DWARFExpression: Fix implementation of DW_OP_pick

[labath]

Summary

The DWARF spec states that the DWARF stack arguments are numbered from
the top. Our implementation of DW_OP_pick was counting them from the
bottom.

This bug probably wasn't noticed because nobody (except my upcoming
postfix-to-DWARF converter) uses DW_OP_pick, but I've cross-checked with
gdb to confirm that counting from the top is the expected behavior.

This patch fixes the implementation to match the spec and gdb behavior
and adds a test.

include/lldb/Utility/Scalar.h

 const Scalar operator>>(const Scalar &lhs, const Scalar &rhs);
 bool operator==(const Scalar &lhs, const Scalar &rhs);
 bool operator!=(const Scalar &lhs, const Scalar &rhs);
 bool operator<(const Scalar &lhs, const Scalar &rhs);
 bool operator<=(const Scalar &lhs, const Scalar &rhs);
 bool operator>(const Scalar &lhs, const Scalar &rhs);
 bool operator>=(const Scalar &lhs, const Scalar &rhs);
 
+llvm::raw_ostream &operator<<(llvm::raw_ostream &os, const Scalar &scalar);
+
 } // namespace lldb_private
 
 #endif // LLDB_UTILITY_SCALAR_H

source/Expression/DWARFExpression.cpp

while (opcodes.ValidOffset(offset) && offset < end_offset) {
 
     // OPCODE: DW_OP_pick
     // OPERANDS: uint8_t index into the current stack
     // DESCRIPTION: The stack entry with the specified index (0 through 255,
     // inclusive) is pushed on the stack
     case DW_OP_pick: {
       uint8_t pick_idx = opcodes.GetU8(&offset);
       if (pick_idx < stack.size())
-        stack.push_back(stack[pick_idx]);
+        stack.push_back(stack[stack.size() - 1 - pick_idx]);
       else {
         if (error_ptr)
           error_ptr->SetErrorStringWithFormat(
               "Index %u out of range for DW_OP_pick.\n", pick_idx);
         return false;
       }
     } break;
 

source/Utility/Scalar.cpp

 //===-- Scalar.cpp ----------------------------------------------*- 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
 //
 //===----------------------------------------------------------------------===//
 
 #include "lldb/Utility/Scalar.h"
 
 #include "lldb/Utility/DataExtractor.h"
 #include "lldb/Utility/Endian.h"
 #include "lldb/Utility/Status.h"
 #include "lldb/Utility/Stream.h"
+#include "lldb/Utility/StreamString.h"
 #include "lldb/lldb-types.h"
 
 #include "llvm/ADT/SmallString.h"
 
 #include <cinttypes>
 #include <cstdio>
 
 using namespace lldb;
case e_uint512:
     return true;
   case e_float:
   case e_double:
   case e_long_double:
     break;
   }
   return false;
 }
+
+llvm::raw_ostream &lldb_private::operator<<(llvm::raw_ostream &os, const Scalar &scalar) {
+  StreamString s;
+  scalar.GetValue(&s, /*show_type*/ true);
+  return os << s.GetString();
+}

unittests/Expression/CMakeLists.txt

 add_lldb_unittest(ExpressionTests
+  DWARFExpressionTest.cpp
   ClangParserTest.cpp
 
   LINK_LIBS
     lldbCore
     lldbPluginExpressionParserClang
     lldbUtility
     lldbUtilityHelpers
+    LLVMTestingSupport
   )

unittests/Expression/DWARFExpressionTest.cpp

+//===-- DWARFExpressionTest.cpp ----------------------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "lldb/Expression/DWARFExpression.h"
+#include "lldb/Core/Value.h"
+#include "lldb/Core/dwarf.h"
+#include "lldb/Utility/StreamString.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/Testing/Support/Error.h"
+#include "gtest/gtest.h"
+
+using namespace lldb_private;
+
+static llvm::Expected<Scalar> Evaluate(llvm::ArrayRef<uint8_t> expr) {
+  DataExtractor extractor(expr.data(), expr.size(), lldb::eByteOrderLittle,
+                          /*addr_size*/ 4);
+
+  Value result;
+  Status status;
+  if (!DWARFExpression::Evaluate(
+          /*exe_ctx*/ nullptr, /*reg_ctx*/ nullptr, /*opcode_ctx*/ nullptr,
+          extractor, /*dwarf_cu*/ nullptr, /*offset*/ 0, expr.size(),
+          lldb::eRegisterKindLLDB, /*initial_value_ptr*/ nullptr,
+          /*object_address_ptr*/ nullptr, result, &status))
+    return status.ToError();
+
+  return result.GetScalar();
+}
+
+TEST(DWARFExpression, DW_OP_pick) {
+  EXPECT_THAT_EXPECTED(Evaluate({DW_OP_lit1, DW_OP_lit0, DW_OP_pick, 0}),
+                       llvm::HasValue(0));
+  EXPECT_THAT_EXPECTED(Evaluate({DW_OP_lit1, DW_OP_lit0, DW_OP_pick, 1}),
+                       llvm::HasValue(1));
+  EXPECT_THAT_EXPECTED(Evaluate({DW_OP_lit1, DW_OP_lit0, DW_OP_pick, 2}),
+                       llvm::Failed());
+}