Added as Issue 240626.1 <https://dwarfstd.org/issues/240626.1.html>.
-cary On Wed, Jun 26, 2024 at 4:06 PM Matthew Lugg via Dwarf-discuss < dwarf-discuss@lists.dwarfstd.org> wrote: > # Add DW_LNS_indirect_line - update `line` to absolute value stored > indirectly > > ## Background > > In many source languages, it is possible for many program-counter > addresses with arbitrary > separation to correspond to the same source line due to features like > templates/generics. When > designing an incremental compiler, the line number program must be > updated when line numbers within > a source file are moved. It would be desirable to have the property that > when moving a source line > corresponding to a large amount of distinct program-counter addresses, > only one line number value in > the DWARF information needs to be updated. For this to be true, the > regions of the line number > program corresponding to each such address must include the line number > of the source construct not > directly, but through an indirect reference. This allows one line number > value stored in the binary > to be shared across arbitrarily many entries in the line number matrix. > > This is not currently possible: all modifications to the `line` register > are given by relative > offsets, and all of these offsets are directly included in the > instruction (or implicit in the case > of a special opcode). > > ## Overview > > Introduce new fields to the line number program header, > `indirect_lines_length` (ULEB128) and > `indirect_lines` (opaque block of bytes containing ULEB128 values). The > `indirect_lines_length` > field is the length in bytes of the `indirect_lines` section, rather > than the number of elements. > Introduce a new standard opcode to the line number program, > `DW_LNS_indirect_line`. This opcode > takes a single ULEB128 operand, which represents a byte offset into the > `indirect_lines` stored in > the header. The effect of this instruction is to set the `line` register > to the ULEB128 value stored > at the given byte offset into `indirect_lines`. Note that > `indirect_lines` is not itself validated > to be a valid sequence of ULEB128 values; decoding only occurs when > `DW_LNS_indirect_line` is used. > This allows an incremental compiler to pre-allocate a large amount of > padding space in > `indirect_lines` to fill in later as needed. > > Note that an incremental compiler would not necessarily wish to use > variable-length integers to > represent this information, since certain changes of line numbers could > cause a line number which > was previously encoded using 1 byte to now require 2. However, since the > stored values need not be > densely packed, an implementation is free to reserve as much space as is > necessary for each entry. > For instance, the downstream Zig compiler (which is the original > motivator for this proposal) may > choose to reserve 4 or 5 bytes for each line number, as line numbers in > Zig source files cannot > exceed 1<<32. The use of ULEB128 allows the compiler to make an > appropriate decision here instead of > codifying such a restriction into the DWARF specification. > > ## Proposed Changes > > Pages and line numbers are given for the 2024-06-16 working draft of > DWARF Version 6, which is the > latest draft at the time of writing. > > 6.2.4 (pg 163; line 27) > > 21. indirect_lines_length (ULEB128) > The length in bytes of the data stored in the `indirect_lines` field. > 22. indirect_lines (block containing ULEB128 entries) > A collection of line numbers, each stored as a ULEB128 integer. > These values are referenced by > DW_LNS_indirect_line instructions to modify the state of the line > number information state > machine. > > The data stored in this field is not checked to be a valid sequence > of ULEB128 entries. The > contained data may include padding bytes or otherwise invalid data. > As such, it is expected that > bytes of this field be accessed only when a DW_LNS_indirect_line > instruction references them. > > 6.2.5.2 (pg 170; line 23) > > 14. DW_LNS_indirect_line > The DW_LNS_indirect_line opcode takes a single unsigned LEB128 > operand. This operand is > interpreted as a byte offset into the `indirect_lines` field of the > line number program header. > An unsigned LEB128 value is read from `indirect_lines` at the given > offset, and this value is > stored into the state machine's `line` register. > > 7.22 (pg 246; table 7.25) > > Opcode name | Value > ----------------------+------- > ... | ... > DW_LNS_indirect_line | 0x0d > > -- > Dwarf-discuss mailing list > Dwarf-discuss@lists.dwarfstd.org > https://lists.dwarfstd.org/mailman/listinfo/dwarf-discuss >
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