[llvm-dev] DW_OP_implicit_pointer design/implementation in general

[Alok Sharma via llvm-dev]

Hi folks,

I am pushing a PoC patch https://reviews.llvm.org/D70833 for review which
includes the case when temporary is promoted.

For such cases it generates IR as

  call void @llvm.dbg.derefval(metadata i32 3, metadata !25, metadata
!DIExpression(DW_OP_LLVM_explicit_pointer, DW_OP_LLVM_arg0)), !dbg !32

And llvm-darfdump output looks like

-------------
0x0000007b:     DW_TAG_inlined_subroutine
                  DW_AT_abstract_origin (0x0000004f "_Z4sinkRKi")
                  DW_AT_low_pc  (0x00000000004004c6)
                  DW_AT_high_pc (0x00000000004004d0)
                  DW_AT_call_file
("/home/alok/openllvm/llvm-project_derefval/build.d/david.cc")
                  DW_AT_call_line       (10)
                  DW_AT_call_column     (0x03)

0x00000088:       DW_TAG_formal_parameter
                    DW_AT_location      (indexed (0x0) loclist = 0x00000010:
                       [0x00000000004004c6, 0x00000000004004d4):
DW_OP_explicit_pointer, DW_OP_lit3)
                    DW_AT_abstract_origin       (0x00000055 "p")
------------

Please note that DW_OP_explicit_pointer denotes that following value
represents de-referenced value of optimized out pointer. With necessary
changes in LLDB debugger this dwarf info can help to detect the explicit
de-referenced value of 'p'.

Hi David,

Should we keep on working for the above case separately and resume the
review of implicit pointer independently now, which is updated with many
suggestions from this discussion?

Regards,
Alok


On Wed, Nov 20, 2019 at 11:24 PM Jeremy Morse <jeremy.morse.llvm at gmail.com>
wrote:

> Hi,
>
> For a new way of representing things,
>
> Adrian wrote:
> > llvm.dbg.value_new(DILocalVariable("y"), DIExpression(DW_OP_LLVM_arg0,
> DW_OP_LLVM_arg1, DW_OP_plus),
> >                    %ptr, %ofs)
>
> I think this would be great -- there're definitely some constructs
> created by the induction-variables pass and similar where one could
> recover an implicit variable value, if you could for example subtract
> one pointer from another.
>
> With the current model of storing DIExpressions as a vector of
> opcodes, it might become a pain to salvage a Value that gets optimised
> out --in the example, if %ofs were salvaged, presumably
> DW_OP_LLVM_arg1 could have to be replaced with several extra
> operations. This isn't insurmountable, but I've repeatedly shied away
> from scanning through DIExpressions to patch them up. A vector of
> opcodes is the final output of the compiler, IMHO richer metadata
> should be used in the meantime.
>
> IMHO the implicit pointer work doesn't need to block on this. As said
> my mild preference would be for a new intrinsic for this form of
> variable location.
>
> ~
>
> Inre PR37682,
>
> > I’ve been reminded of PR37682, where a function with a reference
> parameter might spend all its time computing the “referenced” value in a
> temp, and only move the final value back to the referenced object at the
> end.  This is clearly a situation that could benefit from
> DW_OP_implicit_pointer, and there is really no other-object DIE for it to
> refer to.  Given the current spec, the compiler would need to produce a
> DW_TAG_dwarf_procedure for the parameter DIE to refer to.  Appendix D
> (Figure D.61) has an example of this construction, although it’s a more
> contrived source example.
>
> This has been working through my mind too, and I think it's slightly
> different to what implicit_pointer is trying to achieve. In the case
> implicit_pointer is designed for, it's a strict improvement in debug
> experience because you're recovering information that couldn't be
> expressed. However for PR37682 it's a trade-off between whether the
> user might want to examine the pointer, or the pointed-at integer:
> AFAIUI, we can only express one of the two, not both. Wheras for
> mem2reg'd variables referred to by DIE, there is never a pointer to be
> lost.
>
> I think my preference would always be to see temporarily-promoted
> values as there's no other way of observing them, but others might
> disagree.
>
> --
> Thanks,
> Jeremy
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20191129/368d2139/attachment.html>