Implement DW_OP_convert

This patch adds basic support for DW_OP_convert[1] for integer
types. Recent versions of LLVM's optimizer may insert this opcode into
DWARF expressions. DW_OP_convert is effectively a type cast operation
that takes a reference to a base type DIE (or zero) and then casts the
value at the top of the DWARF stack to that type. Internally this
works by changing the bit size of the APInt that is used as backing
storage for LLDB's DWARF stack.

I managed to write a unit test for this by implementing a mock YAML
object file / module that takes debug info sections in yaml2obj
format.

[1] Typed DWARF stack. http://www.dwarfstd.org/ShowIssue.php?issue=140425.1

<rdar://problem/48167864>

Differential Revision: https://reviews.llvm.org/D67369

llvm-svn: 371532
(cherry picked from commit 9b23df6)

lldb/include/lldb/Core/Section.h

@@ -43,9 +43,8 @@ class SectionList {
   const_iterator begin() { return m_sections.begin(); }
   const_iterator end() { return m_sections.end(); }
 
-  SectionList();
-
-  ~SectionList();
+  /// Create an empty list.
+  SectionList() = default;
 
   SectionList &operator=(const SectionList &rhs);
 

lldb/include/lldb/Utility/Scalar.h

@@ -38,6 +38,7 @@ namespace lldb_private {
 // follows the ANSI C type promotion rules.
 class Scalar {
 public:
+  // FIXME: These are host types which seems to be an odd choice.
   enum Type {
     e_void = 0,
     e_sint,
@@ -98,30 +99,14 @@ class Scalar {
   }
   Scalar(llvm::APInt v) : m_type(), m_float(static_cast<float>(0)) {
     m_integer = llvm::APInt(v);
-    switch (m_integer.getBitWidth()) {
-    case 8:
-    case 16:
-    case 32:
-      m_type = e_sint;
-      return;
-    case 64:
-      m_type = e_slonglong;
-      return;
-    case 128:
-      m_type = e_sint128;
-      return;
-    case 256:
-      m_type = e_sint256;
-      return;
-    case 512:
-      m_type = e_sint512;
-      return;
-    }
-    lldbassert(false && "unsupported bitwidth");
+    m_type = GetBestTypeForBitSize(m_integer.getBitWidth(), true);
   }
   // Scalar(const RegisterValue& reg_value);
   virtual ~Scalar();
 
+  /// Return the most efficient Scalar::Type for the requested bit size.
+  static Type GetBestTypeForBitSize(size_t bit_size, bool sign);
+
   bool SignExtend(uint32_t bit_pos);
 
   bool ExtractBitfield(uint32_t bit_size, uint32_t bit_offset);
@@ -154,6 +139,9 @@ class Scalar {
     return (m_type >= e_sint) && (m_type <= e_long_double);
   }
 
+  /// Convert integer to \p type, limited to \p bits size.
+  void TruncOrExtendTo(Scalar::Type type, uint16_t bits);
+
   bool Promote(Scalar::Type type);
 
   bool MakeSigned();

lldb/source/Core/Section.cpp

@@ -417,10 +417,6 @@ lldb::offset_t Section::GetSectionData(DataExtractor &section_data) {
 
 #pragma mark SectionList
 
-SectionList::SectionList() : m_sections() {}
-
-SectionList::~SectionList() {}
-
 SectionList &SectionList::operator=(const SectionList &rhs) {
   if (this != &rhs)
     m_sections = rhs.m_sections;

lldb/source/Expression/DWARFExpression.cpp

@@ -2559,6 +2559,83 @@ bool DWARFExpression::Evaluate(
       stack.back().SetValueType(Value::eValueTypeScalar);
       break;
 
+    // OPCODE: DW_OP_convert
+    // OPERANDS: 1
+    //      A ULEB128 that is either a DIE offset of a
+    //      DW_TAG_base_type or 0 for the generic (pointer-sized) type.
+    //
+    // DESCRIPTION: Pop the top stack element, convert it to a
+    // different type, and push the result.
+    case DW_OP_convert: {
+      if (stack.size() < 1) {
+        if (error_ptr)
+          error_ptr->SetErrorString(
+              "Expression stack needs at least 1 item for DW_OP_convert.");
+        return false;
+      }
+      const uint64_t die_offset = opcodes.GetULEB128(&offset);
+      Scalar::Type type = Scalar::e_void;
+      uint64_t bit_size;
+      if (die_offset == 0) {
+        // The generic type has the size of an address on the target
+        // machine and an unspecified signedness. Scalar has no
+        // "unspecified signedness", so we use unsigned types.
+        if (!module_sp) {
+          if (error_ptr)
+            error_ptr->SetErrorString("No module");
+          return false;
+        }
+        bit_size = module_sp->GetArchitecture().GetAddressByteSize() * 8;
+        if (!bit_size) {
+          if (error_ptr)
+            error_ptr->SetErrorString("unspecified architecture");
+          return false;
+        }
+        type = Scalar::GetBestTypeForBitSize(bit_size, false);
+      } else {
+        // Retrieve the type DIE that the value is being converted to.
+        // FIXME: the constness has annoying ripple effects.
+        DWARFDIE die = const_cast<DWARFUnit *>(dwarf_cu)->GetDIE(die_offset);
+        if (!die) {
+          if (error_ptr)
+            error_ptr->SetErrorString("Cannot resolve DW_OP_convert type DIE");
+          return false;
+        }
+        uint64_t encoding =
+            die.GetAttributeValueAsUnsigned(DW_AT_encoding, DW_ATE_hi_user);
+        bit_size = die.GetAttributeValueAsUnsigned(DW_AT_byte_size, 0) * 8;
+        if (!bit_size)
+          bit_size = die.GetAttributeValueAsUnsigned(DW_AT_bit_size, 0);
+        if (!bit_size) {
+          if (error_ptr)
+            error_ptr->SetErrorString("Unsupported type size in DW_OP_convert");
+          return false;
+        }
+        switch (encoding) {
+        case DW_ATE_signed:
+        case DW_ATE_signed_char:
+          type = Scalar::GetBestTypeForBitSize(bit_size, true);
+          break;
+        case DW_ATE_unsigned:
+        case DW_ATE_unsigned_char:
+          type = Scalar::GetBestTypeForBitSize(bit_size, false);
+          break;
+        default:
+          if (error_ptr)
+            error_ptr->SetErrorString("Unsupported encoding in DW_OP_convert");
+          return false;
+        }
+      }
+      if (type == Scalar::e_void) {
+        if (error_ptr)
+          error_ptr->SetErrorString("Unsupported pointer size");
+        return false;
+      }
+      Scalar &top = stack.back().ResolveValue(exe_ctx);
+      top.TruncOrExtendTo(type, bit_size);
+      break;
+    }
+
     // OPCODE: DW_OP_call_frame_cfa
     // OPERANDS: None
     // DESCRIPTION: Specifies a DWARF expression that pushes the value of

lldb/source/Utility/Scalar.cpp

@@ -416,6 +416,51 @@ Scalar &Scalar::operator=(llvm::APInt rhs) {
 
 Scalar::~Scalar() = default;
 
+Scalar::Type Scalar::GetBestTypeForBitSize(size_t bit_size, bool sign) {
+  // Scalar types are always host types, hence the sizeof().
+  if (sign) {
+    if (bit_size <= sizeof(int)*8) return Scalar::e_sint;
+    if (bit_size <= sizeof(long)*8) return Scalar::e_slong;
+    if (bit_size <= sizeof(long long)*8) return Scalar::e_slonglong;
+    if (bit_size <= 128) return Scalar::e_sint128;
+    if (bit_size <= 256) return Scalar::e_sint256;
+    if (bit_size <= 512) return Scalar::e_sint512;
+  } else {
+    if (bit_size <= sizeof(unsigned int)*8) return Scalar::e_uint;
+    if (bit_size <= sizeof(unsigned long)*8) return Scalar::e_ulong;
+    if (bit_size <= sizeof(unsigned long long)*8) return Scalar::e_ulonglong;
+    if (bit_size <= 128) return Scalar::e_uint128;
+    if (bit_size <= 256) return Scalar::e_uint256;
+    if (bit_size <= 512) return Scalar::e_uint512;
+  }
+  return Scalar::e_void;
+};
+
+void Scalar::TruncOrExtendTo(Scalar::Type type, uint16_t bits) {
+  switch (type) {
+  case e_sint:
+  case e_slong:
+  case e_slonglong:
+  case e_sint128:
+  case e_sint256:
+  case e_sint512:
+    m_integer = m_integer.sextOrTrunc(bits);
+    break;
+  case e_uint:
+  case e_ulong:
+  case e_ulonglong:
+  case e_uint128:
+  case e_uint256:
+  case e_uint512:
+    m_integer = m_integer.zextOrTrunc(bits);
+    break;
+  default:
+    llvm_unreachable("Promoting a Scalar to a specific number of bits is only "
+                     "supported for integer types.");
+  }
+  m_type = type;
+}
+
 bool Scalar::Promote(Scalar::Type type) {
   bool success = false;
   switch (m_type) {