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140906.1 Cary Coutant Two-Level Line Tables Enhancement Deferred to Ver. 6 Cary Coutant


Section 6.2, pg 128
This proposal introduces a new two-level line number table to
DWARF, splitting the current line number table into two parts: a
"logicals" table, and an "actuals" table. The actuals table would
be optional, and when omitted, the logicals table would
correspond to the DWARF v4 line number table.

In a two-level line number table, the logicals table would
provide a mapping from each logical statement in a program to a
recommended breakpoint location, and the actuals table would
provide a mapping from PC location to a logical statement
represented in the logicals table. The separation allows the line
number table to represent directly the nesting of inline
functions so that consumers would not need to parse the DIE tree
in order to show inlined calls.

For further background information and possible future
extensions, see the DWARF Wiki page:

   http://wiki.dwarfstd.org/index.php?title=TwoLevelLineTables

In Section 6.2 ("Line Number Information"), add the following
non-normative text at the end of the section:

    For optimized code, where instruction scheduling may reorder
    instructions across statement boundaries, where loop
    optimizations may clone code and move instructions across
    loop iterations, and where inlining may introduce
    instructions from an inlined call into the instruction
    stream, an alternate representation with two matrices may be
    used. One matrix (the "logical line table") would have a row
    for each "logical" source statement. If the emitted object
    code for a source statement has been cloned into two
    different locations, there would be a separate row for each.
    The code for a source statement in an inlined function would
    have a separate row for each instance, and would refer to its
    calling context -- i.e., the logical source statement within
    which it was called. The matrix would have columns for:

      * the source file name
      * the source line number
      * the source column number
      * the function name
      * recommended breakpoint location for this statement
      * the calling context (a reference to another row)
      * and so on

    A second matrix (the "actual line table") would have a row
    for each instruction in the emitted object code, and would
    have columns for:

      * the logical statement (a reference to a row in the first
        matrix)
      * whether this instruction is the beginning of a basic
        block
      * and so on

    Both of these matrices can be stored using the same byte-code
    language mentioned above.

In Section 6.2.2 ("State Machine Registers"), add the following
rows to Table 6.2:

    Register name    Meaning
    ------------------------------------------------------------
    context          An unsigned integer indicating a row number
                     in the logical line table that represents
                     the calling context for the current
                     logical statement. Rows are numbered
                     beginning at 1. The value 0 indicates that
                     the current logical statement is not part
                     of an inlined call.

    function_name    An unsigned integer representing the name
                     of the function containing the current
                     logical statement. This value is either a
                     string offset relative to the start of the
                     .debug_str section, or the index of an
                     entry in the .debug_str_offsets section,
                     depending on a flag in the line number
                     program header (see Section 6.2.4).
    ------------------------------------------------------------

Following Table 6.2, add the following notes:

    When only a single line number table is used, the context
    register is not used.

    When both a logical line table and an actual line table are
    used, the basic_block and isa registers are not used in the
    logical line table. The file, column, is_stmt, prologue_end,
    epilogue_begin, discriminator, context, and function_name
    registers are not used in the actual line table. In the
    actual line table, the line register represents a row in the
    logicals table rather than an actual line number. Rows in the
    logical line table are numbered starting at 1.

Following Table 6.2, add the following rows to the (unnumbered)
table showing the state of the registers at the beginning of each
sequence:

    context          0
    function_name    0

In Section 6.2.4 ("The Line Number Program Header"), modify
item 3, insert items 4 and 5, and renumber all subsequent items:

    3.  header_length
        The number of bytes following the header_length field to
        the first byte of the line number program for the logical
        line table (or for the line number table is there is no
        actuals table). In the 32-bit DWARF format, this field is
        a 4-byte unsigned length; in the 64-bit DWARF format,
        this field is an 8-byte unsigned length (see Section 7.4).

    4.  actuals_table_offset
        The offset in bytes from the end of the header to the
        first byte of the line number program for the actual line
        table. If there is only one line number table, this field
        is 0. In the 32-bit DWARF format, this field is a 4-byte
        unsigned length; in the 64-bit DWARF format, this field
        is an 8-byte unsigned length (see Section 7.4).

    5.  function_name_form (ubyte)
        The format of the function_name register. If this value
        is DW_FORM_strp, the function_name register contains an
        offset relative to the start of the .debug_str section.
        If this value  is DW_FORM_strx, the function_name
        register contains an index of an entry in the
        .debug_str_offsets table.

(In Section 6.2.5.1, "Special Opcodes", no change is necessary,
unless we want to explicitly point out that the new context and
function_name registers are left unchanged by the special
opcodes.)

In Section 6.2.5.2 ("Standard Opcodes"), add the following new
opcode:

    13. DW_LNS_inlined_call
        The DW_LNS_inlined_call opcode takes two unsigned LEB128
        operands. It sets the context register to the value of
        the first operand, and the function_name register to the
        value of the second operand.

        This opcode may also be used to set the context register
        to zero, to indicate that a logical statement is not part
        of an inlined call.

In Section 6.2.5.3 ("Extended Opcodes"), add the following new
opcode:

    6.  DW_LNE_set_function_name
        The DW_LNE_set_function_name opcode takes a single
        unsigned LEB128 operand. It sets the function_name
        register to the new value.

--
10/21/2014 - Deferred to DWARF Version 6


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