[torque] Emit labels only if they are used.

This CL changes the generated C++ code for LabeledStatementBlocks to
only emit labels if they are used.

Prior to this CL, when a label was only used on one path of an
if constexpr expression, and not at all anywhere else,
the try/label construct would BIND a label that was not used,
causing a CSA verification error.

R=tebbi@chromium.org

Change-Id: Ia81a0cd081b84528c95bbdbdb98b9ab51928e13f
Reviewed-on: https://chromium-review.googlesource.com/1057247Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#53173}

src/compiler/code-assembler.h

@@ -1244,6 +1244,7 @@ class CodeAssemblerLabel {
   ~CodeAssemblerLabel();
 
   inline bool is_bound() const { return bound_; }
+  inline bool is_used() const { return merge_count_ != 0; }
 
  private:
   friend class CodeAssembler;

src/torque/implementation-visitor.cc

@@ -1408,6 +1408,11 @@ bool ImplementationVisitor::GenerateLabeledStatementBlocks(
   bool live = false;
   auto label_iterator = statement_labels.begin();
   for (Statement* block : blocks) {
+    GenerateIndent();
+    source_out() << "if (" << (*label_iterator)->generated() << "->is_used())"
+                 << std::endl;
+    ScopedIndent indent(this);
+
     GenerateLabelBind(*label_iterator++);
     if (!Visit(block)->IsNever()) {
       GenerateLabelGoto(merge_label);

test/cctest/torque/test-torque.cc

@@ -92,6 +92,15 @@ TEST(TestGotoLabelWithTwoParameters) {
   ft.CheckCall(ft.true_value());
 }
 
+TEST(TestPartiallyUnusedLabel) {
+  Isolate* isolate(CcTest::InitIsolateOnce());
+  CodeAssemblerTester asm_tester(isolate, 0);
+  TestBuiltinsFromDSLAssembler m(asm_tester.state());
+  { m.Return(m.TestPartiallyUnusedLabel()); }
+  FunctionTester ft(asm_tester.GenerateCode(), 0);
+  ft.CheckCall(ft.true_value());
+}
+
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8

test/torque/test-torque.tq

@@ -82,4 +82,29 @@ macro TestGotoLabelWithTwoParameters(): Boolean {
   }
 }
 
+macro LabelTestHelper4(flag: constexpr bool): never labels Label4, Label5 {
+  if constexpr (flag) goto Label4;
+  else goto Label5;
+}
+
+macro CallLabelTestHelper4(flag: constexpr bool): bool {
+  try {
+    LabelTestHelper4(flag) otherwise Label4, Label5;
+  }
+  label Label4 {
+    return true;
+  }
+  label Label5 {
+    return false;
+  }
+}
+
+macro TestPartiallyUnusedLabel(): Boolean {
+  let r1: bool = CallLabelTestHelper4(true);
+  let r2: bool = CallLabelTestHelper4(false);
+
+  if (r1 && !r2) return True;
+  else return False;
+}
+
 }