[Bug middle-end/105867] New: incorrect dangling-pointer warning

[Bug middle-end/105867] New: incorrect dangling-pointer warning

[gman at chromium dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105867

            Bug ID: 105867
           Summary: incorrect dangling-pointer warning
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gman at chromium dot org
  Target Milestone: ---

Created attachment 53094
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53094&action=edit
dangling-pointer warning repo

This code (attached) could be doing something wrong but I was asked to look
into why this code gets an dangling-pointer warning in gcc 12.1 and after
looking though it it seems like maybe the code is fine and the warning is
incorrect?

compiled with 

-std=c++17 -Wall -Wextra -pedantic-errors -Wdangling-pointer -O2

note that there is a function, TIntermRebuild::printStack, which is called in
several places. It's inclusion doesn't seem like it should affect the warning.
As it is it is compiled out via a macro

    #define printStack(msg)

compiling it back in (delete the line above) makes the warning go away.

[Bug tree-optimization/105867] [12/13 Regression] incorrect dangling-pointer warning

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105867

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-06-07
            Summary|incorrect dangling-pointer  |[12/13 Regression]
                   |warning                     |incorrect dangling-pointer
                   |                            |warning
             Status|UNCONFIRMED                 |NEW
          Component|middle-end                  |tree-optimization
   Target Milestone|---                         |12.2
     Ever confirmed|0                           |1
           Keywords|                            |diagnostic

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Looks the warning does not notice the store was "undone" with later on.
We have:
  MEM[(struct ConsList *)this_3(D) + 8B].value = currNode_5(D);
  MEM[(struct ConsList *)this_3(D) + 8B].tail = &guard.oldNodeStack;

....
  <bb 5> [local count: 118111600]:
  MEM[(struct ConsList *)this_3(D) + 8B] = guard.oldNodeStack;
  guard ={v} {CLOBBER};
  guard ={v} {CLOBBER(eol)};
  return;

The store to this removes the stored address of the local variable.

Confirmed.

[Bug tree-optimization/105867] [12/13 Regression] incorrect dangling-pointer warning

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105867

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
           Priority|P3                          |P2

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  I don't know how the dataflow of the diagnostic is implemented but
to fix this it would need to go backwards from function exits and maintain a
set of "killed" locations it would not diagnose in earlier code.  Of course
it's difficult in general considering

  global = &local;
  tem = global;
  other_global = tem;
  global = NULL;

and here &local escapes to other_global but we'd have to either rely on
optimization forwarding &local to tem or we'd have to conservatively
assume escaping (what the current code seems to do).  The former gets
you false negatives while the latter false positives.

Note for the testcase the store we diagnose is not dead since it has
possible uses via function calls also getting 'this', so optimization
is of no help in avoiding the false positive here.

[Bug tree-optimization/105867] [12/13 Regression] incorrect dangling-pointer warning

[agriff at tin dot it]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105867

Andrea Griffini <agriff at tin dot it> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |agriff at tin dot it

--- Comment #4 from Andrea Griffini <agriff at tin dot it> ---
Created attachment 53891
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53891&action=edit
Simpler version that triggers the bug

This code compiled on 12.2.0 with -O3 -Wall generates a warning about storing
the address of a local variable. Surprisingly (for me) adding either of the two
`printf` statements makes the warning go away.

The code seems correct to me; a doubly linked list of all instances of Node is
kept by inserting nodes in constructor and removing them in destructor.

[Bug tree-optimization/105867] [12/13 Regression] incorrect dangling-pointer warning

[wielkiegie at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105867

Gustaw Smolarczyk <wielkiegie at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |wielkiegie at gmail dot com

--- Comment #5 from Gustaw Smolarczyk <wielkiegie at gmail dot com> ---
This warning is triggered just by std::set::swap(std::set&) when optimizing.

https://godbolt.org/z/e84xPMdf6


--------
#include <set>

void foo()
{
  std::set<int> x, y;
  x.swap(y);
}
--------

[Bug tree-optimization/105867] [12/13/14 Regression] incorrect dangling-pointer warning

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105867

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

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

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 12.3 is being released, retargeting bugs to GCC 12.4.