[flang-dev] Question WRT to debug info in flang.

[Sourabh Singh Tomar via flang-dev]

Hi Gary,

Thank you for quick response!
Since I was working/investigating this, so before filing this bug could you
please help me clarify a couple of things? WRT to flang.

Are their Blocks(Lexical Blocks) even present at ILM or ILI level, because
consider the case of *value* removed(pass by reference) flang also emit
*DW_TAG_lexical_block* in this case also.

And these lexical blocks have nearly ranges as the parent subroutine. To be
exact high_pc are same, but low_pc of lexical block corresponds to prologue
end of the subroutine.

0x0000002f:   DW_TAG_subprogram
                DW_AT_low_pc    (0x0000000000201ae0)
                DW_AT_high_pc   (0x0000000000201b75)
0x000000**:   DW_TAG_lexical_block
                DW_AT_low_pc    (0x0000000000201a**) -- marking prologue
end of subroutine
                DW_AT_high_pc   (0x0000000000201b75)

Correct me if I'm wrong here, Fortran Doesn't have lexical scoping? If it
doesn't have we may have to strip out the *Lexical Block* creation part, in
debug info.

Regarding bug filing should we create a specific bug WRT to *Lexical block*
or a something more generic *unable to retrieve formal parameter in case of
pass  by value arguments* or third option will be to keep both of these
issues orthogonal to each other.

Please share your thoughts on this.

Thank You,
Sourabh Singh Tomar.




On Wed, Mar 25, 2020 at 3:47 AM Gary Klimowicz <gklimowicz at nvidia.com>
wrote:

> Hi, Sourabh.
>
>
>
> We've confirmed the bug. It can be worked around by removing the VALUE
> attribute.
>
>
>
> Can you file an issue for us on the (current) Flang GitHub site?
>
> https://github.com/flang-compiler/flang/issues
>
>
>
> Thank you.
>
>
>
>
>
> On 3/24/20, 12:52 PM, "flang-dev on behalf of Sourabh Singh Tomar via
> flang-dev" <flang-dev-bounces at lists.llvm.org on behalf of
> flang-dev at lists.llvm.org> wrote:
>
>
>
> *External email: Use caution opening links or attachments*
>
>
>
> Hello Everyone,
>
>
>
> For a typical Fortran program compiled with flang with -g(debuginfo
> enabled), I'm noticing *DW_TAG_lexical_block* is getting created as child
> of the *DW_TAG_subprogram*.
>
> As a consequence of this debuggers like GDB  is not able to show the
> arguments of the subprogram.
>
> Program snippet -
>
> subroutine sub(foo)
>         integer, value :: foo
>         print*,foo
>       end subroutine
> program main
>       interface
>         subroutine sub(a)
>               integer, value :: a
>          end subroutine
>       end interface
>              call sub(11)
>  end program
>
> --------------------------------
>
> Snippet from GDB --
>
> Breakpoint 1, sub () at test.f90:1
> 1             subroutine sub(foo)
> (gdb) info args
> No arguments.
>
> -----------------------------------
>
> Furthermore, *DW_TAG_formal_parameter* is created as a child of this
> *DW_TAG_lexical_scope*. Hence GDB is showing arguments of the subprogram as
> locals variables.
>
> Snippet DWARF--
>
> 0x0000002f:   DW_TAG_subprogram
>
> DW_AT_name      ("sub")
>
>  DW_AT_low_pc    (0x00000000004011b0)
>  DW_AT_high_pc   (0x0000000000401245
> .....
> 0x00000049:     DW_TAG_lexical_block
>
>  DW_AT_low_pc    (0x00000000004011b0)
>  DW_AT_high_pc   (0x0000000000401245
>  ....
> 0x0000005a:       DW_TAG_formal_parameter
>
>   DW_AT_name      ("foo")
>
> 0x0000005a:       DW_TAG_variable
>
>   DW_AT_name      ("foo")
>
> ...
>
> ---------------------------------------
>
> Is this behavior intended ?
>
>
>
> Anyone working on this area, Please share your understanding on this!
>
>
>
> *Please note here that same hierarchy of debug info is also present at
> LLVM IR level, hence the final DWARF is shared here.
>
>
>
> Thank You,
>
> Sourabh.
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