spirv-opt: Implement opt::Function::HasEarlyReturn function (#3711)

source/opt/function.cpp

@@ -227,6 +227,18 @@ BasicBlock* Function::InsertBasicBlockBefore(
   return nullptr;
 }
 
+bool Function::HasEarlyReturn() const {
+  auto post_dominator_analysis =
+      blocks_.front()->GetLabel()->context()->GetPostDominatorAnalysis(this);
+  for (auto& block : blocks_) {
+    if (spvOpcodeIsReturn(block->tail()->opcode()) &&
+        !post_dominator_analysis->Dominates(block.get(), entry().get())) {
+      return true;
+    }
+  }
+  return false;
+}
+
 bool Function::IsRecursive() const {
   IRContext* ctx = blocks_.front()->GetLabel()->context();
   IRContext::ProcessFunction mark_visited = [this](Function* fp) {

source/opt/function.h

@@ -158,7 +158,10 @@ class Function {
   BasicBlock* InsertBasicBlockBefore(std::unique_ptr<BasicBlock>&& new_block,
                                      BasicBlock* position);
 
-  // Return true if the function calls itself either directly or indirectly.
+  // Returns true if the function has a return block other than the exit block.
+  bool HasEarlyReturn() const;
+
+  // Returns true if the function calls itself either directly or indirectly.
   bool IsRecursive() const;
 
   // Pretty-prints all the basic blocks in this function into a std::string.

test/opt/function_test.cpp

@@ -29,6 +29,60 @@ namespace {
 
 using ::testing::Eq;
 
+TEST(FunctionTest, HasEarlyReturn) {
+  std::string shader = R"(
+          OpCapability Shader
+     %1 = OpExtInstImport "GLSL.std.450"
+          OpMemoryModel Logical GLSL450
+          OpEntryPoint Vertex %6 "main"
+
+; Types
+     %2 = OpTypeBool
+     %3 = OpTypeVoid
+     %4 = OpTypeFunction %3
+
+; Constants
+     %5 = OpConstantTrue %2
+
+; main function without early return
+     %6 = OpFunction %3 None %4
+     %7 = OpLabel
+          OpBranch %8
+     %8 = OpLabel
+          OpBranch %9
+     %9 = OpLabel
+          OpBranch %10
+    %10 = OpLabel
+          OpReturn
+          OpFunctionEnd
+
+; function with early return
+    %11 = OpFunction %3 None %4
+    %12 = OpLabel
+          OpSelectionMerge %15 None
+          OpBranchConditional %5 %13 %14
+    %13 = OpLabel
+          OpReturn
+    %14 = OpLabel
+          OpBranch %15
+    %15 = OpLabel
+          OpReturn
+          OpFunctionEnd
+  )";
+
+  const auto context =
+      BuildModule(SPV_ENV_UNIVERSAL_1_1, nullptr, shader,
+                  SPV_TEXT_TO_BINARY_OPTION_PRESERVE_NUMERIC_IDS);
+
+  // Tests |function| without early return.
+  auto* function = spvtest::GetFunction(context->module(), 6);
+  ASSERT_FALSE(function->HasEarlyReturn());
+
+  // Tests |function| with early return.
+  function = spvtest::GetFunction(context->module(), 11);
+  ASSERT_TRUE(function->HasEarlyReturn());
+}
+
 TEST(FunctionTest, IsNotRecursive) {
   const std::string text = R"(
 OpCapability Shader