[Bug middle-end/65686] New: incorrect warning maybe-uninitialized

[Bug middle-end/65686] New: incorrect warning maybe-uninitialized

[dimhen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65686

            Bug ID: 65686
           Summary: incorrect warning maybe-uninitialized
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dimhen at gmail dot com

gcc version 5.0.0 20150404 (experimental) [trunk revision 221866] (GCC)

$ cat x.i
typedef unsigned mytype; // 'int' fixes issue

struct S {
  mytype *pu;
};

mytype f(struct S *e)
{
  mytype x;
  if(&x != e->pu)
    __builtin_memcpy(&x, e->pu, sizeof(unsigned));
  return x;
}

$ gcc -fpreprocessed -Werror -Wall -O2 -c x.i
x.i: In function ‘f’:
x.i:9:10: error: ‘x’ may be used uninitialized in this function
[-Werror=maybe-uninitialized]
   mytype x;
          ^
cc1: all warnings being treated as error
>From gcc-bugs-return-482861-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Tue Apr 07 12:29:45 2015
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From: "rainer@emrich-ebersheim.de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug lto/65559] [5 Regression] lto1.exe: internal compiler error: in read_cgraph_and_symbols, at lto/lto.c:2947
Date: Tue, 07 Apr 2015 12:29:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ide559

--- Comment #10 from Rainer Emrich <rainer@emrich-ebersheim.de> ---
(In reply to Richard Biener from comment #8)
> Unfortunately -Wl,-debug is missing ;)
Ok, I uploaded a version including -Wl,-debug

>
> It would be interesting to see the lto-wrapper invocation (is there sth like
> strace on mingw?) and to eventually attach to lto-wrapper with a debugger...
I can't tell how to do this. Kai can you have a look?

[Bug middle-end/65686] [4.9 regression] inconsistent warning maybe-uninitialized: warn about 'unsigned', not warn about 'int'

[dimhen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65686

--- Comment #2 from Dmitry G. Dyachenko <dimhen at gmail dot com> ---
r212420 PASS
r212492 FAIL

gcc version 4.10.0 20140710 (experimental) [trunk revision 212420] (GCC)
gcc version 4.10.0 20140713 (experimental) [trunk revision 212492] (GCC)

[Bug middle-end/65686] [5/6 regression] inconsistent warning maybe-uninitialized: warn about 'unsigned', not warn about 'int'

[dimhen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65686

--- Comment #3 from Dmitry G. Dyachenko <dimhen at gmail dot com> ---
start FAIL r212452 (fix for PR middle-end/61473)

[Bug middle-end/65686] [5/6 regression] inconsistent warning maybe-uninitialized: warn about 'unsigned', not warn about 'int'

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65686

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic,
                   |                            |missed-optimization
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2015-04-15
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org
   Target Milestone|---                         |5.2
     Ever confirmed|0                           |1

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
It's (good)

  <bb 2>:
  _4 = e_3(D)->pu;
  if (&x != _4)
    goto <bb 3>;
  else
    goto <bb 5>;

  <bb 5>:
  goto <bb 4>;

  <bb 3>:
  _5 = MEM[(char * {ref-all})_4];
  MEM[(char * {ref-all})&x] = _5;

  <bb 4>:
  _7 = x;
  x ={v} {CLOBBER};
  return _7;

vs. (bad)

  <bb 2>:
  _4 = e_3(D)->pu;
  if (&x != _4)
    goto <bb 4>;
  else
    goto <bb 3>;

  <bb 3>:
  pretmp_9 = x;
  goto <bb 5>;

  <bb 4>:
  _5 = MEM[(char * {ref-all})_4];

  <bb 5>:
  # prephitmp_10 = PHI <pretmp_9(3), _5(4)>
  x ={v} {CLOBBER};
  return prephitmp_10;

that's a missed PRE opportunity in the good case (not sure why we use
unsigned int for the memcpy inlining - ah, because it goes the new
single load/store case in gimple_fold_builtin_memory_op).  PRE should
still be able to handle.

Only after PRE we hit the very special case where uninit warns about memory
reads (no store before the reads).  For this case inconsistencies are
very much expected...

But the testcase also shows a missed optimization opportunity because
&x can never be equal to e->pu (PR13962).

[Bug middle-end/65686] [5/6 regression] inconsistent warning maybe-uninitialized: warn about 'unsigned', not warn about 'int'

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65686

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Depends on|                            |13962

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Note that you are basically relying on optimization to remove the uninitialized
load.

Where does this obfuscated 'return *(e->pu)' come from?

Related to PR13962 (to simplify the compare to false)


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=13962
[Bug 13962] [tree-ssa] make "fold" use alias information to optimize pointer
comparisons

[Bug middle-end/65686] [5/6 regression] inconsistent warning maybe-uninitialized: warn about 'unsigned', not warn about 'int'

[dimhen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65686

--- Comment #6 from Dmitry G. Dyachenko <dimhen at gmail dot com> ---
(In reply to Richard Biener from comment #5)
> 
> Where does this obfuscated 'return *(e->pu)' come from?

If I remove aggregate then there are no warning.


i.e. this code produce no warning

mytype f(mytype *pu)
{
  mytype x;
  if(&x != pu)
    __builtin_memcpy(&x, pu, sizeof(unsigned));
  return x;
}

[Bug middle-end/65686] [5/6 regression] inconsistent warning maybe-uninitialized: warn about 'unsigned', not warn about 'int'

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65686

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug middle-end/65686] [5/6 regression] inconsistent warning maybe-uninitialized: warn about 'unsigned', not warn about 'int'

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65686

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|5.2                         |5.3

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 5.2 is being released, adjusting target milestone to 5.3.

[Bug middle-end/65686] [5/6/7 regression] inconsistent warning maybe-uninitialized: warn about 'unsigned', not warn about 'int'

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65686
Bug 65686 depends on bug 13962, which changed state.

Bug 13962 Summary: [tree-ssa] make "fold" use alias information to optimize pointer comparisons
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=13962

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