead_const_int_c = OpConstant %int 3",
+                "%dead_const_int_c_vec = OpConstantComposite %v2int %dead_const_int_c %dead_const_int_c",
+
+                // Add
+                "%add_a_b = OpSpecConstantOp %int IAdd %dead_spec_int_a %dead_spec_int_b",
+                "%add_a_b_vec = OpSpecConstantOp %v2int IAdd %dead_spec_int_a_vec %dead_spec_int_b_vec",
+
+                // Sub
+                "%sub_a_b = OpSpecConstantOp %int ISub %dead_spec_int_a %dead_spec_int_b",
+                "%sub_a_b_vec = OpSpecConstantOp %v2int ISub %dead_spec_int_a_vec %dead_spec_int_b_vec",
+
+                // Mul
+                "%mul_a_b = OpSpecConstantOp %int IMul %dead_spec_int_a %dead_spec_int_b",
+                "%mul_a_b_vec = OpSpecConstantOp %v2int IMul %dead_spec_int_a_vec %dead_spec_int_b_vec",
+
+                // Div
+                "%div_a_b = OpSpecConstantOp %int SDiv %dead_spec_int_a %dead_spec_int_b",
+                "%div_a_b_vec = OpSpecConstantOp %v2int SDiv %dead_spec_int_a_vec %dead_spec_int_b_vec",
+
+                // Bitwise Xor
+                "%xor_a_b = OpSpecConstantOp %int BitwiseXor %dead_spec_int_a %dead_spec_int_b",
+                "%xor_a_b_vec = OpSpecConstantOp %v2int BitwiseXor %dead_spec_int_a_vec %dead_spec_int_b_vec",
+
+                // Scalar Comparison
+                "%less_a_b = OpSpecConstantOp %int SLessThan %dead_spec_int_a %dead_spec_int_b",
+            },
+        },
+
+        // Vectors without used swizzles should be removed.
+        {
+            /* .used_consts = */
+            {
+                "%used_int = OpConstant %int 3",
+            },
+            /* .main_insts = */
+            {
+                "%int_var = OpVariable %_pf_int Function",
+                "OpStore %int_var %used_int",
+            },
+            /* .dead_consts = */
+            {
+                "%dead_int = OpConstant %int 3",
+
+                "%dead_spec_int_a = OpSpecConstant %int 1",
+                "%vec_a = OpSpecConstantComposite %v4int %dead_spec_int_a %dead_spec_int_a %dead_int %dead_int",
+
+                "%dead_spec_int_b = OpSpecConstant %int 2",
+                "%vec_b = OpSpecConstantComposite %v4int %dead_spec_int_b %dead_spec_int_b %used_int %used_int",
+
+                // Extract scalar
+                "%a_x = OpSpecConstantOp %int CompositeExtract %vec_a 0",
+                "%b_x = OpSpecConstantOp %int CompositeExtract %vec_b 0",
+
+                // Extract vector
+                "%a_xy = OpSpecConstantOp %v2int VectorShuffle %vec_a %vec_a 1 0",
+                "%b_xy = OpSpecConstantOp %v2int VectorShuffle %vec_b %vec_b 1 0",
+            },
+        },
+        // Vectors with used swizzles should not be removed.
+        {
+            /* .used_consts = */
+            {
+                "%used_int = OpConstant %int 3",
+                "%used_spec_int_a = OpSpecConstant %int 1",
+                "%used_spec_int_b = OpSpecConstant %int 2",
+                // Create vectors
+                "%vec_a = OpSpecConstantComposite %v4int %used_spec_int_a %used_spec_int_a %used_int %used_int",
+                "%vec_b = OpSpecConstantComposite %v4int %used_spec_int_b %used_spec_int_b %used_int %used_int",
+                // Extract vector
+                "%a_xy = OpSpecConstantOp %v2int VectorShuffle %vec_a %vec_a 1 0",
+                "%b_xy = OpSpecConstantOp %v2int VectorShuffle %vec_b %vec_b 1 0",
+            },
+            /* .main_insts = */
+            {
+                "%v2int_var_a = OpVariable %_pf_v2int Function",
+                "%v2int_var_b = OpVariable %_pf_v2int Function",
+                "OpStore %v2int_var_a %a_xy",
+                "OpStore %v2int_var_b %b_xy",
+            },
+            /* .dead_consts = */ {},
+        },
+        // clang-format on
+    })));
+
+INSTANTIATE_TEST_CASE_P(
+    LongDefUseChain, EliminateDeadConstantTest,
+    ::testing::ValuesIn(std::vector<EliminateDeadConstantTestCase>({
+        // clang-format off
+        // Long Def-Use chain with binary operations.
+        {
+            /* .used_consts = */
+            {
+              "%array_size = OpConstant %int 4",
+              "%type_arr_int_4 = OpTypeArray %int %array_size",
+              "%used_int_0 = OpConstant %int 100",
+              "%used_int_1 = OpConstant %int 1",
+              "%used_int_2 = OpSpecConstantOp %int IAdd %used_int_0 %used_int_1",
+              "%used_int_3 = OpSpecConstantOp %int ISub %used_int_0 %used_int_2",
+              "%used_int_4 = OpSpecConstantOp %int IAdd %used_int_0 %used_int_3",
+              "%used_int_5 = OpSpecConstantOp %int ISub %used_int_0 %used_int_4",
+              "%used_int_6 = OpSpecConstantOp %int IAdd %used_int_0 %used_int_5",
+              "%used_int_7 = OpSpecConstantOp %int ISub %used_int_0 %used_int_6",
+              "%used_int_8 = OpSpecConstantOp %int IAdd %used_int_0 %used_int_7",
+              "%used_int_9 = OpSpecConstantOp %int ISub %used_int_0 %used_int_8",
+              "%used_int_10 = OpSpecConstantOp %int IAdd %used_int_0 %used_int_9",
+              "%used_int_11 = OpSpecConstantOp %int ISub %used_int_0 %used_int_10",
+              "%used_int_12 = OpSpecConstantOp %int IAdd %used_int_0 %used_int_11",
+              "%used_int_13 = OpSpecConstantOp %int ISub %used_int_0 %used_int_12",
+              "%used_int_14 = OpSpecConstantOp %int IAdd %used_int_0 %used_int_13",
+              "%used_int_15 = OpSpecConstantOp %int ISub %used_int_0 %used_int_14",
+              "%used_int_16 = OpSpecConstantOp %int ISub %used_int_0 %used_int_15",
+              "%used_int_17 = OpSpecConstantOp %int IAdd %used_int_0 %used_int_16",
+              "%used_int_18 = OpSpecConstantOp %int ISub %used_int_0 %used_int_17",
+              "%used_int_19 = OpSpecConstantOp %int IAdd %used_int_0 %used_int_18",
+              "%used_int_20 = OpSpecConstantOp %int ISub %used_int_0 %used_int_19",
+              "%used_vec_a = OpSpecConstantComposite %v2int %used_int_18 %used_int_19",
+              "%used_vec_b = OpSpecConstantOp %v2int IMul %used_vec_a %used_vec_a",
+              "%used_int_21 = OpSpecConstantOp %int VectorShuffle %used_vec_b %used_vec_b 0",
+              "%used_array = OpConstantComposite %type_arr_int_4 %used_int_20 %used_int_20 %used_int_21 %used_int_21",
+            },
+            /* .main_insts = */
+            {
+              "%int_var = OpVariable %_pf_int Function",
+              "%used_array_2 = OpCompositeExtract %int %used_array 2",
+              "OpStore %int_var %used_array_2",
+            },
+            /* .dead_consts = */
+            {
+              "%dead_int_1 = OpConstant %int 2",
+              "%dead_int_2 = OpSpecConstantOp %int IAdd %used_int_0 %dead_int_1",
+              "%dead_int_3 = OpSpecConstantOp %int ISub %used_int_0 %dead_int_2",
+              "%dead_int_4 = OpSpecConstantOp %int IAdd %used_int_0 %dead_int_3",
+              "%dead_int_5 = OpSpecConstantOp %int ISub %used_int_0 %dead_int_4",
+              "%dead_int_6 = OpSpecConstantOp %int IAdd %used_int_0 %dead_int_5",
+              "%dead_int_7 = OpSpecConstantOp %int ISub %used_int_0 %dead_int_6",
+              "%dead_int_8 = OpSpecConstantOp %int IAdd %used_int_0 %dead_int_7",
+              "%dead_int_9 = OpSpecConstantOp %int ISub %used_int_0 %dead_int_8",
+              "%dead_int_10 = OpSpecConstantOp %int IAdd %used_int_0 %dead_int_9",
+              "%dead_int_11 = OpSpecConstantOp %int ISub %used_int_0 %dead_int_10",
+              "%dead_int_12 = OpSpecConstantOp %int IAdd %used_int_0 %dead_int_11",
+              "%dead_int_13 = OpSpecConstantOp %int ISub %used_int_0 %dead_int_12",
+              "%dead_int_14 = OpSpecConstantOp %int IAdd %used_int_0 %dead_int_13",
+              "%dead_int_15 = OpSpecConstantOp %int ISub %used_int_0 %dead_int_14",
+              "%dead_int_16 = OpSpecConstantOp %int ISub %used_int_0 %dead_int_15",
+              "%dead_int_17 = OpSpecConstantOp %int IAdd %used_int_0 %dead_int_16",
+              "%dead_int_18 = OpSpecConstantOp %int ISub %used_int_0 %dead_int_17",
+              "%dead_int_19 = OpSpecConstantOp %int IAdd %used_int_0 %dead_int_18",
+              "%dead_int_20 = OpSpecConstantOp %int ISub %used_int_0 %dead_int_19",
+              "%dead_vec_a = OpSpecConstantComposite %v2int %dead_int_18 %dead_int_19",
+              "%dead_vec_b = OpSpecConstantOp %v2int IMul %dead_vec_a %dead_vec_a",
+              "%dead_int_21 = OpSpecConstantOp %int VectorShuffle %dead_vec_b %dead_vec_b 0",
+              "%dead_array = OpConstantComposite %type_arr_int_4 %dead_int_20 %used_int_20 %dead_int_19 %used_int_19",
+            },
+        },
+        // Long Def-Use chain with swizzle
+        // clang-format on
+    })));
+}  // anonymous namespace

tools/opt/opt.cpp

@@ -88,6 +88,8 @@ int main(int argc, char** argv) {
         pass_manager.AddPass<opt::StripDebugInfoPass>();
       } else if (0 == strcmp(cur_arg, "--freeze-spec-const")) {
         pass_manager.AddPass<opt::FreezeSpecConstantValuePass>();
+      } else if (0 == strcmp(cur_arg, "--eliminate-dead-const")) {
+        pass_manager.AddPass<opt::EliminateDeadConstantPass>();
       } else if ('\0' == cur_arg[1]) {
         // Setting a filename of "-" to indicate stdin.
         if (!in_file) {