002-clarifications-loc.txt

Part 2: Clarifications for Location Descriptions

This is part 2 in a series of proposals related to support for debugging
on heterogeneous architectures. Although this proposal is not directly
related to heterogeneous architectures, it lays the groundwork for
subsequent proposals in this series.


BACKGROUND
----------

This proposal is editorial in nature: it does not intend to change the
meaning of any DWARF constructs, but merely to clarify aspects of
DWARF expression evaluation that were unclear to the teams implementing
DWARF consumers.

The changes proposed below alter the taxonomy of location descriptions.


PROPOSED CHANGES
----------------

In Section 2.6 Location Descriptions, in the first (non-normative)
paragraph, change "whose location changes over the object's lifetime" to
"whose location changes over the object's lifetime, and may reside at
multiple locations simultaneously during parts of an object's lifetime."

Replace these paragraphs:

    --------------------------------------------------------------------
    Information about the location of program objects is provided by
    location descriptions. Location descriptions can be either of two
    forms:

    1. Single location descriptions...

    2. Location lists...
    --------------------------------------------------------------------

With the following:

    --------------------------------------------------------------------
    Information about the location of program objects is provided by
    location descriptions.

    Location descriptions can consist of one or more single location
    descriptions.

    A single location description specifies the location storage that
    holds a program object and a position within the location storage
    where the program object starts. The position within the location
    storage is expressed as a bit offset relative to the start of the
    location storage.

    A location storage is a linear stream of bits that can hold values.
    Each location storage has a size in bits and can be accessed using a
    zero-based bit offset. The ordering of bits within a location
    storage uses the bit numbering and direction conventions that are
    appropriate to the current language on the target architecture.

    There are five kinds of location storage:

     1. memory location storage

        Corresponds to the target architecture memory address spaces.

     2. register location storage

        Corresponds to the target architecture registers.

     3. implicit location storage

        Corresponds to fixed values that can only be read.

     4. undefined location storage

        Indicates no value is available and therefore cannot be read or
        written.

     5. composite location storage

        Allows a mixture of these where some bits come from one location
        storage and some from another location storage, or from disjoint
        parts of the same location storage.

        > [For further discussion...]
        > It may be better to add an implicit pointer location storage
        > kind used by the DW_OP_implicit_pointer operation. It would
        > specify the debugging information entry and byte offset
        > provided by the operations.

    [non-normative] Location descriptions are a language independent
    representation of addressing rules.

      * [non-normative] They can be the result of evaluating a debugging
        information entry attribute that specifies an operation expression of
        arbitrary complexity. In this usage they can describe the
        location of an object as long as its lifetime is either static
        or the same as the lexical block (see 3.5 Lexical Block Entries)
        that owns it, and it does not move during its lifetime.

      * [non-normative] They can be the result of evaluating a debugging
        information entry attribute that specifies a location list expression.
        In this usage they can describe the location of an object that has
        a limited lifetime, changes its location during its lifetime, or
        has multiple locations over part or all of its lifetime.

    If a location description has more than one single location
    description, the DWARF expression is ill-formed if the object value
    held in each single location description’s position within the
    associated location storage is not the same value, except for the
    parts of the value that are uninitialized.

    [non-normative] A location description that has more than one single
    location description can only be created by a location list
    expression that has overlapping program location ranges, or certain
    expression operations that act on a location description that has
    more than one single location description. There are no operation
    expression operations that can directly create a location
    description with more than one single location description.

    [non-normative] A location description with more than one single
    location description can be used to describe objects that reside in
    more than one piece of storage at the same time. An object may have
    more than one location as a result of optimization. For example, a
    value that is only read may be promoted from memory to a register
    for some region of code, but later code may revert to reading the
    value from memory as the register may be used for other purposes.
    For the code region where the value is in a register, any change to
    the object value must be made in both the register and the memory so
    both regions of code will read the updated value.

    [non-normative] A consumer of a location description with more than
    one single location description can read the object’s value from any
    of the single location descriptions (since they all refer to
    location storage that has the same value), but must write any
    changed value to all the single location descriptions.

    Updating a location description L by a bit offset B is defined as
    adding the value of B to the bit offset of each single location
    description SL of L. It is an evaluation error if the updated bit
    offset of any SL is less than 0 or greater than or equal to the size
    of the location storage specified by SL.

    The evaluation of an expression may require context elements to
    create a location description. If such a location description is
    accessed, the storage it denotes is that associated with the context
    element values specified when the location description was created,
    which may differ from the context at the time it is accessed.

    [non-normative] For example, creating a register location
    description requires the thread context: the location storage is for
    the specified register of that thread. Creating a memory location
    description for an address space may required a thread context: the
    location storage is the memory associated with that thread.

    If any of the context elements required to create a location
    description change, the location description becomes invalid and
    accessing it is undefined.

    [non-normative] Examples of context that can invalidate a location
    description are:

      * [non-normative] The thread context is required and execution causes the
        thread to terminate.

      * [non-normative] The call frame context is required and further execution
        causes the call frame to return to the calling frame.

      * [non-normative] The program location is required and further execution
        of the thread occurs. That could change the location list entry or call
        frame information entry that applies.

      * [non-normative] An operation uses call frame information:

          * [non-normative] Any of the frames used in the virtual call frame
            unwinding return.

          * [non-normative] The top call frame is used, the program location is
            used to select the call frame information entry, and further
            execution of the thread occurs.

    [non-normative] A DWARF expression can be used to compute a location
    description for an object. A subsequent DWARF expression evaluation
    can be given the object location description as the object context
    or initial stack context to compute a component of the object. The
    final result is undefined if the object location description becomes
    invalid between the two expression evaluations.

    A change of a thread’s program location may not make a location
    description invalid, yet may still render it as no longer
    meaningful. Accessing such a location description, or using it as
    the object context or initial stack context of an expression
    evaluation, may produce an undefined result.

    [non-normative] For example, a location description may specify a
    register that no longer holds the intended program object after a
    program location change. One way to avoid such problems is to
    recompute location descriptions associated with threads when their
    program locations change.
    --------------------------------------------------------------------

Remove the top-level Section 2.6.1 Single Location Descriptions and
2.6.1.1 Simple Location Descriptions, and promote the subsections of
2.6.1.1 to the level of 2.6.1 (i.e., 2.6.1.1.1 becomes 2.6.1).

In Section 2.6.1.1.1 (now 2.6.1), change “Empty Location Descriptions”
to “Undefined Location Description Operations,” and replace the single
paragraph in that section with the following:

    --------------------------------------------------------------------
    [non-normative] The undefined location storage represents a piece or
    all of an object that is present in the source but not in the object
    code (perhaps due to optimization). Neither reading nor writing to
    the undefined location storage is meaningful.

    An undefined location description specifies the undefined location
    storage. There is no concept of the size of the undefined location
    storage, nor of a bit offset for an undefined location description.
    The DW_OP_*piece operations can implicitly specify an undefined
    location description, allowing any size and offset to be specified,
    and results in a part with all undefined bits.
    --------------------------------------------------------------------

Promote Section 2.6.1.2 Composite Location Descriptions up one level, so
that it becomes Section 2.6.5.

Renumber Section 2.6.2 Location Lists to become 2.6.6.