[Bug middle-end/39390] [4.4 regression] Bogus aliasing warning with std::set

[Bug middle-end/39390] [4.4 regression] Bogus aliasing warning with std::set

[reichelt at gcc dot gnu dot org]

-- 

reichelt at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.4.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug middle-end/39390] New: [4.4 regression] Bogus aliasing warning with std::set

[reichelt at gcc dot gnu dot org]

Another problem with aliasing: The following code snippet triggers a bogus
warning when compiled on trunk with "-O -Wall":

==========================================================================
#include<set>

struct A
{
  A() : i() {}
  int i;
};

struct B
{
  bool operator() (const A& x, const A& y) const { return x.i < y.i; }
};

void foo()
{
  std::set<A, B> s;
  s.insert(A());
}
==========================================================================

bug.cc: In function 'void foo()':
bug.cc:11: warning: dereferencing pointer '__x.15' does break strict-aliasing
rules
.../gcc-4.4-20090305/include/c++/4.4.0/bits/stl_tree.h:530: note: initialized
from here
bug.cc:11: warning: dereferencing pointer '__x.15' does break strict-aliasing
rules
.../gcc-4.4-20090305/include/c++/4.4.0/bits/stl_tree.h:530: note: initialized
from here

This probably results in wrong code generation.


-- 
           Summary: [4.4 regression] Bogus aliasing warning with std::set
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Keywords: wrong-code, diagnostic
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: reichelt at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug middle-end/39390] [4.4 regression] Bogus aliasing warning with std::set

[reichelt at gcc dot gnu dot org]

------- Comment #1 from reichelt at gcc dot gnu dot org  2009-03-06 13:10 -------
Created an attachment (id=17405)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17405&action=view)
Partially reduced testcase

Somewhat reduced testcase.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug middle-end/39390] [4.4 regression] Bogus aliasing warning with std::set

[reichelt at gcc dot gnu dot org]

------- Comment #2 from reichelt at gcc dot gnu dot org  2009-03-06 13:27 -------
Oops, typo: I meant the bug is triggered with "-O3 -Wall".

The reduced testcase already triggers the warning with "-O2 -Wall" thanks to
some additional "inline"s.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug middle-end/39390] [4.4 regression] Bogus aliasing warning with std::set

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2009-03-06 13:36 -------
Confirmed with the reduced testcase only, at -O2 -Wall.  The warning happens
after PTA triggered by SRA.

Relevant constraints:

__y_7 = &s.64+64
D.2839_21 = __y_7
__x.8_43 = D.2839_21

and code:

  D.2839_21 = &__y_7->D.2545;
  D.2832_22 ={v} &0B->D.2545;
  if (D.2832_22 != 0B)
    goto <bb 6>;
  else
    goto <bb 4>;

<bb 4>:
  D.2888_42 = &__y_7->D.2545;
  if (D.2839_21 == D.2888_42)
    goto <bb 6>;
  else
    goto <bb 5>;

<bb 5>:
  __x.8_43 = (const struct _Rb_tree_node *) D.2839_21;
  D.2878_45 = __x.8_43->_M_value_field;

which shows that the warning is for dead code.  Thus this is not a
wrong-code problem.

VRP could figure out the equivalencies but doesn't.  The next DOM
pass figures out the second but not the first non-equivalency - that
survives until the next forwprop pass.

In the end the problem is that VRP doesn't do a good job cleaning up
after itself (there are single-argument PHIs left after it as well as
non-simplified comparisons):

<bb 3>:
Invalid sum of incoming frequencies 900, should be 10000
  # __y_8 = PHI <0B(2)>
  # __y_18 = PHI <__y_7(2)>
...

<bb 4>:
Invalid sum of incoming frequencies 3900, should be 102
  D.2837 = 1;
  D.2839_21 = &__y_18->D.2545;
  D.2832_22 ={v} &0B->D.2545;
  if (D.2832_22 != 0B)
    goto <bb 8>;
  else
    goto <bb 5>;

<bb 5>:
  D.2888_42 = &__y_7->D.2545;
  if (D.2839_21 == D.2888_42)
    goto <bb 8>;
  else
    goto <bb 6>;


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|wrong-code                  |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2009-03-06 13:36:52
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 regression] Bogus aliasing warning with std::set

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2009-03-06 14:16 -------
Scheduling another pass_phi_only_cprop after VRP removes the single-argument
PHI nodes (I think that really cfg-cleanup should do this, as they usually
result from edge removal).  While this is reasonably cheap it doesn't get
rid of the conditionals but that would require a forwprop run which is not
that cheap (it requires only the forward_propagate_into_gimple_cond () pieces,
but has to run after removing single-argument PHIs, thus cannot run at
VRP substitution time).

Thus, the following would fix it:

Index: passes.c
===================================================================
--- passes.c    (revision 144665)
+++ passes.c    (working copy)
@@ -611,6 +611,8 @@ init_optimization_passes (void)
       NEXT_PASS (pass_copy_prop);
       NEXT_PASS (pass_merge_phi);
       NEXT_PASS (pass_vrp);
+      NEXT_PASS (pass_phi_only_cprop);
+      NEXT_PASS (pass_forwprop);
       NEXT_PASS (pass_dce);
       NEXT_PASS (pass_cselim);
       NEXT_PASS (pass_tree_ifcombine);

alternatively only adding pass_phi_only_cprop and calling
forward_propagate_into_gimple_cond from pass_tree_ifcombine.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 Regression] Bogus aliasing warning with std::set

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2009-03-08 15:42 -------
Only the diagnostic part is a regression.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
            Summary|[4.4 regression] Bogus      |[4.4 Regression] Bogus
                   |aliasing warning with       |aliasing warning with
                   |std::set                    |std::set


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4/4.5 Regression] Bogus aliasing warning with std::set

[jakub at gcc dot gnu dot org]

-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.0                       |4.4.1


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4/4.5 Regression] Bogus aliasing warning with std::set

[jakub at gcc dot gnu dot org]

-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.1                       |4.4.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 Regression] Bogus aliasing warning with std::set

[pinskia at gcc dot gnu dot org]

------- Comment #6 from pinskia at gcc dot gnu dot org  2009-10-05 20:16 -------
This is fixed on the trunk


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[4.4/4.5 Regression] Bogus  |[4.4 Regression] Bogus
                   |aliasing warning with       |aliasing warning with
                   |std::set                    |std::set


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 Regression] Bogus aliasing warning with std::set

[rguenther at suse dot de]

------- Comment #7 from rguenther at suse dot de  2009-10-06 09:01 -------
Subject: Re:  [4.4 Regression] Bogus aliasing
 warning with std::set

On Mon, 5 Oct 2009, pinskia at gcc dot gnu dot org wrote:

> ------- Comment #6 from pinskia at gcc dot gnu dot org  2009-10-05 20:16 -------
> This is fixed on the trunk

Indeed - the warning code was removed ;)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 Regression] Bogus aliasing warning with std::set

[paolo dot carlini at oracle dot com]

------- Comment #8 from paolo dot carlini at oracle dot com  2009-10-06 09:23 -------
Then it's easy to fix in 4_4 too ;)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 Regression] Bogus aliasing warning with std::set

[jakub at gcc dot gnu dot org]

-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.2                       |4.4.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 Regression] Bogus aliasing warning with std::set

[paolo dot carlini at oracle dot com]

------- Comment #9 from paolo dot carlini at oracle dot com  2009-11-17 11:09 -------
*** Bug 42077 has been marked as a duplicate of this bug. ***


-- 

paolo dot carlini at oracle dot com changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 Regression] Bogus aliasing warning with std::set

[scovich at gmail dot com]

------- Comment #10 from scovich at gmail dot com  2009-11-17 11:16 -------
(In reply to comment #3)
> the warning is for dead code.  Thus this is not a
> wrong-code problem.

Just to verify, does this (and comment #7) mean that the warning is harmless
and can be ignored?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 Regression] Bogus aliasing warning with std::set

[paolo dot carlini at oracle dot com]

------- Comment #11 from paolo dot carlini at oracle dot com  2009-11-17 15:15 -------
Yes.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 Regression] Bogus aliasing warning with std::set

[pinskia at gcc dot gnu dot org]

------- Comment #12 from pinskia at gcc dot gnu dot org  2009-11-18 04:02 -------
*** Bug 42087 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |riddikulus at verizon dot
                   |                            |net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 Regression] Bogus aliasing warning with std::set

[jakub at gcc dot gnu dot org]

-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.3                       |4.4.4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390

[Bug tree-optimization/39390] [4.4 Regression] Bogus aliasing warning with std::set

[jakub at gcc dot gnu dot org]

-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.4.4                       |4.4.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39390