[Bug c++/107290] New: False positive -Wuninitialized with Eigen

[Bug c++/107290] New: False positive -Wuninitialized with Eigen

[carlosgalvezp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107290

            Bug ID: 107290
           Summary: False positive -Wuninitialized with Eigen
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: carlosgalvezp at gmail dot com
  Target Milestone: ---

Created attachment 53713
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53713&action=edit
Preprocessed source

Hi, 

I'm getting a false positive in the following code (preprocessed attached):

#include <Eigen/Core>

using Points = Eigen::Array<float, 3, 1>;

struct Foo
{
    explicit Foo(Points p);
    Points p_{};
};

void foo()
{
    Points p{};
    Foo Foo(p);
}

Compiling with flags:
-std=c++14 -Wall -Wextra -O3  

In file included from /opt/compiler-explorer/libs/eigen/v3.3.7/Eigen/Core:450,
                 from <source>:1:
In copy constructor 'Eigen::DenseStorage<T, Size, _Rows, _Cols,
_Options>::DenseStorage(const Eigen::DenseStorage<T, Size, _Rows, _Cols,
_Options>&) [with T = float; int Size = 3; int _Rows = 3; int _Cols = 1; int
_Options = 0]',
    inlined from 'Eigen::PlainObjectBase<Derived>::PlainObjectBase(const
Eigen::PlainObjectBase<Derived>&) [with Derived = Eigen::Array<float, 3, 1>]'
at
/opt/compiler-explorer/libs/eigen/v3.3.7/Eigen/src/Core/PlainObjectBase.h:520:17,
    inlined from 'Eigen::Array<_Scalar, _Rows, _Cols, _Options, _MaxRows,
_MaxCols>::Array(const Eigen::Array<_Scalar, _Rows, _Cols, _Options, _MaxRows,
_MaxCols>&) [with _Scalar = float; int _Rows = 3; int _Cols = 1; int _Options =
0; int _MaxRows = 3; int _MaxCols = 1]' at
/opt/compiler-explorer/libs/eigen/v3.3.7/Eigen/src/Core/Array.h:229:25,
    inlined from 'void foo()' at <source>:14:14:
/opt/compiler-explorer/libs/eigen/v3.3.7/Eigen/src/Core/DenseStorage.h:194:47:
warning: 'p.Eigen::Array<float, 3, 1, 0, 3,
1>::<unnamed>.Eigen::PlainObjectBase<Eigen::Array<float, 3, 1, 0, 3, 1>
>::m_storage.Eigen::DenseStorage<float, 3, 3, 1, 0>::m_data' is used
uninitialized [-Wuninitialized]
  194 |     DenseStorage(const DenseStorage& other) : m_data(other.m_data) {
      |                                               ^~~~~~~~~~~~~~~~~~~~
<source>: In function 'void foo()':
<source>:13:12: note: 'p' declared here
   13 |     Points p{};
      |            ^

Repro on Godbolt: https://godbolt.org/z/TaTdT33vr

Interestingly, if I define the constructor inline, then the warning disappears:

-     explicit Foo(Points p);
+     explicit Foo(Points p) : p_(std::move(p)) {}

[Bug c++/107290] False positive -Wuninitialized with Eigen

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107290

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |24639
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-10-17
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I can confirm the diagnostics at -O{1,2,3}.  We diagnose the aggregate copy
in

<bb 2> [local count: 1073741824]:
MEM[(struct plain_array *)&p] ={v} {CLOBBER};
MEM[(struct DenseStorage *)&D.103583] ={v} {CLOBBER};
MEM[(struct DenseStorage *)&D.103583].m_data = MEM[(const struct DenseStorage
&)&p].m_data;
Foo::Foo (&Foo, &D.103583);
D.103583 ={v} {CLOBBER(eol)};
p ={v} {CLOBBER(eol)};
Foo ={v} {CLOBBER(eol)};
return;

clearly 'p' is completely uninitialized and we copy m_data from it to
the value argument of Foo::Foo.

'p' gets initialized by

    struct Points p;
  <<cleanup_point <<< Unknown tree: expr_stmt
    Eigen::Array<float, 3, 1>::Array (&p) >>>>>;

I don't know enough C++ to decipher what supposedly initializes m_data.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24639
[Bug 24639] [meta-bug] bug to track all Wuninitialized issues