This originates from https://github.com/rust-lang/rust/issues/46698
This Rust program has a function that takes a struct type as an argument. The compiler is clever enough to rewrite the function so that a subset of the struct's fields are passed as arguments at the machine code level (in this case, the struct consists of two 8 byte fields and only the second is needed). LLVM emits the following DW_AT_location expression for the argument: DW_OP_piece 0x8, DW_OP_reg5 RDI, DW_OP_piece 0x8 The DWARF spec does not explicitly contemplate this sequence, merely saying: The DW_OP_piece operation takes a single operand, which is an unsigned LEB128 number. The number describes the size in bytes of the piece of the object referenced by the preceding simple location description. If the piece is located in a register, but does not occupy the entire register, the placement of the piece within that register is defined by the ABI. The text for DW_OP_bit_piece, on the other hand, does explicitly contemplate this, containing the language: If the location description is empty, the offset doesn’t matter and the DW_OP_bit_piece operation describes a piece consisting of the given number of bits whose values are undefined. I propose adding similar language to the description of DW_OP_piece. e.g. The DW_OP_piece operation takes a single operand, which is an unsigned LEB128 number. The number describes the size in bytes of the piece of the object referenced by the preceding simple location description. If the location description is empty the DW_OP_piece operation describes a piece consisting of the given number of bytes whose values are undefined. If the piece is located in a register, but does not occupy the entire register, the placement of the piece within that register is defined by the ABI. (In practical use, gdb simply errors out on that location expression, while lldb substitutes the undefined bytes with zeroes, neither of which are correct IMO.) - Kyle -- Dwarf-discuss mailing list Dwarf-discuss@lists.dwarfstd.org https://lists.dwarfstd.org/mailman/listinfo/dwarf-discuss
