https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85301
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed|2018-04-09 00:00:00 |2021-4-5
Known to fail| |10.2.0, 11.0, 8.3.0, 9.3.0
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=99919
CC| |msebor at gcc dot gnu.org
--- Comment #8 from Martin Sebor <msebor at gcc dot gnu.org> ---
Reconfirmed with GCC 11 with the ever-so-slightly slightly simplified program
below and enhanced output. The warning has been issued since at least GCC 4.1
and so is not a regression.
$ cat pr85301.c && gcc -O2 -S -Wall -DUSE_BITFIELD pr85301.c
struct A
{
#ifdef USE_BITFIELD
unsigned i : 1;
unsigned j : 1;
#else
unsigned i;
unsigned j;
#endif
};
int z, f (void);
struct A a;
void h (void)
{
int y;
if (a.j || a.i)
y = f ();
if (a.i)
z = y;
}
pr85301.c: In function ‘h’:
pr85301.c:24:7: warning: ‘y’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
24 | z = y;
| ~~^~~
pr85301.c:18:7: note: when ‘!(((unsigned char*)&a)[0] & 3)’
18 | int y;
| ^
pr85301.c:18:7: note: used when ‘prephitmp_15 = PHI <_1(7), pretmp_14(3)> & 1’
pr85301.c:18:7: note: ‘y’ was declared here
Jump threading and other optimization opportunities aside (I have raised
pr99918 for one of those), there does also seem to be a limitation in the
warning code in that it doesn't understand BIT_FIELD_REF expressions. The
warning sees the IL below from which it should be able to determine that y's
use is conditional on its definition (i.e., the use predicate a strict subset
of the predicate controlling the definition).
void h ()
{
int y;
unsigned char _1;
unsigned char _2;
unsigned char _4;
unsigned char pretmp_14;
unsigned char prephitmp_15;
<bb 2> [local count: 1073741824]:
# VUSE <.MEM_8(D)>
_1 = BIT_FIELD_REF <a, 8, 0>;
_2 = _1 & 3;
if (_2 != 0)
goto <bb 3>; [33.00%]
else
goto <bb 7>; [67.00%]
<bb 7> [local count: 719407024]:
goto <bb 4>; [100.00%]
<bb 3> [local count: 354334800]:
# .MEM_10 = VDEF <.MEM_8(D)>
y_11 = f ();
# VUSE <.MEM_10>
pretmp_14 = BIT_FIELD_REF <a, 8, 0>;
<bb 4> [local count: 1073741824]:
# y_5 = PHI <y_9(D)(7), y_11(3)>
# .MEM_6 = PHI <.MEM_8(D)(7), .MEM_10(3)>
# prephitmp_15 = PHI <_1(7), pretmp_14(3)>
_4 = prephitmp_15 & 1;
if (_4 != 0)
goto <bb 5>; [50.00%]
else
goto <bb 8>; [50.00%]
<bb 8> [local count: 536870912]:
goto <bb 6>; [100.00%]
<bb 5> [local count: 536870913]:
## y_5 = PHI <y_9(D)(7), y_11(3)>
## uninit when: _2 == 0
## : BIT_FIELD_REF <a, 8, 0> & 3 == 0
##
## used when: prephitmp_15 & != 0
## : PHI <_1(7), pretmp_14(3)>
## : _1(7) != 0 || pretmp_14 != 0
## : BIT_FIELD_REF <a, 8, 0> != 0 || BIT_FIELD_REF <a, 8, 0> != 0
## : BIT_FIELD_REF <a, 8, 0> != 0
# .MEM_12 = VDEF <.MEM_6>
z = y_5;
<bb 6> [local count: 1073741824]:
# .MEM_7 = PHI <.MEM_6(8), .MEM_12(5)>
# VUSE <.MEM_7>
return;
}
