Issue 221105.1: Add a mechanism for specifying subprogram return value locations

Author:	Kyle Huey
Champion:	Andrew Cagney
Date submitted:	2022-11-05
Date revised:	2024-03-04
Date closed:
Type:	Enhancement
Status:	Open
DWARF Version:	6

Section 3.3.2, pg 78

Background

A debug feature is to print the result of a subroutine at exit.

One way this is implemented is:

run the called subroutine until it exits

i.e., the return instruction has been completed and the program is in the calling subprogram
extract the result from the calling subprogram's frame

While DWARF makes available the type of the return value (DW_AT_type attached to DW_TAG_subprogram, as detailed in Section 3.3.2.) it does not provide any information about the return value's location. Debuggers instead use the platform's calling convention to determine the return value's location.

However, for subprograms that do not follow the standard calling convention (i.e., (hopefully) have DW_AT_calling_convention DW_CC_nocall attached to DW_TAG_subprogram), locating the return value may not be possible.

Inline functions are similar (using DW_TAG_inlined_subroutine via DW_AT_abstract_origin). Run the code block until the inlined function has "exited", and then extract the returned value. However, here things are even more challenging. It is very unlikely that the inlined and optimized code ever follows anything approaching an ABI.

Proposal

Attach a location description to the subprogram's debug information describing the location of the return value immediately after the subprogram has returned.

Why not describe the value at the return instruction?

this is addressing the above existing practice
informally, the completion of the return instruction acts as a synchronization point
for delayed branch and VLIW architectures, it is often only after the completion of the return instruction (bundle) that the subprogram result is in a known location

Change Details

Called Subprograms

In 3.3.2 Subroutine and Entry Point Return Types, change the section heading to:

3.3.2 Subroutine and Entry Point Return Type and Value

At the end of the section, which currently reads:

If the subroutine or entry point is a function that returns a value, then its debugging information entry has a DW_AT_type attribute to denote the type returned by that function.

[non-normative text]

append the text:

A subroutine or entry point that is a function that returns a value and has a DW_AT_type attribute may also have a DW_AT_location attribute. This attribute specifies the location of the return value after the called function has returned and the calling subprogram is about to be resumed.

[begin non-normative]

Since the called subprogram has returned, the location description is relative to the calling subprogram. For instance, for a sliding window architecture such as SPARC, the location description will refer to the output registers of the caller, and not the input registers of the callee.

Since the called subprogram is expected to return the value in a specific location, the DW_AT_location should be a Single Location Description.

When the both the attribute and DW_CC_nocall are both omitted, the subprogram is assumed to comply with ABI calling conventions.

[end non-normative]

Notes:

should this attribute get a new name other than DW_AT_location or stick with that since it goes with DW_AT_type?
Since DW_AT_location is useless with out DW_AT_type I made DW_AT_type a predicate. Should it instead be non-normative text?
Is my assumption that the DW_AT_location can only be for a Single Location (i.e., not a location list) correct?
If a subprogram were to be included in multiple objects then, presumably, each would define its own DW_AT_location.
This is a technical violation of:

Single location descriptions, [...]. They are sufficient for describing the location of any object as long as its lifetime is either static or the same as the lexical block that owns it, [...]

Call Sites

In 3.4.2 Call Site Parameters, change the section title to:

3.4.2 Call Site Parameters and Return Value

(or a new section?)

At the end of the section add the text:

A call site entry that has a DW_AT_type attribute for the type of the called function, may have a DW_AT_location attribute specifying the location of the return value after the called function has returned and the calling subprogram is about to be resumed.

[non-normative] Since Call Site Entry's DW_AT_type attribute can describe an abstract function, the return type and location may be different between the Call Site and the subprogram.

Inlined Subprograms

In 3.3.8.1 Abstract Instances, in the non-normative text:

For example, the DW_AT_low_pc, [...] attributes typically should be omitted;

add DW_AT_location.

In 3.3.8.2 Concrete Instances, following the paragraph:

An inlined subroutine entry may have a DW_AT_const_expr attribute, [...], represented as it would be on the target architecture.

Add the paragraph and non-normative text:

An inlined subroutine entry that is a function that returns a value and and has DW_AT_type defined in the Abstract Instance may have a DW_AT_location attribute. This attribute describes the location of the return value immediately after the concrete instance has completed execution of the contiguous or non-contiguous machine instructions generated for the inlined subroutine.

[non-normative] Since the concrete instance may consist of non-contiguous instructions, DW_AT_location may be defined using a Location List. Within that list, each bounded entry identifies an instruction immediately after the concrete instance has completed execution. That list of locations may not be exhaustive.

Notes:

Since DW_AT_location is useless with out DW_AT_type I made DW_AT_type a predicate. Should it instead be non-normative text?

Appendix A. Attributes by Tag (Informative)

Under TAG name DW_TAG_subprogram, add DW_AT_location to the applicable attributes.

Appendix D

Questions:

do we want to attempt an example?

2023-11-05: Rewrote.

2024-01-08: Revised with more non-normative text.

2024-02-05: Added section on call sites.

2024-03-04: Revised to add ABI fallback and address questions about call sites.