Fix a buffer-size bug when the first DW_OP_piece is undefined

and document the shortcomings of LLDB's partially defined DW_OP_piece
handling.

This would manifest as "DW_OP_piece for offset foo but top of stack is
of size bar".

rdar://problem/46262998

Differential Revision: https://reviews.llvm.org/D72880

(cherry picked from commit f55ab6f90b7317a6bb85303a6102702bdae1199e)

1 files changed, 5 insertions, 1 deletions

diff --git a/lldb/source/Expression/DWARFExpression.cpp b/lldb/source/Expression/DWARFExpression.cpp
index 0f8eea754c2..08cc70c201f 100644
--- a/lldb/source/Expression/DWARFExpression.cpp
+++ b/lldb/source/Expression/DWARFExpression.cpp
@@ -2071,6 +2071,10 @@ bool DWARFExpression::Evaluate(
           // not available. Fill with zeros for now by resizing the data and
           // appending it
           curr_piece.ResizeData(piece_byte_size);
+          // Note that "0" is not a correct value for the unknown bits.
+          // It would be better to also return a mask of valid bits together
+          // with the expression result, so the debugger can print missing
+          // members as "<optimized out>" or something.
           ::memset(curr_piece.GetBuffer().GetBytes(), 0, piece_byte_size);
           pieces.AppendDataToHostBuffer(curr_piece);
         } else {
@@ -2193,8 +2197,8 @@ bool DWARFExpression::Evaluate(
               return false;
             }
           }
-          op_piece_offset += piece_byte_size;
         }
+        op_piece_offset += piece_byte_size;
       }
     } break;