[lldb] Handle deref of register and implicit locations

lldb/source/Expression/DWARFExpression.cpp

@@ -861,13 +861,30 @@ ResolveLoadAddress(ExecutionContext *exe_ctx, lldb::ModuleSP &module_sp,
   return load_addr;
 }
 
-static llvm::Error Evaluate_DW_OP_deref(DWARFExpression::Stack &stack,
-                                        ExecutionContext *exe_ctx,
-                                        lldb::ModuleSP module_sp,
-                                        Process *process) {
+static llvm::Error Evaluate_DW_OP_deref(
+    DWARFExpression::Stack &stack, ExecutionContext *exe_ctx,
+    lldb::ModuleSP module_sp, Process *process,
+    LocationDescriptionKind &dwarf4_location_description_kind) {
   if (stack.empty())
     return llvm::createStringError("expression stack empty for DW_OP_deref");
 
+  // Handle deref of a register or implicit location.
+  // When the current value is a register or implicit location description then
+  // a deref operation should read the value from that location. We eagerly
+  // read the register and implicit values and so its value is already on top of
+  // the stack. We just need to reset the value context and description to their
+  // defaults.
+  if (dwarf4_location_description_kind == Register ||
+      dwarf4_location_description_kind == Implicit) {
+    // Reset context to default values.
+    dwarf4_location_description_kind = Memory;
+    stack.back().ClearContext();
+
+    // The value is already on top of the stack so there is nothing
+    // more to do here.
+    return llvm::Error::success();
+  }
+
   const Value::ValueType value_type = stack.back().GetValueType();
   switch (value_type) {
   case Value::ValueType::HostAddress: {
@@ -1080,7 +1097,8 @@ llvm::Expected<Value> DWARFExpression::Evaluate(
     // target machine.
     case DW_OP_deref: {
       if (llvm::Error err =
-              Evaluate_DW_OP_deref(stack, exe_ctx, module_sp, process))
+              Evaluate_DW_OP_deref(stack, exe_ctx, module_sp, process,
+                                   dwarf4_location_description_kind))
         return err;
     } break;
 
@@ -1106,6 +1124,25 @@ llvm::Expected<Value> DWARFExpression::Evaluate(
         return llvm::createStringError(
             "Invalid address size for DW_OP_deref_size: %d\n", size);
       }
+
+      // Deref a register or implicit location and truncate the value to `size`
+      // bytes. See the corresponding comment in DW_OP_deref for more details on
+      // why we deref these locations this way.
+      if (dwarf4_location_description_kind == Register ||
+          dwarf4_location_description_kind == Implicit) {
+        // Reset context to default values.
+        dwarf4_location_description_kind = Memory;
+        stack.back().ClearContext();
+
+        // Truncate the value on top of the stack to *size* bytes then
+        // extend to the size of an address (e.g. generic type).
+        Scalar scalar = stack.back().GetScalar();
+        scalar.TruncOrExtendTo(size * 8, /*sign=*/false);
+        scalar.TruncOrExtendTo(opcodes.GetAddressByteSize() * 8,
+                               /*sign=*/false);
+        stack.back().GetScalar() = scalar;
+        break;
+      }
       Value::ValueType value_type = stack.back().GetValueType();
       switch (value_type) {
       case Value::ValueType::HostAddress: {

lldb/unittests/Expression/DWARFExpressionTest.cpp

@@ -1216,3 +1216,107 @@ TEST_F(DWARFExpressionMockProcessTestWithAArch, DW_op_deref_no_ptr_fixing) {
   llvm::Expected<Value> result_deref = evaluate_expr(expr_deref);
   EXPECT_THAT_EXPECTED(result_deref, ExpectLoadAddress(expected_value));
 }
+
+TEST_F(DWARFExpressionMockProcessTest, deref_register) {
+  TestContext test_ctx;
+  constexpr uint32_t reg_r0 = 0x504;
+  MockMemory::Map memory = {
+      {{0x004, 4}, {0x1, 0x2, 0x3, 0x4}},
+      {{0x504, 4}, {0xa, 0xb, 0xc, 0xd}},
+      {{0x505, 4}, {0x5, 0x6, 0x7, 0x8}},
+  };
+  ASSERT_TRUE(CreateTestContext(&test_ctx, "i386-pc-linux",
+                                RegisterValue(reg_r0), memory, memory));
+
+  ExecutionContext exe_ctx(test_ctx.process_sp);
+  MockDwarfDelegate delegate = MockDwarfDelegate::Dwarf5();
+  auto Eval = [&](llvm::ArrayRef<uint8_t> expr_data) {
+    ExecutionContext exe_ctx(test_ctx.process_sp);
+    return Evaluate(expr_data, {}, &delegate, &exe_ctx,
+                    test_ctx.reg_ctx_sp.get());
+  };
+
+  // Reads from the register r0.
+  // Sets the context to RegisterInfo so we know this is a register location.
+  EXPECT_THAT_EXPECTED(Eval({DW_OP_reg0}),
+                       ExpectScalar(reg_r0, Value::ContextType::RegisterInfo));
+
+  // Reads from the location(register r0).
+  // Clears the context so we know this is a value not a location.
+  EXPECT_THAT_EXPECTED(Eval({DW_OP_reg0, DW_OP_deref}),
+                       ExpectLoadAddress(reg_r0, Value::ContextType::Invalid));
+
+  // Reads from the location(register r0) and adds the value to the host buffer.
+  // The evaluator should implicitly convert it to a memory location when
+  // added to a composite value and should add the contents of memory[r0]
+  // to the host buffer.
+  EXPECT_THAT_EXPECTED(Eval({DW_OP_reg0, DW_OP_deref, DW_OP_piece, 4}),
+                       ExpectHostAddress({0xa, 0xb, 0xc, 0xd}));
+
+  // Reads from the location(register r0) and truncates the value to one byte.
+  // Clears the context so we know this is a value not a location.
+  EXPECT_THAT_EXPECTED(
+      Eval({DW_OP_reg0, DW_OP_deref_size, 1}),
+      ExpectLoadAddress(reg_r0 & 0xff, Value::ContextType::Invalid));
+
+  // Reads from the location(register r0) and truncates to one byte then adds
+  // the value to the host buffer. The evaluator should implicitly convert it to
+  // a memory location when added to a composite value and should add the
+  // contents of memory[r0 & 0xff] to the host buffer.
+  EXPECT_THAT_EXPECTED(Eval({DW_OP_reg0, DW_OP_deref_size, 1, DW_OP_piece, 4}),
+                       ExpectHostAddress({0x1, 0x2, 0x3, 0x4}));
+
+  // Reads from the register r0 + 1.
+  EXPECT_THAT_EXPECTED(
+      Eval({DW_OP_breg0, 1}),
+      ExpectLoadAddress(reg_r0 + 1, Value::ContextType::Invalid));
+
+  // Reads from address r0 + 1, which contains the bytes [5,6,7,8].
+  EXPECT_THAT_EXPECTED(
+      Eval({DW_OP_breg0, 1, DW_OP_deref}),
+      ExpectLoadAddress(0x08070605, Value::ContextType::Invalid));
+}
+
+TEST_F(DWARFExpressionMockProcessTest, deref_implicit_value) {
+  TestContext test_ctx;
+  MockMemory::Map memory = {
+      {{0x4, 1}, {0x1}},
+      {{0x4, 4}, {0x1, 0x2, 0x3, 0x4}},
+  };
+  ASSERT_TRUE(CreateTestContext(&test_ctx, "i386-pc-linux", {}, memory));
+
+  ExecutionContext exe_ctx(test_ctx.process_sp);
+  MockDwarfDelegate delegate = MockDwarfDelegate::Dwarf5();
+  auto Eval = [&](llvm::ArrayRef<uint8_t> expr_data) {
+    ExecutionContext exe_ctx(test_ctx.process_sp);
+    return Evaluate(expr_data, {}, &delegate, &exe_ctx,
+                    test_ctx.reg_ctx_sp.get());
+  };
+
+  // Creates an implicit location with a value of 4.
+  EXPECT_THAT_EXPECTED(Eval({DW_OP_lit4, DW_OP_stack_value}),
+                       ExpectScalar(0x4));
+
+  // Creates an implicit location with a value of 4. The deref reads the value
+  // out of the location and implicitly converts it to a load address.
+  EXPECT_THAT_EXPECTED(Eval({DW_OP_lit4, DW_OP_stack_value, DW_OP_deref}),
+                       ExpectLoadAddress(0x4));
+
+  // Creates an implicit location with a value of 0x504 (uleb128(0x504) =
+  // 0xa84). The deref reads the low byte out of the location and implicitly
+  // converts it to a load address.
+  EXPECT_THAT_EXPECTED(
+      Eval({DW_OP_constu, 0x84, 0xa, DW_OP_stack_value, DW_OP_deref_size, 1}),
+      ExpectLoadAddress(0x4));
+
+  // The tests below are similar to the ones above, but there is no implicit
+  // location created by a stack_value operation. They are provided here as a
+  // reference to contrast with the above tests.
+  EXPECT_THAT_EXPECTED(Eval({DW_OP_lit4}), ExpectLoadAddress(0x4));
+
+  EXPECT_THAT_EXPECTED(Eval({DW_OP_lit4, DW_OP_deref}),
+                       ExpectLoadAddress(0x04030201));
+
+  EXPECT_THAT_EXPECTED(Eval({DW_OP_lit4, DW_OP_deref_size, 1}),
+                       ExpectLoadAddress(0x01));
+}