* [Bug middle-end/84078] false positive for -Wmaybe-uninitialized with __asm__
[not found] <bug-84078-4@http.gcc.gnu.org/bugzilla/>
@ 2021-04-05 17:10 ` msebor at gcc dot gnu.org
2022-11-20 4:16 ` law at gcc dot gnu.org
2022-11-28 22:35 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 3+ messages in thread
From: msebor at gcc dot gnu.org @ 2021-04-05 17:10 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84078
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Known to fail| |10.2.0, 11.0, 4.1.0, 4.8.4,
| |4.9.4, 5.5.0, 6.4.0, 7.2.0,
| |8.3.0, 9.1.0
Keywords| |missed-optimization
Last reconfirmed|2018-01-29 00:00:00 |2021-4-5
CC| |msebor at gcc dot gnu.org
--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Reconfirmed with GCC 11 and -O1/-O2 and the slightly reduced test case below.
I don't see anything in the IL that could be used to prove the uninitialized
variable isn't used: a_3 is uninitialized when b_6 != 0 || x == 0, and a_3 is
used when b_2 == 0 && c_9 == 0. The two predicates aren't mutually exclusive
so the warning triggers.
$ cat pr84078.c && gcc -O2 -S -Wall -fdump-tree-uninit=/dev/stdout pr84078.c
int x, z;
int f (void);
void g (int b)
{
int a;
if (!b)
{
if (x)
a = x;
else
b = 1;
}
{
int c;
__asm __volatile("" : "=r" (c));
if (c)
f ();
}
if (b)
return;
z = a;
}
;; Function g (g, funcdef_no=0, decl_uid=1947, cgraph_uid=1, symbol_order=2)
[AFTER NORMALIZATION -- [DEF]:
(.NOT.) b_6(D) == 0
[AFTER NORMALIZATION -- [DEF]:
a_3 = PHI <a_7(D)(10), x.0_1(11), a_7(D)(4)>
is guarded by :
x.0_1 != 0 (.AND.) b_6(D) == 0
[AFTER NORMALIZATION -- [USE]:
z = a_3;
is guarded by :
(.NOT.) b_2 != 0
pr84078.c: In function ‘g’:
pr84078.c:27:5: warning: ‘a’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
27 | z = a;
| ~~^~~
pr84078.c:7:7: note: ‘a’ was declared here
7 | int a;
| ^
void g (int b)
{
int c;
int a;
int x.0_1;
<bb 2> [local count: 1073741824]:
if (b_6(D) == 0)
goto <bb 3>; [50.00%]
else
goto <bb 10>; [50.00%]
<bb 10> [local count: 536870912]:
goto <bb 5>; [100.00%]
<bb 3> [local count: 536870913]:
x.0_1 = x;
if (x.0_1 != 0)
goto <bb 11>; [50.00%]
else
goto <bb 4>; [50.00%]
<bb 11> [local count: 268435457]:
goto <bb 5>; [100.00%]
<bb 4> [local count: 268435457]:
<bb 5> [local count: 1073741824]:
# b_2 = PHI <b_6(D)(10), 0(11), 1(4)>
# a_3 = PHI <a_7(D)(10), x.0_1(11), a_7(D)(4)>
__asm__ __volatile__("" : "=r" c_9);
if (c_9 != 0)
goto <bb 6>; [33.00%]
else
goto <bb 12>; [67.00%]
<bb 12> [local count: 719407024]:
goto <bb 7>; [100.00%]
<bb 6> [local count: 354334800]:
f ();
<bb 7> [local count: 1073741824]:
if (b_2 != 0)
goto <bb 13>; [34.00%]
else
goto <bb 8>; [66.00%]
<bb 13> [local count: 365072224]:
goto <bb 9>; [100.00%]
<bb 8> [local count: 708669601]:
z = a_3;
<bb 9> [local count: 1073741824]:
return;
}
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