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Document the status quo of DIExpression semantics in LangRef.rst

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Adrian Prantl 2017-03-22 18:01:01 +00:00
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llvm/docs/LangRef.rst
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@ -4366,29 +4366,42 @@ parameter, and it will be included in the ``variables:`` field of its
DIExpression
""""""""""""
``DIExpression`` nodes represent DWARF expression sequences. They are used in
:ref:`debug intrinsics<dbg_intrinsics>` (such as ``llvm.dbg.declare``) to
describe how the referenced LLVM variable relates to the source language
variable.
``DIExpression`` nodes represent expressions that are inspired by the DWARF
expression language. They are used in :ref:`debug intrinsics<dbg_intrinsics>`
(such as ``llvm.dbg.declare`` and ``llvm.dbg.value``) to describe how the
referenced LLVM variable relates to the source language variable.
The current supported vocabulary is limited:
- ``DW_OP_deref`` dereferences the working expression.
- ``DW_OP_plus, 93`` adds ``93`` to the working expression.
- ``DW_OP_bit_piece, 16, 8`` specifies the offset and size (``16`` and ``8``
here, respectively) of the variable piece from the working expression.
- ``DW_OP_LLVM_fragment, 16, 8`` specifies the offset and size (``16`` and ``8``
here, respectively) of the variable fragment from the working expression. Note
that contrary to DW_OP_bit_piece, the offset is describing the the location
within the described source variable.
- ``DW_OP_swap`` swaps top two stack entries.
- ``DW_OP_xderef`` provides extended dereference mechanism. The entry at the top
of the stack is treated as an address. The second stack entry is treated as an
address space identifier.
- ``DW_OP_stack_value`` marks a constant value.
DIExpression nodes that contain a ``DW_OP_stack_value`` operator are standalone
location descriptions that describe constant values. This form is used to
describe global constants that have been optimized away. All other expressions
are modifiers to another location: A debug intrinsic ties a location and a
DIExpression together. Contrary to DWARF expressions, a DIExpression always
describes the *value* of a source variable and never its *address*. In DWARF
terminology, a DIExpression can always be considered an implicit location
description regardless whether it contains a ``DW_OP_stack_value`` or not.
.. code-block:: text
!0 = !DIExpression(DW_OP_deref)
!1 = !DIExpression(DW_OP_plus, 3)
!2 = !DIExpression(DW_OP_bit_piece, 3, 7)
!3 = !DIExpression(DW_OP_deref, DW_OP_plus, 3, DW_OP_bit_piece, 3, 7)
!3 = !DIExpression(DW_OP_deref, DW_OP_plus, 3, DW_OP_LLVM_fragment, 3, 7)
!4 = !DIExpression(DW_OP_constu, 2, DW_OP_swap, DW_OP_xderef)
!5 = !DIExpression(DW_OP_constu, 42, DW_OP_stack_value)
DIObjCProperty
""""""""""""""