[Bug bootstrap/95970] New: gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’ pointer null

[Bug bootstrap/95970] New: gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’ pointer null

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95970

            Bug ID: 95970
           Summary: gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’
                    pointer null
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: bootstrap
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Since r11-1697 Go bootstrap fails due to the following warning turned error:

In static member function ‘static Expression* Type::type_descriptor(Gogo*,
Type*)’,
    inlined from ‘virtual Expression* Named_type::do_type_descriptor(Gogo*,
Named_type*)’ at /ssd/test/src/gcc/trunk/gcc/go/gofrontend/types.cc:11114:53,
    inlined from ‘virtual Expression* Named_type::do_type_descriptor(Gogo*,
Named_type*)’ at /ssd/test/src/gcc/trunk/gcc/go/gofrontend/types.cc:11105:1:
/ssd/test/src/gcc/trunk/gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’
pointer null [-Wnonnull]
 1474 |   return type->do_type_descriptor(gogo, NULL);


The warning, issued from tree-ssa-ccp.c, is triggered by the use of the null
pointer as the first argument in the following call for the IL below.

  _31 = OBJ_TYPE_REF(_30;(struct Type)0B->12) (0B, gogo_12(D), 0B);

Named_type::do_type_descriptor (struct Named_type * const this, struct Gogo *
gogo, struct Named_type * name)
{
  struct Expression * D.404029;
  struct Expression * ret;
  bool _1;
  bool _2;
  bool _3;
  struct Type * _4;
  struct Expression * _5;
  struct Named_type * iftmp.1259_6;
  struct Expression * _14;
  struct Expression * _17;
  struct Expression * _19;
  struct Expression * _20;
  struct Named_type * iftmp.1259_21;
  struct Named_type * iftmp.1259_22;
  struct Expression * _24;
  struct Expression * _25;
  struct Expression * _27;
  struct Expression * _28;
  int (*) () * _29;
  int (*) () _30;
  struct Expression * _31;

  <bb 2> :
  _1 = this_9(D)->is_error_;
  if (_1 != 0)
    goto <bb 3>; [INV]
  else
    goto <bb 4>; [INV]

  <bb 3> :
  _27 = Expression::make_error (this_9(D)->location_);
  _28 = _27;
  goto <bb 13>; [INV]

  <bb 4> :
  if (name_10(D) == 0B)
    goto <bb 5>; [INV]
  else
    goto <bb 9>; [INV]

  <bb 5> :
  _2 = this_9(D)->is_alias_;
  if (_2 != 0)
    goto <bb 6>; [INV]
  else
    goto <bb 9>; [INV]

  <bb 6> :
  _3 = this_9(D)->seen_alias_;
  if (_3 != 0)
    goto <bb 7>; [INV]
  else
    goto <bb 8>; [INV]

  <bb 7> :
  _19 = Expression::make_error (this_9(D)->location_);
  _20 = _19;
  goto <bb 13>; [INV]

  <bb 8> :
  this_9(D)->seen_alias_ = 1;
  _29 = MEM[(struct Type *)0B]._vptr.Type;
  _30 = MEM[(int (*) () *)_29 + 96B];
  _31 = OBJ_TYPE_REF(_30;(struct Type)0B->12) (0B, gogo_12(D), 0B);
  _33 = _31;
  _14 = _33;
  ret_15 = _14;
  this_9(D)->seen_alias_ = 0;
  _17 = ret_15;
  // predicted unlikely by early return (on trees) predictor.
  goto <bb 13>; [INV]

[Bug bootstrap/95970] gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’ pointer null

[jamborm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95970

Martin Jambor <jamborm at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-06-29
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
                 CC|                            |ian at airs dot com

--- Comment #1 from Martin Jambor <jamborm at gcc dot gnu.org> ---
I hit this today too (and it indeed prevents go bootstrap), so I guess it's
confirmed.  Ian, can you have a look whether the warning is correct?  I glanced
at the code only for a little while but it looks so to me.

[Bug bootstrap/95970] gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’ pointer null

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95970

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
I think the warning is a false positive.  The problem call is introduced by
early inlining and is eliminated by path isolation, but the -Wnonnull warning
code runs before that happens (by the post_ipa_warn pass).

Here's the relevant portion of the path isolation dump showing the removal:

;; Function Named_type::do_type_descriptor
(_ZN10Named_type18do_type_descriptorE
P4GogoPS_, funcdef_no=8125, decl_uid=214219, cgraph_uid=3913,
symbol_order=4835)


Pass statistics of "isolate-paths": ----------------
...
Removing basic block 11
;; basic block 11, loop depth 0
;;  pred:      
_20 = MEM[(int (*) () *)_19 + 96B];
_21 = OBJ_TYPE_REF(_20;(struct Type)0B->12) (0B, gogo_9(D), 0B);
this_7(D)->seen_alias_ = 0;
goto <bb 10>; [100.00%]
;;  succ:       10

[Bug bootstrap/95970] gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’ pointer null

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95970

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
On second thought, I think the warning might be correct.  The function it's
issued for is a static member of class Type (I at first thought it was
virtual):

Expression*
Type::type_descriptor(Gogo* gogo, Type* type)
{
  return type->do_type_descriptor(gogo, NULL);   <<< -Wnonnull
}

and the call to it is below:

Expression*
Named_type::do_type_descriptor(Gogo* gogo, Named_type* name)
{
  if (this->is_error_)
    return Expression::make_error(this->location_);
  if (name == NULL && this->is_alias_)
    {
      if (this->seen_alias_)
        return Expression::make_error(this->location_);
      this->seen_alias_ = true;
      Expression* ret = this->type_->type_descriptor(gogo, NULL);   <<<
      this->seen_alias_ = false;
      return ret;
    }

I'll leave it to Ian to confirm one way or the other.

[Bug bootstrap/95970] gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’ pointer null

[ian at airs dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95970

--- Comment #4 from Ian Lance Taylor <ian at airs dot com> ---
I'm having a hard time parsing the warning.  It says

gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’ pointer null [-Wnonnull]

The line is

  return type->do_type_descriptor(gogo, NULL);

Is this trying to say that "type" may be NULL?

That's true, it can be.  This does look like a bug.  There must be some reason
that this code is never executed.

[Bug bootstrap/95970] gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’ pointer null

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95970

--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
Yes, that's what it's saying.  Unfortunately, the locations of null pointer
constants isn't available in the middle end so all we have left is the function
calls they're used in.

[Bug bootstrap/95970] gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’ pointer null

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95970

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Ian Lance Taylor <ian@gcc.gnu.org>:

https://gcc.gnu.org/g:9bca676cc7a1670be021567ae4a128a5082229d3

commit r11-1721-g9bca676cc7a1670be021567ae4a128a5082229d3
Author: Ian Lance Taylor <iant@golang.org>
Date:   Mon Jun 29 11:30:43 2020 -0700

    compiler: remove some erroneous code that was never run

    The code accidentally called Type::type_descriptor rather than the
    do_type_descriptor method.  Calling Type::type_descriptor with a second
    argument of NULL would always crash.  Since that never happened,
    it revealed that this code was never actually executed.

    Fixes PR go/95970

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/240477

[Bug bootstrap/95970] gcc/go/gofrontend/types.cc:1474:34: warning: ‘this’ pointer null

[ian at airs dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95970

Ian Lance Taylor <ian at airs dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED

--- Comment #7 from Ian Lance Taylor <ian at airs dot com> ---
Should be fixed now.