[Dwarf-Discuss] Use of Location Description operations in DWARF Expressions?

[Michael Eager]

On 3/23/20 6:28 AM, Robinson, Paul via Dwarf-Discuss wrote:
>> From: Dwarf-Discuss <dwarf-discuss-bounces at lists.dwarfstd.org> On Behalf
>> Of Adrian Prantl via Dwarf-Discuss
>>> On Mar 19, 2020, at 5:49 PM, Michael Eager via Dwarf-Discuss <dwarf-
>> discuss at lists.dwarfstd.org> wrote:
>>>
>>> My reading of sections 2.5 & 2.6 is that you cannot have a DW_OP_piece
>> in an DWARF expression.
>>>
>>
>> I wonder if this is an intentional part of the design because of
>> ambiguity/correctness issues or is this just something that happens to
>> fall out of the way the text is worded? I can see how such a restriction
>> might simplify DWARF consumers, but it also seems like an arbitrary
>> restriction for which there may not be a technical reason.
> 
> My intuition (clearly I wasn't there at the time) is that this is like
> a C expression being an rvalue (DWARF expression) or lvalue (location
> description).  Values and locations aren't the same thing.

It is somewhat an L-value vs R-value issue.

You can craft a DWARF expression to extract a value (an R-value) from 
arbitrary memory locations or registers (for example, using DW_OP_and, 
DW_OP_sh?, etc.) and place it on the top of the stack.  A DW_OP_piece 
operator doesn't do this.  (There might be value in an operator which 
extracts a value from a composite location.)

A location (an L-value) which includes potentially multiple register or 
memory locations and multiple DW_OP_piece or DW_OP_bit_piece operations 
can't be evaluated by a simple stack-based expression interpreter.

The design is intentional, AFAIK, not accidental.

I think that the description has become a bit less clear with the 
addition of the Implicit Location Descriptions in Section 2.6.1.1.4, 
which do compute values, rather than locations.  Perhaps these should 
have been described in Section 2.5 as parts of a DWARF expression, not 
as parts of a Location Description.

The description (and implementation) of DWARF expressions and locations 
are somewhat muddled together.  This can be seen in the first sentence 
of Section 2.5:
    DWARF expressions describe how to compute a value or specify a
    location.
A clearer definition would specify that the DWARF expression only 
computes a value, and leaving what that value means (e.g., 
register/memory contents, arbitrary computation, memory address) to the 
context in which the expression is used.  A more precise definition of a 
location, especially a composite location, would help.

Part of this is historical, with the expression evaluation initially 
used to compute a memory address and then extended to accommodate 
composite locations and further extended to compute arbitrary values. 
These extensions were in divergent directions, affecting both location 
composition and value computation.  It would have been better if the two 
concepts were clearly distinguished.

-- 
Michael Eager