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

1 files changed, 77 insertions, 0 deletions

diff --git a/lldb/source/Expression/DWARFExpression.cpp b/lldb/source/Expression/DWARFExpression.cpp
index 44aacfde43c..c11800dc4a0 100644
--- a/lldb/source/Expression/DWARFExpression.cpp
+++ b/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