[lldb] Add a return opcode to the formatter bytecode (#121602)

In LLVM we love our early exists and this opcode allows for simpler code
generation.

lldb/docs/resources/formatterbytecode.rst

@@ -75,6 +75,7 @@ These manipulate the control stack and program counter. Both `if` and `ifelse` e
  0x12      `ifelse`    `(UInt -> )` pop two blocks from the control stack, if
                        the top of the data stack is nonzero, execute the first,
                        otherwise the second.
+ 0x13      `return`    pop the entire control stack and return
 ========  ==========  ============================================================
 
 Literals for basic types

lldb/examples/python/formatter_bytecode.py

@@ -35,6 +35,7 @@ define_opcode(6, "rot", "rot")
 define_opcode(0x10, "{", "begin")
 define_opcode(0x11, "if", "if")
 define_opcode(0x12, "ifelse", "ifelse")
+define_opcode(0x13, "return", "return")
 
 define_opcode(0x20, None, "lit_uint")
 define_opcode(0x21, None, "lit_int")
@@ -342,6 +343,9 @@ def interpret(bytecode: bytearray, control: list, data: list, tracing: bool = Fa
             else:
                 frame.append(control.pop())
                 control.pop()
+        elif b == op_return:
+            control.clear()
+            return data[-1]
 
         # Literals.
         elif b == op_lit_uint:

lldb/source/DataFormatters/FormatterBytecode.cpp

@@ -304,6 +304,9 @@ llvm::Error Interpret(std::vector<ControlStackElement> &control,
       control.pop_back();
       activate_block();
       continue;
+    case op_return:
+      control.clear();
+      return pc.takeError();
 
     // Literals.
     case op_lit_uint:

lldb/source/DataFormatters/FormatterBytecode.def

@@ -27,6 +27,7 @@ DEFINE_OPCODE(0x06, "rot",  rot)
 DEFINE_OPCODE(0x10, "{", begin)
 DEFINE_OPCODE(0x11, "if", if)
 DEFINE_OPCODE(0x12, "ifelse", ifelse)
+DEFINE_OPCODE(0x13, "return", return)
 
 DEFINE_OPCODE(0x20, nullptr, lit_uint)
 DEFINE_OPCODE(0x21, nullptr, lit_int)

lldb/unittests/DataFormatter/FormatterBytecodeTest.cpp

@@ -97,6 +97,16 @@ TEST_F(FormatterBytecodeTest, ControlOps) {
                           data));
     ASSERT_EQ(data.Pop<uint64_t>(), 42u);
   }
+  {
+    DataStack data;
+    ASSERT_TRUE(Interpret({op_lit_uint, 1, op_begin, 3, op_lit_uint, 42,
+                           op_return, op_if, op_lit_uint, 23},
+                          data));
+    ASSERT_EQ(data.Pop<uint64_t>(), 42u);
+  }
+}
+
+TEST_F(FormatterBytecodeTest, ConversionOps) {
   {
     DataStack data(lldb::ValueObjectSP{});
     ASSERT_TRUE(Interpret({op_is_null}, data));