[Bug tree-optimization/53802] New: Spurious 'may be used uninitialized' related to shifts

[Bug tree-optimization/53802] New: Spurious 'may be used uninitialized' related to shifts

[lukeocamden at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53802

             Bug #: 53802
           Summary: Spurious 'may be used uninitialized' related to shifts
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: lukeocamden@gmail.com


Trivial changes in the following program suppress the warning:

inline void foo(const unsigned char* mem_, unsigned int* value_)
{
  unsigned int value = *mem_;
  value <<= 8;
  value |= *++mem_;
  value <<= 8;
  value |= *++mem_;
  value <<= 8;
  value |= *++mem_;
  value <<= 8;
  value |= *++mem_;
  *value_ = value;
}

inline void bar(unsigned char* dest_, unsigned int value_)
{
  if (!value_)
    return;
  dest_[0] = 0;
  dest_[1] = value_ >> 24;
  dest_[2] = value_ >> 16;
  dest_[3] = value_ >> 8;
  dest_[4] = value_;
}

int main()
{
  for (unsigned int number = 0xFFFFFFFF; number; number >>= 1)
  {
    unsigned char buf[5];
    bar(buf, number);
    unsigned int val;
    foo(buf, &val);
    if (number != val)
      return 0;
  }
}

$ gcc main.cpp -O -Wall
main.cpp: In function 'int main()':
main.cpp:6:19: warning: '*((void*)& buf +1)' may be used uninitialized in this
function [-Wmaybe-uninitialized]
main.cpp:31:19: note: '*((void*)& buf +1)' was declared here
main.cpp:12:19: warning: '*((void*)& buf +4)' may be used uninitialized in this
function [-Wmaybe-uninitialized]
main.cpp:31:19: note: '*((void*)& buf +4)' was declared here
main.cpp:10:19: warning: '*((void*)& buf +3)' may be used uninitialized in this
function [-Wmaybe-uninitialized]
main.cpp:31:19: note: '*((void*)& buf +3)' was declared here
main.cpp:8:19: warning: '*((void*)& buf +2)' may be used uninitialized in this
function [-Wmaybe-uninitialized]
main.cpp:31:19: note: '*((void*)& buf +2)' was declared here

[Bug tree-optimization/53802] Spurious 'may be used uninitialized' related to shifts

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53802

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-06-29
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-06-29 09:57:17 UTC ---
Confirmed.  Reason is a missed "jump-threading" (peeling off one iteration):

<bb 2>:

<bb 3>:
  # number_59 = PHI <number_7(6), 4294967295(2)>
  # buf$2_60 = PHI <buf$2_44(6), buf$2_51(D)(2)>
  # buf$3_61 = PHI <buf$3_2(6), buf$3_52(D)(2)>
  # buf$4_64 = PHI <buf$4_6(6), buf$4_53(D)(2)>
  # buf$1_65 = PHI <buf$1_46(6), buf$1_50(D)(2)>
  if (number_59 == 0)
    goto <bb 5>;
  else
    goto <bb 4>;

<bb 4>:
  D.2247_18 = number_59 >> 24;
  buf$1_19 = (unsigned char) D.2247_18;
  D.2245_20 = number_59 >> 16;
  buf$2_21 = (unsigned char) D.2245_20;
  D.2243_22 = number_59 >> 8;
  buf$3_23 = (unsigned char) D.2243_22;
  buf$4_24 = (unsigned char) number_59;

<bb 5>:
  # buf$1_46 = PHI <buf$1_19(4), buf$1_65(3)>
  # buf$2_44 = PHI <buf$2_21(4), buf$2_60(3)>
  # buf$3_2 = PHI <buf$3_23(4), buf$3_61(3)>
  # buf$4_6 = PHI <buf$4_24(4), buf$4_64(3)>
  D.2257_29 = (unsigned int) buf$1_46;

so we decide there is a path from entry to BB 5 where buf$1_46 is not
initialized.

[Bug tree-optimization/53802] Spurious 'may be used uninitialized' related to shifts

[lukeocamden at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53802

lukeocamden at gmail dot com changed:

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

--- Comment #2 from lukeocamden at gmail dot com ---
Works fine with g++ (GCC) 4.9.0 20140202 (experimental)