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050701.1 Ron Brender and Jim Blandy Improve description of DW_OP_piece, with small extension Improvement Accepted Ron Brender


Section: 2.4.1.6, 2.5
Page: 21-25

This proposal is the result of a joint effort between Jim Blandy
and myself over the last week of June. Additional discussion may
be found in the DWARF-WORKGROUP mail archive under the Subj: 060628.1


brender@gemevn.zko.dec.com (Ron 603-884-2088) writes:
> One issue that remains unanswered is in
>
>     "Keith.Walker@a 26-JAN-2001  DW_OP_piece and optimisations
>
> which comes down to how to describe an object one (or more) of whose
> pieces, but not all, is optimized away. I had no suggestion then and
> still offer none.
>
>
> PROPOSAL
> --------

I like where all this is going.  Let me offer something which is a bit
more formal (in the sense of "related to form", not in the sense of
"stuffy"), and which I think also addresses Keith Walker's concerns.

Keep these two changes as they are:

>  1) In Section 2.4.1.6, remove bullet 1. Keep the plural section
>     title ("Special Operations") even though there is now only one,
>     but change the introductory sentence to read
>
>       "There is one special operation currently defined:"
>
>
>  2) In Section 2.5.1, delete the (third) introductory sentence
>     that reads
>
>       "Each register name operator must be used alone..."

And keep these --- but with some deletions, marked with <omit>...</omit>:

>  3) Between Sections 2.5.1 and 2.5.2, insert a new section titled
>     "Composition Operators". (Yes, plural--with or without
>     DW_OP_bit_piece.)
>
>  4) Begin this new section with
>
>       "A composition operator describes the location of a piece
>       of a value which may be contained in part of a register or
>       stored in more than one location; it does not compute a
>       value nor store any result on the DWARF stack. <omit>There
>       may be one or more composition operators in a single
>       DWARF location expression. A series of such operators
>       describes the parts of a value in memory address order.
>
>       Each composition operator is immediately preceeded by
>       a register name operator or an addressing operation
>       which describes the location where part of the resultant
>       value is contained.</omit>
>
>  5) Follow this with the text removed from 2.4.1.6 per 1) above.
>
>  6) If Issue 050518.1 is adopted, the DW_OP_bit_piece operator
>     can follow as bullet 2.

Then:

7) After "Composition Operators", insert a second new section titled
   "Simple Location Expressions":

        A <definition>simple location expression</definition> describes
        the location of one contiguous piece of a value.  A simple
        location expression is either:

        - a non-empty DWARF expression, whose value is the address of
          a piece of an object in memory,

        - a DWARF expression containing zero operations, which
          represents a piece of an object that is present in the
          source code but not in the object code (perhaps due to
          optimization), or

        - a register name operator, appearing alone as a single
          opcode, without other DWARF expression operators,
          representing a piece of the object located in the given
          register.

8) Replace the first paragraph of "Location Expressions" with the
   following text:

       A location expression is either:

       - a simple location expression, representing an object which
         exists in one piece at the given location, or

       - two or more simple location expressions, each of which is
         followed by one composition operator.  Each simple location
         expression describes the location of one piece of the object;
         each composition operator describes which part of the object
         is located there.

   (If issue 050518.1 is adopted, then that last sentence might need
   to be loosened a little bit.)

   I think this would address Keith Walker's concerns: we simply allow
   the existing designation for an unavailable value --- an empty
   expression --- to be part of a piece list, like anything else.

9) Add an example to section 2.5.3:

        DW_OP_reg0 DW_OP_piece 4 DW_OP_piece 4 DW_OP_fbreg -12 DW_OP_piece 4

                A twelve-byte value whose first four bytes reside in
                register zero, whose middle four bytes are unavailable
                (perhaps due to optimization), and whose last four
                bytes are in memory, twelve bytes before the frame
                base.

   This example shows an empty DWARF expression being used as a piece,
   and also shows a piece that lives in memory, of which we have no
   other examples.

==================================

Revised proposal:


2.4.1.6	Special Operations

There is one special operation currently defined: 

    1.  DW_OP_nop 
    The DW_OP_nop operation may be a place holder. It has no effect on
    the location stack or any of its values. 

...

2.5.2	Composition Operators

A composition operator describes the location of a piece of a
value which may be contained in part of a register or stored in
more than one location; it does not compute a value nor store any
result on the DWARF stack. There may be one or more composition
operators in a single DWARF location expression. A series of such
operators described the parts of a value in memory address order.

Each composition operator is immediately preceeded by a register
name operator or an addressing operation which describes the
location where part of the resultant value is contained.

    1.	DW_OP_piece 
    DW_OP_piece 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 DWARF expression whose result is at
    the top of the stack. 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.

    
    Many compilers store a single variable in sets of registers, or
    store a variable partially in memory and partially in registers.
    DW_OP_piece provides a way of describing how large a part of a
    variable a particular DWARF expression refers to.
    

    2.	DW_OP_bit_piece
    DW_OP_bit_piece takes two operands. The first is an unsigned
    LEB128 number that gives the size in bits of the piece. The
    second is an unsigned LEB128 number that gives the offset in bits
    from the location defined by the preceding DWARF location
    expression.

    Interpretation of the offset depends on the kind of location
    expression. If the location is null, the offset doesnt matter and
    the DW_OP_bit_piece operator describes a piece consisting of the
    given number of bits whose values are undefined. If the location
    is a register, the offset is from the least significant bit end
    of the register. If the location is a memory address, the
    DW_OP_bit_piece operator describes a sequence of bits relative to
    the location whose address is on the top of the DWARF stack using
    the bit numbering and direction conventions that are appropriate
    to the current language on the target system.


DW_OP_bit_piece is used instead of DW_OP_piece when the piece to
be assembled into a value or assigned to is not byte-sized or is
not at the start of a register or addressable unit of memory.


2.5.3	Simple Location Expressions

A simple location expression describes the location of one
contiguous piece of a value. A simple location expression is
either:

    1.	A non-empty DWARF expression, whose value is the address
    of the piece of the value in memory

    2.	A DWARF expression containing no operations, which
    represents a piece of an object that is present in the source
    code but not in the object code (perhaps due to optimization), or

    3.	A register name operator, appearing alone as a single
    opcode, without other DWARF expression operators, representing a
    piece of the object located in the given register.

2.5.4	Location Expressions

A location expression is either:

    1.	A simple location expression, representing an object
    which exists in one piece at the given location, or

    2.	Two or more simple location expressions, each of which is
    followed by one composition operator. Each simple location
    expression describes the location of one piece of the object;
    each composition operator describes which part of the object is
    located there. Each simple location expression that is a DWARF
    expression is evaluated independently of any others (as though on
    its own separate stack).

In the case of locations used for structure members, the
computation implicitly pushes the base address of the immediately
containing structure on the stack before evaluation of the
addressing operation. 

2.5.5	Example Location Expressions

...

DW_OP_reg0 DW_OP_piece 4 DW_OP_piece 4 DW_OP_fbreg -12 DW_OP_piece 4
    A twelve byte value whose first four bytes reside in register
    zero, whose middle four bytes are unavailable (perhaps due to
    optimizaqtion), and whose last four bytes are in memory, 12 bytes
    before the frame base.

Accepted as revised.



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