Remove redundant stores.

The code patterns generated by DXC around function calls can cause many
store to be storing the same value that was just loaded from the same
location:

```
%10 = OpLoad %type %var
OpStore %var %10
```

We want to clean these up very early on because they can cause other
transformations to do a lot of work.  For the cases I see, they can be
removed during local-single-block-elim.

For one set of shaders the compile time goes from 248s to 182s.  A 26%
improvement.

Part of https://github.com/KhronosGroup/SPIRV-Tools/issues/1494.

source/opt/local_single_block_elim_pass.cpp

@@ -73,16 +73,35 @@ bool LocalSingleBlockLoadStoreElimPass::LocalSingleBlockLoadStoreElim(
           if (ptrInst->opcode() == SpvOpVariable) {
             // If a previous store to same variable, mark the store
             // for deletion if not still used.
-            ir::Instruction* prev_store = var2store_[varId];
-            if (prev_store != nullptr &&
-                instructions_to_save.count(prev_store) == 0)
-              instructions_to_kill.push_back(prev_store);
-            var2store_[varId] = &*ii;
+            auto prev_store = var2store_.find(varId);
+            if (prev_store != var2store_.end() &&
+                instructions_to_save.count(prev_store->second) == 0) {
+              instructions_to_kill.push_back(prev_store->second);
+            }
+
+            bool kill_store = false;
+            auto li = var2load_.find(varId);
+            if (li != var2load_.end()) {
+              if (ii->GetSingleWordInOperand(kStoreValIdInIdx) ==
+                  li->second->result_id()) {
+                // We are storing the same value that already exists in the
+                // memory location.  The store does nothing.
+                kill_store = true;
+              }
+            }
+
+            if (!kill_store) {
+              var2store_[varId] = &*ii;
+              var2load_.erase(varId);
+            } else {
+              instructions_to_kill.push_back(&*ii);
+              modified = true;
+            }
           } else {
             assert(IsNonPtrAccessChain(ptrInst->opcode()));
             var2store_.erase(varId);
+            var2load_.erase(varId);
           }
-          var2load_.erase(varId);
         } break;
         case SpvOpLoad: {
           // Verify store variable is target type

test/opt/local_single_block_elim.cpp

@@ -867,6 +867,120 @@ OpFunctionEnd
   SinglePassRunAndCheck<opt::LocalSingleBlockLoadStoreElimPass>(before, after,
                                                                 true, true);
 }
+
+TEST_F(LocalSingleBlockLoadStoreElimTest, RedundantStore) {
+  // Test that checks if a pointer variable is removed.
+  const std::string predefs_before =
+      R"(OpCapability Shader
+%1 = OpExtInstImport "GLSL.std.450"
+OpMemoryModel Logical GLSL450
+OpEntryPoint Fragment %main "main" %BaseColor %gl_FragColor
+OpExecutionMode %main OriginUpperLeft
+OpSource GLSL 140
+OpName %main "main"
+OpName %v "v"
+OpName %BaseColor "BaseColor"
+OpName %gl_FragColor "gl_FragColor"
+%void = OpTypeVoid
+%7 = OpTypeFunction %void
+%float = OpTypeFloat 32
+%v4float = OpTypeVector %float 4
+%_ptr_Function_v4float = OpTypePointer Function %v4float
+%_ptr_Input_v4float = OpTypePointer Input %v4float
+%BaseColor = OpVariable %_ptr_Input_v4float Input
+%_ptr_Output_v4float = OpTypePointer Output %v4float
+%gl_FragColor = OpVariable %_ptr_Output_v4float Output
+)";
+
+  const std::string before =
+      R"(%main = OpFunction %void None %7
+%13 = OpLabel
+%v = OpVariable %_ptr_Function_v4float Function
+%14 = OpLoad %v4float %BaseColor
+OpStore %v %14
+OpBranch %16
+%16 = OpLabel
+%15 = OpLoad %v4float %v
+OpStore %v %15
+OpReturn
+OpFunctionEnd
+)";
+
+  const std::string after =
+      R"(%main = OpFunction %void None %7
+%13 = OpLabel
+%v = OpVariable %_ptr_Function_v4float Function
+%14 = OpLoad %v4float %BaseColor
+OpStore %v %14
+OpBranch %16
+%16 = OpLabel
+%15 = OpLoad %v4float %v
+OpReturn
+OpFunctionEnd
+)";
+
+  SetAssembleOptions(SPV_TEXT_TO_BINARY_OPTION_PRESERVE_NUMERIC_IDS);
+  SinglePassRunAndCheck<opt::LocalSingleBlockLoadStoreElimPass>(
+      predefs_before + before, predefs_before + after, true, true);
+}
+
+TEST_F(LocalSingleBlockLoadStoreElimTest, RedundantStore2) {
+  // Test that checks if a pointer variable is removed.
+  const std::string predefs_before =
+      R"(OpCapability Shader
+%1 = OpExtInstImport "GLSL.std.450"
+OpMemoryModel Logical GLSL450
+OpEntryPoint Fragment %main "main" %BaseColor %gl_FragColor
+OpExecutionMode %main OriginUpperLeft
+OpSource GLSL 140
+OpName %main "main"
+OpName %v "v"
+OpName %BaseColor "BaseColor"
+OpName %gl_FragColor "gl_FragColor"
+%void = OpTypeVoid
+%7 = OpTypeFunction %void
+%float = OpTypeFloat 32
+%v4float = OpTypeVector %float 4
+%_ptr_Function_v4float = OpTypePointer Function %v4float
+%_ptr_Input_v4float = OpTypePointer Input %v4float
+%BaseColor = OpVariable %_ptr_Input_v4float Input
+%_ptr_Output_v4float = OpTypePointer Output %v4float
+%gl_FragColor = OpVariable %_ptr_Output_v4float Output
+)";
+
+  const std::string before =
+      R"(%main = OpFunction %void None %7
+%13 = OpLabel
+%v = OpVariable %_ptr_Function_v4float Function
+%14 = OpLoad %v4float %BaseColor
+OpStore %v %14
+OpBranch %16
+%16 = OpLabel
+%15 = OpLoad %v4float %v
+OpStore %v %15
+%17 = OpLoad %v4float %v
+OpStore %v %17
+OpReturn
+OpFunctionEnd
+)";
+
+  const std::string after =
+      R"(%main = OpFunction %void None %7
+%13 = OpLabel
+%v = OpVariable %_ptr_Function_v4float Function
+%14 = OpLoad %v4float %BaseColor
+OpStore %v %14
+OpBranch %16
+%16 = OpLabel
+%15 = OpLoad %v4float %v
+OpReturn
+OpFunctionEnd
+)";
+
+  SetAssembleOptions(SPV_TEXT_TO_BINARY_OPTION_PRESERVE_NUMERIC_IDS);
+  SinglePassRunAndCheck<opt::LocalSingleBlockLoadStoreElimPass>(
+      predefs_before + before, predefs_before + after, true, true);
+}
 // TODO(greg-lunarg): Add tests to verify handling of these cases:
 //
 //    Other target variable types