[Bug tree-optimization/90710] Bogus Wmaybe-uninitialized caused by __builtin_expect when compiled with -Og

[Bug tree-optimization/90710] Bogus Wmaybe-uninitialized caused by __builtin_expect when compiled with -Og

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90710

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2019-06-02 00:00:00         |2021-4-6
      Known to fail|                            |10.2.1, 11.0, 9.3.0
                 CC|                            |msebor at gcc dot gnu.org

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
Reconfirmed with GCC 11.  My enhanced version of trunk under test prints:

pr90710.c: In function ‘testFunction’:
pr90710.c:22:17: warning: ‘value’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
   22 |                 printf("My if() causes -Wmaybe-uninitialized for my use
of `value': %d\n",value);
      |                
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pr90710.c:17:22: note: when ‘x == 0’
   17 |         unsigned int value;
      |                      ^~~~~
pr90710.c:17:22: note: used when ‘_11 = PHI <0(3), 1(9)> != 0’
pr90710.c:17:22: note: ‘value’ was declared here


The IL the warning sees (annotated with my comments) is below.  The predicate
that guards the possibly uninitialized use is (was_ok_8 != 0 && was_ok_8 == 0),
which the warning should be able to determine is false.  It doesn't because, as
a result of the __builtin_expect() intrinsic which converts the first argument
to long int, the assignment 'was_ok_8 = (int) _11' in bb 4 involves a
conversion that the warning code doesn't handle.  Changing the type of the
variables to long avoids the warning.

__attribute__((noinline))
void testFunction ()
{
  int was_ok;
  unsigned int value;
  volatile unsigned int x;
  unsigned int x.0_1;
  long int _2;
  unsigned int _11;

  <bb 2> [local count: 1073741824]:
  # .MEM_7 = VDEF <.MEM_6(D)>
  x ={v} 1;
  # VUSE <.MEM_7>
  x.0_1 ={v} x;
  if (x.0_1 == 0)
    goto <bb 3>; [34.00%]
  else
    goto <bb 9>; [66.00%]

  <bb 9> [local count: 708669600]:
  goto <bb 4>; [100.00%]

  <bb 3> [local count: 365072224]:

  <bb 4> [local count: 1073741824]:
  # _11 = PHI <0(3), 1(9)>
  # value_12 = PHI <value_13(D)(3), x.0_1(9)>
  was_ok_8 = (int) _11;            <<< int to long conversion gets in the way
  if (was_ok_8 != 0)
    goto <bb 5>; [33.00%]
  else
    goto <bb 10>; [67.00%]          <<< was_ok_8 == 0

  <bb 10> [local count: 719407024]:
  goto <bb 6>; [100.00%]

  <bb 5> [local count: 354334800]:
  # .MEM_9 = VDEF <.MEM_7>
  printf ("My if() compiles fine: %d\n", value_12);

  <bb 6> [local count: 1073741824]:
  # .MEM_4 = PHI <.MEM_7(10), .MEM_9(5)>
  _2 = (long int) was_ok_8;
  if (_2 != 0)
    goto <bb 7>; [90.00%]           <<< was_ok_8 != 0
  else
    goto <bb 11>; [10.00%]

  <bb 11> [local count: 107374184]:
  goto <bb 8>; [100.00%]

  <bb 7> [local count: 966367641]:
  ## value_12 used when was_ok_8 != 0 AND was_ok_8 == 0   <<< must be false
  # .MEM_10 = VDEF <.MEM_4>
  printf ("My if() causes -Wmaybe-uninitialized for my use of `value\': %d\n",
value_12);

  <bb 8> [local count: 1073741824]:
  # .MEM_5 = PHI <.MEM_4(11), .MEM_10(7)>
  # VUSE <.MEM_5>
  return;

}

[Bug tree-optimization/90710] Bogus Wmaybe-uninitialized caused by __builtin_expect when compiled with -Og

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90710

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
Here's a smaller test case showing both the problem (first gcc invocation) and
how it can be avoided (second invocation):

$ (set -x && cat pr90710.c && gcc -Og -S -Wall pr90710.c && gcc -Dint=long -Og
-S -Wall pr90710.c)
+ cat pr90710.c
static inline int f (int x, int *p)
{
  if (x == 0)
    return 0;

  *p = x;
  return 1;
}

volatile int z;

void g (int x)
{
  int b, a = f (x, &b);

  if (a)
    z = b;

  if (__builtin_expect (a, 1))
    z = b;
}
+ gcc -Og -S -Wall pr90710.c
pr90710.c: In function ‘g’:
pr90710.c:20:7: warning: ‘b’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
   20 |     z = b;
      |     ~~^~~
pr90710.c:14:7: note: ‘b’ was declared here
   14 |   int b, a = f (x, &b);
      |       ^
+ gcc -Dint=long -Og -S -Wall pr90710.c