[Bug 1192742] New: Fix DWARF for variable references

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=1192742

Bug ID: 1192742
           Summary: Fix DWARF for variable references
           Product: Fedora
           Version: rawhide
         Component: golang
          Assignee: [e-mail]
          Reporter: [e-mail]
        QA Contact: [e-mail]
                CC: [e-mail], [e-mail],
                    [e-mail], [e-mail],
                    [e-mail], [e-mail], [e-mail]

Created attachment 991800
  --> https://bugzilla.redhat.com/attachment.cgi?id=991800&action=edit
A bit hacky fix

Description of problem:
https://sourceware.org/gdb/wiki/GoDebugging#golang_Wrong_DW_TAG_variable
golang provides &name variable names and the DWARF interface uses one more
pointer than in the source.

Version-Release number of selected component (if applicable):
golang-1.4.1-1.fc22.x86_64

How reproducible:
Always.

Steps to Reproduce:
-------------------
package main
func main () {
  var i int
  ip:=&i
  ipp:=&ip
  go func() {
      **ipp++
  }()
}
-------------------

Actual results:
(gdb) b 8
(gdb) r
(gdb) p i
No symbol "i" in current context.
(gdb) p &i
No symbol "i" in current context.
(gdb) p '&i'
Invalid character constant.
(gdb) p ' &i'
$1 = (int *) 0xc20800a080
(gdb) p *' &i'
$2 = 0
(gdb) p ' &ip'
$3 = (int **) 0xc20803a000
(gdb) p *' &ip'
$4 = (int *) 0xc20800a080
(gdb) p **' &ip'
$5 = 0
(gdb) p ' &ipp'
$6 = (int ***) 0xc20803a008
(gdb) p *' &ipp'
$7 = (int **) 0xc20803a000
(gdb) p **' &ipp'
$8 = (int *) 0xc20800a080
(gdb) p ***' &ipp'
$9 = 0

Expected results:
(gdb) b 8
(gdb) r
(gdb) p i
$1 = (int &) @0xc20800a080: 0
(gdb) p ip
$2 = (int *&) @0xc20803a000: 0xc20800a080
(gdb) p *ip
$3 = 0
(gdb) p ipp
$4 = (int **&) @0xc20803a008: 0xc20803a000
(gdb) p *ipp
$5 = (int *) 0xc20800a080
(gdb) p **ipp
$6 = 0
(gdb) whatis i
type = int &
(gdb) whatis ip
type = int *&
(gdb) whatis ipp
type = int **&

Additional info:
But you said golang upstream is writing some new linker so maybe this hack is
not much relevant for upstream.  DW_AT_name of the reference types is IMO
incorrect but I do not think it matters much for GDB; for 'ipp' with this patch
there is:
    <fe78>   DW_AT_name        : &(***int)

[Bug 1192742] Fix DWARF for variable references

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=1192742

Jan Kratochvil  changed:

What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |[e-mail]

--- Comment #1 from Jan Kratochvil  ---
BTW that DW_AT_name should not be there at all for such DW_TAG_reference_type,
according to DWARF spec:

If a name has been given to the modified type in the source program, then the
corresponding modified type entry has a DW_AT_name attribute whose value is a
null-terminated string containing the modified type name as it appears in the
source program.

[Bug 1192742] Fix DWARF for variable references

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=1192742

--- Comment #2 from Vincent Batts  ---
so here is a comparison of your reproduction on latest stable (go1.4.2) and
master (c25c371 / go1.5-dev)

go version go1.4.2 linux/amd64

(gdb) b 8
Breakpoint 1 at 0x400c8e: /home/vbatts/bz1192742/bz1192742.go:8. (2 locations)
(gdb) r
Starting program: /home/vbatts/bz1192742/bz1192742

Breakpoint 1, main.main () at /home/vbatts/bz1192742/bz1192742.go:8
8         }()
(gdb) p i
No symbol "i" in current context.
(gdb) p &i
No symbol "i" in current context.
(gdb) p ' &i'
$1 = (int *) 0xc20800a080
(gdb) p *' &i'
$2 = 0
(gdb) p ' &ip'
$3 = (int **) 0xc20803c000
(gdb) p *' &ip'
$4 = (int *) 0xc20800a080
(gdb) p **' &ip'
$5 = 0
(gdb) p ' &ipp'
$6 = (int ***) 0xc20803c008
(gdb) p *' &ipp'
$7 = (int **) 0xc20803c000
(gdb) p **' &ipp'
$8 = (int *) 0xc20800a080
(gdb) p ***' &ipp'
$9 = 0
(gdb) whatis ' &i'
type = int *
(gdb) whatis ' &ip'
type = int **
(gdb) whatis ' &ipp'
type = int **

go version devel +c25c371 Thu Feb 19 17:00:30 2015 +0000 linux/amd64

(gdb) b 8
Breakpoint 1 at 0x400c66: /home/vbatts/bz1192742/bz1192742.go:8. (2 locations)
(gdb) r
Starting program: /home/vbatts/bz1192742/bz1192742

Breakpoint 1, main.main () at /home/vbatts/bz1192742/bz1192742.go:8
8         }()
(gdb) p i
No symbol "i" in current context.
(gdb) p &i
No symbol "i" in current context.
(gdb) p ' &i'
$1 = (int *) 0xc20800a080
(gdb) p *' &i'
$2 = 0
(gdb) p ' &ip'
$3 = (int **) 0xc20801e008
(gdb) p *' &ip'
$4 = (int *) 0xc20800a080
(gdb) p **' &ip'
$5 = 0
(gdb) p ' &ipp'
No symbol " &ipp" in current context.
(gdb) p *' &ipp'
No symbol " &ipp" in current context.
(gdb) p **' &ipp'
No symbol " &ipp" in current context.
(gdb) p ***' &ipp'
No symbol " &ipp" in current context.
(gdb) whatis ' &i'
type = int *
(gdb) whatis ' &ip'
type = int **
(gdb) whatis ' &ipp'
No symbol " &ipp" in current context.

[Bug 1192742] Fix DWARF for variable references

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=1192742

--- Comment #3 from Jan Kratochvil  ---
= The only difference is that 'ipp' is missing there completely.

Apparently 'ipp' has been optimized out, it took me some tuning of the example
code to keep it small and prevent 'ipp' being optimized out, apparently new
golang needs a bit more complicated usage of the variable to keep it in place.

I do not think the specific testcase is a major issue in this Bug but thanks
for the notice I should update the testcase.

I am not sure if you are going to accept it as a downstream patch in Fedora
and/or if I should submit it upstream myself.

[Bug 1192742] Fix DWARF for variable references

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=1192742

--- Comment #4 from Vincent Batts  ---
(In reply to Jan Kratochvil from comment #1)
> BTW that DW_AT_name should not be there at all for such
> DW_TAG_reference_type, according to DWARF spec:
>
> If a name has been given to the modified type in the source program, then
> the corresponding modified type entry has a DW_AT_name attribute whose value
> is a null-terminated string containing the modified type name as it appears
> in the source program.

so, the following should not reference 'ipp' ?:

<2><86>: Abbrev Number: 5 (DW_TAG_formal_parameter)
    <87>   DW_AT_name        : ipp      
    <8b>   DW_AT_location    : 1 byte block: 9c         (DW_OP_call_frame_cfa)
    <8d>   DW_AT_type        : <0x1951b>

[Bug 1192742] Fix DWARF for variable references

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=1192742

--- Comment #5 from Vincent Batts  ---
(In reply to Jan Kratochvil from comment #3)
> I am not sure if you are going to accept it as a downstream patch in Fedora
> and/or if I should submit it upstream myself.

I've just been looking for issues related to this upstream.
https://github.com/golang/go/issues

Something like this we should work with them upstream first.

[Bug 1192742] Fix DWARF for variable references

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=1192742

--- Comment #6 from Jan Kratochvil  ---
(In reply to Vincent Batts from comment #4)
> so, the following should not reference 'ipp' ?:

A different case:

<2><40>: Abbrev Number: 4 (DW_TAG_variable)
    <41>   DW_AT_name        : &ipp     
    <46>   DW_AT_location    : 4 byte block: 9c 11 60 22       
(DW_OP_call_frame_cfa; DW_OP_consts: -32; DW_OP_plus)
    <4b>   DW_AT_type        : <0xe45a> 
 <1><e45a>: Abbrev Number: 17 (DW_TAG_pointer_type)
    <e45b>   DW_AT_name        : ***int 
    <e462>   DW_AT_type        : <0xe46b>       
    <e46a>   Unknown AT value: 2900: 22 
 <1><e46b>: Abbrev Number: 17 (DW_TAG_pointer_type)
    <e46c>   DW_AT_name        : **int  
    <e472>   DW_AT_type        : <0xe47b>       
    <e47a>   Unknown AT value: 2900: 22 
[...]

Only this line should be omitted:
    <e45b>   DW_AT_name        : ***int

Because there is no triple-pointer in the .go source, it is only internal
implementation detail how golang has compiled it.

With my patch this DIE
 <1><e45a>: Abbrev Number: 17 (DW_TAG_pointer_type)
    <e45b>   DW_AT_name        : ***int 
    <e462>   DW_AT_type        : <0xe46b>       
    <e46a>   Unknown AT value: 2900: 22 
is replaced by this DIE:
 <1><fe77>: Abbrev Number: 18 (DW_TAG_reference_type)
    <fe78>   DW_AT_name        : &(***int)      
    <fe82>   DW_AT_type        : <0xe462>       
    <fe8a>   Unknown AT value: 2900: 22 
But still this line should be in fact omitted for DWARF compliance:
    <fe78>   DW_AT_name        : &(***int)

[Bug 1192742] Fix DWARF for variable references

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=1192742

--- Comment #7 from Jan Kratochvil  ---
(In reply to Vincent Batts from comment #5)
> I've just been looking for issues related to this upstream.
> https://github.com/golang/go/issues

OK, there are some open DWARF issues.
> Something like this we should work with them upstream first.

You said the 'linker' is going to be replaced by something new but I expect for
RH Go development it still makes sense to patch the current 'linker' instance.

[Bug 1192742] Fix DWARF for variable references

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=1192742

--- Comment #8 from Vincent Batts  ---
https://go-review.googlesource.com/#/c/7150/ is the upstream change review