[Bug middle-end/95681] New: False positive uninitialized variable usage in decNumberCompareTotalMag

[Bug middle-end/95681] New: False positive uninitialized variable usage in decNumberCompareTotalMag

[stefansf at linux dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95681

            Bug ID: 95681
           Summary: False positive uninitialized variable usage in
                    decNumberCompareTotalMag
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: build, diagnostic
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stefansf at linux dot ibm.com
  Target Milestone: ---
            Target: s390

The following error/warning shows up on S/390 while bootstrapping:

libdecnumber/decNumber.c: In function 'decNumberCompareTotalMag':
libdecnumber/decNumber.c:953:14: error: '*(allocbufa).bits' may be used
uninitialized [-Werror=maybe-uninitialized]
  953 |       a->bits&=~DECNEG;   /* .. and clear the sign */
      |              ^~
libdecnumber/decNumber.c:967:14: error: '*(allocbufb).bits' may be used
uninitialized [-Werror=maybe-uninitialized]
  967 |       b->bits&=~DECNEG;   /* .. and clear the sign */
      |              ^~

The part of interest is:

if (decNumberIsNegative(lhs)) {     /* lhs<0 */
  a=bufa;
  needbytes=sizeof(decNumber)+(D2U(lhs->digits)-1)*sizeof(Unit);
  if (needbytes>sizeof(bufa)) {     /* need malloc space */
    allocbufa=(decNumber *)malloc(needbytes);
    if (allocbufa==NULL) {          /* hopeless -- abandon */
      status|=DEC_Insufficient_storage;
      break;}
    a=allocbufa;                    /* use the allocated space */
    }
  decNumberCopy(a, lhs);            /* copy content */
  a->bits&=~DECNEG;                 /* .. and clear the sign */
  lhs=a;                            /* use copy from here on */
  }

While calling `decNumberCopy` variable `a` either points to `bufa` or to
`allocbufa`. Since `bufa` and `allocbufa` are both allocated inside function
`decNumberCompareTotalMag` and argument `lhs` is not changed, it is guaranteed
that `a != lhs` holds prior call `decNumberCopy(a, lhs)`. Function
`decNumberCopy` initializes `a->bits`, if `a != lhs` holds. Since this is the
case here, no warning should be printed.

Note, no warning is printed if `dest->bits=src->bits;` is shifted above `if
(dest==src) return dest;` of function `decNumberCopy` which endorses that this
is a false positive resulting from a wrong assumption that `a == lhs` may hold.

Analogous for the second warning/error which speaks about variable `b`.

[Bug middle-end/95681] False positive uninitialized variable usage in decNumberCompareTotalMag

[trnsz at pobox dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95681

--- Comment #1 from Jeffrey H. Johnson <trnsz at pobox dot com> ---
Confirm this behavior and reached same analysis with gcc version 11.1.1
20210531 (Red Hat 11.1.1-3) on x86_64.

[Bug middle-end/95681] False positive uninitialized variable usage in decNumberCompareTotalMag

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95681

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |msebor at gcc dot gnu.org
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2021-07-09

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Could you please reduce the warning to a standalone test case (or translation
unit) and attach it to the report so that it can be more easily reproduced
(with a cross-compiler) and analyzed?

[Bug middle-end/95681] False positive uninitialized variable usage in decNumberCompareTotalMag

[stefansf at linux dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95681

--- Comment #3 from Stefan Schulze Frielinghaus <stefansf at linux dot ibm.com> ---
Created attachment 51160
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51160&action=edit
Reduced program

[Bug middle-end/95681] False positive uninitialized variable usage in decNumberCompareTotalMag

[stefansf at linux dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95681

--- Comment #4 from Stefan Schulze Frielinghaus <stefansf at linux dot ibm.com> ---
Running todays mainline (d97d71a1989) using options -O3 -Wall against the
reduced program on x86 as well as s390x results in

t.c: In function 'decNumberCompareTotalMag':
t.c:55:14: warning: '*allocbufa.bits' may be used uninitialized
[-Wmaybe-uninitialized]
   55 |       a->bits&=~0x80;
      |              ^~
t.c:70:14: warning: '*allocbufb.bits' may be used uninitialized
[-Wmaybe-uninitialized]
   70 |       b->bits&=~0x80;
      |              ^~