[Dwarf-discuss] Proposal: Error: DW_OP_entry_value description and examples

[Cary Coutant]

I've added this as issue 230808.1.

https://dwarfstd.org/issues/230808.1.html

-cary

On Tue, Aug 8, 2023 at 9:02 AM Robinson, Paul via Dwarf-discuss
<dwarf-discuss@lists.dwarfstd.org> wrote:
>
> # Error: DW_OP_entry_value description and examples
>
> ## Overview
>
> DW_OP_entry_value provides a way to compute a value as if the value
> had been computed on entry to the current subprogram. Its operand is
> either a DWARF expression, which assumes nothing is already on the
> DWARF stack and leaves one value on the DWARF stack; or, it is a
> register location description, and the content of the register (as it
> was on entry to the subprogram) is implicitly read and pushed on the
> DWARF stack. The register location description is simply a more compact
> representation of the common case of reading a register; for example,
> the producer can emit "DW_OP_reg1" instead of "DW_OP_breg1 0" with the
> same result.
>
> However, the description and examples aren't completely consistent, and
> in some cases are wrong. This proposal corrects the language and examples.
>
> ## Proposed Changes
>
> ### Section 2.5.1.7 Special Operations, p.37
>
> The first sentence of the description of DW_OP_entry_value reads:
>
>     The DW_OP_entry_value operation pushes the value that the described
>     location held upon entering the current subprogram.
>
> A DWARF expression does not describe a location, so this should read:
>
>     The DW_OP_entry_value operation evaluates an expression or register
>     location description as if it had been evaluated upon entering the
>     current subprogram, and pushes the value of the expression or content
>     of the register, respectively.
>
> ### Appendix D.1.3, pp.291-292
>
> There are six examples of DW_OP_entry_value, some of which are not
> consistent with the description and some of which are just plain wrong.
> The six examples are:
>
>     DW_OP_entry_value 2 DW_OP_breg1 0
>     DW_OP_entry_value 1 DW_OP_reg1
>     DW_OP_entry_value 2 DW_OP_breg1 0 DW_OP_stack_value
>     DW_OP_entry_value 1 DW_OP_reg1 DW_OP_stack_value
>     DW_OP_entry_value 3 DW_OP_breg4 16 DW_OP_deref DW_OP_stack_value
>     DW_OP_entry_value 1 DW_OP_reg5 DW_OP_plus_uconst 16
>
> The first two illustrate the smaller expression allowed by the special
> case for a register location description; they are fine.
>
> The third is just wrong and should be deleted. DW_OP_stack_value converts
> a memory location description into a value, but what precedes it is not
> a memory location description. Without the stack_value, it's identical to
> the first example.
>
> The fourth is just wrong and should be deleted. DW_OP_reg1 names a register
> but pushes nothing on the stack, so DW_OP_stack_value is incorrect; and
> without that, it's identical to the second example.
>
> The fifth should not have DW_OP_stack_value at the end; the expression
> already leaves a value on the stack.
>
> The sixth is incorrectly using DW_OP_reg5, which is allowed in a location
> description but not in a simple DWARF expression. The expression could be:
>     DW_OP_entry_value 2 DW_OP_breg5 16
>
> The textual description of the sixth example should also be revised.
> Currently it reads:
>     The address of the memory location is calculated by adding 16 to the
>     value contained in register 5 upon erringing the current subprogram.
>     <i>Note that unlike the previous DW_OP_entry_value examples, this one
>     does not end with DW_OP_stack_value.</i>
>
> This should be changed to:
>     Add 16 to the value register 5 had upon entering the current subprogram
>     and push the result.
>
> (The italicized text after the sixth example should be removed entirely;
> there is no reason ever to use DW_OP_stack_value in these expressions.)
>
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