[Bug middle-end/78391] g++ (any version) at O0 (for O1, O2, O3 is ok) doesn't warn when class members are used uninitialized.

[Bug middle-end/78391] g++ (any version) at O0 (for O1, O2, O3 is ok) doesn't warn when class members are used uninitialized.

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78391

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |10.2.0, 11.0, 7.3.0, 8.3.0,
                   |                            |9.2.0
                 CC|                            |msebor at gcc dot gnu.org
   Last reconfirmed|2016-11-17 00:00:00         |2021-3-31

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
Reconfirmed with GCC 11.

GCC does issue -Wuninitialized without optimization on an equivalent test case
with an explicit ctor.  The difference between the two ctors is that B's has a
clobber:

$ cat pr78391.C && PRED_DUMP=euninit gcc -S -Wall pr78391.C
struct A { int x = w; int w = 10; };

int f ()
{
  A a;
  return a.x;
}

struct B { int x, w; B (): x(w), w (10) { } };

int g ()
{
  B b;
  return b.x;
}

void A::A (struct A * const this)
{
  int _1;

  <bb 2> :
  # VUSE <.MEM_2(D)>           <<< no clobber
  _1 = this_3(D)->w;           <<< -Wuninitialized
  # .MEM_4 = VDEF <.MEM_2(D)>
  this_3(D)->x = _1;
  # .MEM_5 = VDEF <.MEM_4>
  this_3(D)->w = 10;
  # VUSE <.MEM_5>
  return;

}


int f ()
{
  struct A a;
  int D.2439;
  int _3;

  <bb 2> :
  # .MEM_2 = VDEF <.MEM_1(D)>
  A::A (&a);
  # VUSE <.MEM_2>
  _3 = a.x;
  # .MEM_4 = VDEF <.MEM_2>
  a ={v} {CLOBBER};

  <bb 3> :
<L1>:
  # VUSE <.MEM_4>
  return _3;

}


void B::B (struct B * const this)
{
  int _1;

  <bb 2> :
  # .MEM_4 = VDEF <.MEM_2(D)>
  *this_3(D) ={v} {CLOBBER};   <<< clobber here
  # VUSE <.MEM_4>
  _1 = this_3(D)->w;           <<< -Wuninitialized
  # .MEM_5 = VDEF <.MEM_4>
  this_3(D)->x = _1;
  # .MEM_6 = VDEF <.MEM_5>
  this_3(D)->w = 10;
  # VUSE <.MEM_6>
  return;

}


pr78391.C: In constructor ‘B::B()’:
pr78391.C:9:30: warning: ‘*this.B::w’ is used uninitialized [-Wuninitialized]
    9 | struct B { int x, w; B (): x(w), w (10) { } };
      |                              ^
int g ()
{
  struct B b;
  int D.2442;
  int _3;

  <bb 2> :
  # .MEM_2 = VDEF <.MEM_1(D)>
  B::B (&b);
  # VUSE <.MEM_2>
  _3 = b.x;
  # .MEM_4 = VDEF <.MEM_2>
  b ={v} {CLOBBER};

  <bb 3> :
<L1>:
  # VUSE <.MEM_4>
  return _3;

}

[Bug c++/78391] g++ (any version) at O0 (for O1, O2, O3 is ok) doesn't warn when class members are used uninitialized.

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78391

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |c++
           Severity|enhancement                 |normal

--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
So the missing warning seems like it might be due to a bug in the C++ front
end.

[Bug middle-end/78391] g++ (any version) at O0 (for O1, O2, O3 is ok) doesn't warn when class members are used uninitialized.

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78391

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0
           Assignee|unassigned at gcc dot gnu.org      |msebor at gcc dot gnu.org
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=19808
             Status|NEW                         |ASSIGNED
          Component|c++                         |middle-end

--- Comment #6 from Martin Sebor <msebor at gcc dot gnu.org> ---
In bug 19808 comment #38 Jason explains why the CLOBBER isn't present in the
constructor, so that's not a bug.  In light of that, I'm not sure the request
can be implemented without introducing what are strictly speaking false
positives like those Clang suffers from.  In the test case below, the global a
is implicitly zeroed out first and then its members are explicitly assigned, so
there is no uninitialized read.

At the same time, I suspect the result of this initialization is probably not
going to be what the author intended, and so warning on it regardless might
actually be useful (admittedly, issuing -Wuninitialized in this case would be a
bit misleading).  So with that, -Wuninitialized or -Wmaybe-uninitialized in the
middle end could be probably enhanced to detect this pattern of intialization
dependencies.  Marek submitted a C++ patch for a subset of pr19808 last
November but I'm not sure it handled this case.  The patch was also not
approved as Jason's preference was to detect these things in the middle end. 
So let me reassign this back to the middle end and try to remember to get to it
for GCC 12.

$ cat t.C && clang -S t.C
struct A { int x = w; int w = 10; };

A a;
t.C:1:20: warning: field 'w' is uninitialized when used here [-Wuninitialized]
struct A { int x = w; int w = 10; };
                   ^
t.C:3:3: note: in implicit default constructor for 'A' first required here
A a;
  ^
t.C:1:8: note: during field initialization in the implicit default constructor
struct A { int x = w; int w = 10; };
       ^
1 warning generated.

[Bug middle-end/78391] g++ (any version) at O0 (for O1, O2, O3 is ok) doesn't warn when class members are used uninitialized.

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78391

--- Comment #7 from Martin Sebor <msebor at gcc dot gnu.org> ---
Patch for pr19808:
https://gcc.gnu.org/pipermail/gcc-patches/2020-November/559162.html

[Bug middle-end/78391] g++ (any version) at O0 (for O1, O2, O3 is ok) doesn't warn when class members are used uninitialized.

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78391

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org

--- Comment #8 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #7)
> Patch for pr19808:
> https://gcc.gnu.org/pipermail/gcc-patches/2020-November/559162.html

That's fixed now.

[Bug middle-end/78391] g++ (any version) at O0 (for O1, O2, O3 is ok) doesn't warn when class members are used uninitialized.

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78391

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|msebor at gcc dot gnu.org          |unassigned at gcc dot gnu.org
             Status|ASSIGNED                    |NEW

--- Comment #9 from Martin Sebor <msebor at gcc dot gnu.org> ---
This has been improved in GCC 12 but GCC still doesn't diagnose the original
test case at -O0.  It seems that it should be able to, either in the C++ front
end or even in the middle end.  I'm not working on this anymore.

[Bug middle-end/78391] g++ (any version) at O0 (for O1, O2, O3 is ok) doesn't warn when class members are used uninitialized.

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78391

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|12.0                        |12.2

--- Comment #10 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 12.1 is being released, retargeting bugs to GCC 12.2.

[Bug middle-end/78391] g++ (any version) at O0 (for O1, O2, O3 is ok) doesn't warn when class members are used uninitialized.

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78391

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|12.3                        |12.4

--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 12.3 is being released, retargeting bugs to GCC 12.4.