[Lldb-commits] [lldb] [lldb] DWARF expression evaluator to test unified stack semantics (PR #205532)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lldb

Author: Rajveer Singh Bharadwaj (Rajveer100)

<details>
<summary>Changes</summary>

Part of #<!-- -->181509

This evaluator uses a unified stack for expressions as a DWARF6Value/DWARF6Location and appropriately pushes/pops from stacks depending on the context of the stack frame for various operations like `DW_OP_deref`, `DW_OP_plus` and more.

---
Full diff: https://github.com/llvm/llvm-project/pull/205532.diff


3 Files Affected:

- (modified) lldb/source/Commands/CMakeLists.txt (+1) 
- (added) lldb/source/Commands/CommandObjectDWARFExpression.cpp (+130) 
- (added) lldb/source/Commands/CommandObjectDWARFExpression.h (+8) 


``````````diff
diff --git a/lldb/source/Commands/CMakeLists.txt b/lldb/source/Commands/CMakeLists.txt
index f2e62243e999e..c4f2a842a0fda 100644
--- a/lldb/source/Commands/CMakeLists.txt
+++ b/lldb/source/Commands/CMakeLists.txt
@@ -10,6 +10,7 @@ add_lldb_library(lldbCommands NO_PLUGIN_DEPENDENCIES
   CommandObjectCommands.cpp
   CommandObjectDiagnostics.cpp
   CommandObjectDisassemble.cpp
+  CommandobjectDWARFExpression.cpp
   CommandObjectDWIMPrint.cpp
   CommandObjectExpression.cpp
   CommandObjectFrame.cpp
diff --git a/lldb/source/Commands/CommandObjectDWARFExpression.cpp b/lldb/source/Commands/CommandObjectDWARFExpression.cpp
new file mode 100644
index 0000000000000..46a0229e9201a
--- /dev/null
+++ b/lldb/source/Commands/CommandObjectDWARFExpression.cpp
@@ -0,0 +1,130 @@
+//===-- CommandObjectDWARFExpression.cpp
+//-----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "lldb/Target/ExecutionContext.h"
+#include "lldb/Target/Process.h"
+#include "lldb/Target/RegisterContext.h"
+#include "lldb/Utility/RegisterValue.h"
+#include "lldb/Utility/Status.h"
+#include "lldb/lldb-private-types.h"
+#include "llvm/BinaryFormat/Dwarf.h"
+#include <variant>
+
+namespace lldb_private {
+
+struct DWARF6Value {
+  uint64_t IntegerLiteral;
+};
+
+struct DWARF6Location {
+  enum Kind { MemoryAddress, Register } Type;
+  uint64_t Payload;
+};
+
+using StackItem = std::variant<DWARF6Value, DWARF6Location>;
+
+class DWARF6Evaluator {
+public:
+  std::vector<StackItem> Stack;
+
+  Status Run(const std::vector<uint8_t> &opcodes, ExecutionContext &exe_ctx) {
+    size_t pc = 0;
+    RegisterContext *reg_ctx = exe_ctx.GetRegisterContext();
+
+    while (pc < opcodes.size()) {
+      uint8_t op = opcodes[pc++];
+      switch (op) {
+      case llvm::dwarf::DW_OP_lit0:
+      case llvm::dwarf::DW_OP_lit1:
+      case llvm::dwarf::DW_OP_lit2:
+      case llvm::dwarf::DW_OP_lit3:
+      case llvm::dwarf::DW_OP_lit4: {
+        Stack.push_back(
+            DWARF6Value{static_cast<uint64_t>(op - llvm::dwarf::DW_OP_lit0)});
+        break;
+      }
+
+      case llvm::dwarf::DW_OP_reg0: {
+        Stack.push_back(DWARF6Location{DWARF6Location::Register, 0});
+        break;
+      }
+
+      case llvm::dwarf::DW_OP_deref: {
+        if (Stack.empty())
+          return Status::FromErrorString("Stack underflow on DW_OP_deref");
+
+        StackItem top = Stack.back();
+        Stack.pop_back();
+
+        if (std::holds_alternative<DWARF6Location>(top)) {
+          DWARF6Location loc = std::get<DWARF6Location>(top);
+          if (loc.Type == DWARF6Location::Register) {
+            if (!reg_ctx)
+              return Status::FromErrorString("Missing Register Context");
+
+            const RegisterInfo *reg_info =
+                reg_ctx->GetRegisterInfoAtIndex(loc.Payload);
+            if (!reg_info)
+              return Status::FromErrorString("Failed to get Register Info");
+
+            RegisterValue reg_val;
+            if (!reg_ctx->ReadRegister(reg_info, reg_val))
+              return Status::FromErrorString(
+                  "Failed to read register value from frame");
+
+            Stack.push_back(DWARF6Value{reg_val.GetAsUInt64()});
+          } else if (loc.Type == DWARF6Location::MemoryAddress) {
+            Process *process = exe_ctx.GetProcessPtr();
+            if (!process)
+              return Status::FromErrorString(
+                  "No active target process context available");
+
+            uint64_t mem_val = 0;
+            Status error;
+            process->ReadMemory(loc.Payload, &mem_val, sizeof(mem_val), error);
+
+            Stack.push_back(DWARF6Value{mem_val});
+          }
+        } else {
+          return Status::FromErrorString(
+              "DWARF 6 Type Mismatch: Cannot dereference");
+        }
+        break;
+      }
+
+      case llvm::dwarf::DW_OP_plus: {
+        if (Stack.size() < 2)
+          return Status::FromErrorString("Stack underflow on DW_OP_plus");
+
+        if (!std::holds_alternative<DWARF6Value>(Stack.back()))
+          return Status::FromErrorString(
+              "Right-hand operand must be a Value type");
+        auto right = std::get<DWARF6Value>(Stack.back());
+        Stack.pop_back();
+
+        if (!std::holds_alternative<DWARF6Value>(Stack.back()))
+          return Status::FromErrorString(
+              "Left-hand operand must be a Value type");
+        auto left = std::get<DWARF6Value>(Stack.back());
+        Stack.pop_back();
+
+        Stack.push_back(
+            DWARF6Value{left.IntegerLiteral + right.IntegerLiteral});
+        break;
+      }
+
+      default:
+        return Status::FromErrorStringWithFormat(
+            "Opcode 0x%02X is currently unimplemented", op);
+      }
+    }
+    return Status();
+  }
+};
+} // namespace lldb_private
diff --git a/lldb/source/Commands/CommandObjectDWARFExpression.h b/lldb/source/Commands/CommandObjectDWARFExpression.h
new file mode 100644
index 0000000000000..d4a348bcda32e
--- /dev/null
+++ b/lldb/source/Commands/CommandObjectDWARFExpression.h
@@ -0,0 +1,8 @@
+//===-- CommandObjectDWARFExpression.h ---------------------------*- C++
+//-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//

``````````

</details>


https://github.com/llvm/llvm-project/pull/205532