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* [Bug c/89990] request warning: Use of out of scope compound literals
[not found] <bug-89990-4@http.gcc.gnu.org/bugzilla/>
@ 2020-03-31 2:34 ` modchipv12 at gmail dot com
2024-04-09 7:32 ` pinskia at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 5+ messages in thread
From: modchipv12 at gmail dot com @ 2020-03-31 2:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89990
Andrew D'Addesio <modchipv12 at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |modchipv12 at gmail dot com
--- Comment #5 from Andrew D'Addesio <modchipv12 at gmail dot com> ---
GCC already warns about this at compile time, but the warning sometimes doesn't
appear (due to PR88058, as Andrew Pinski just mentioned). Plus, the warning is
a bit confusing and could be reworded.
For example, create the following files:
foo.c:
int foo(const unsigned char *buf)
{
(void)buf; /* unused parameter */
return 1;
}
test.c:
int foo(const unsigned char *buf);
struct mytype {
char c;
};
static struct mytype d = { 42 };
int test(int x)
{
const unsigned char buf[32];
const struct mytype *ptr = &d;
if (x != 0)
ptr = &(const struct mytype){ 43 };
foo(buf);
#ifdef CALL_FOO_TWICE
foo(buf);
#endif
return ptr->c;
}
int main()
{
return test(1); /* returns 43 on GCC8, 0 on GCC9+ */
}
Compiling with one foo() call gives us a warning:
$ gcc -std=c99 -Wall -Wextra -pedantic -O1 -o test test.c foo.c
test.c: In function ‘main’:
test.c:26:15: warning: ‘<U2cf0>.c’ is used uninitialized in this function
[-Wuninitialized]
26 | return ptr->c;
| ~~~^~~
$ ./test
$ echo $?
0
However, compiling with two foo() calls makes the warning disappear, for some
reason:
$ gcc -DCALL_FOO_TWICE -std=c99 -Wall -Wextra -pedantic -O1 -o test test.c
foo.c
$ ./test
$ echo $?
0
My GCC version is 9.3.1 20200317 (Red Hat 9.3.1-1) on Fedora 31 x86-64.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/89990] request warning: Use of out of scope compound literals
[not found] <bug-89990-4@http.gcc.gnu.org/bugzilla/>
2020-03-31 2:34 ` [Bug c/89990] request warning: Use of out of scope compound literals modchipv12 at gmail dot com
@ 2024-04-09 7:32 ` pinskia at gcc dot gnu.org
2024-04-09 7:33 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-04-09 7:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89990
Bug 89990 depends on bug 88058, which changed state.
Bug 88058 Summary: gcc fails to detect use of out of scope variable ?
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88058
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |DUPLICATE
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/89990] request warning: Use of out of scope compound literals
[not found] <bug-89990-4@http.gcc.gnu.org/bugzilla/>
2020-03-31 2:34 ` [Bug c/89990] request warning: Use of out of scope compound literals modchipv12 at gmail dot com
2024-04-09 7:32 ` pinskia at gcc dot gnu.org
@ 2024-04-09 7:33 ` pinskia at gcc dot gnu.org
2024-05-07 2:42 ` modchipv12 at gmail dot com
2024-05-07 4:55 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-04-09 7:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89990
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |DUPLICATE
Status|NEW |RESOLVED
--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The warning is now included in GCC 12.
And this makes this a dup of bug 63272.
*** This bug has been marked as a duplicate of bug 63272 ***
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/89990] request warning: Use of out of scope compound literals
[not found] <bug-89990-4@http.gcc.gnu.org/bugzilla/>
` (2 preceding siblings ...)
2024-04-09 7:33 ` pinskia at gcc dot gnu.org
@ 2024-05-07 2:42 ` modchipv12 at gmail dot com
2024-05-07 4:55 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 5+ messages in thread
From: modchipv12 at gmail dot com @ 2024-05-07 2:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89990
--- Comment #7 from Andrew D'Addesio <modchipv12 at gmail dot com> ---
(In reply to Andrew Pinski from comment #6)
> The warning is now included in GCC 12.
> And this makes this a dup of bug 63272.
>
> *** This bug has been marked as a duplicate of bug 63272 ***
Yep, the new warning is working nicely on my test case (comment #5) on GCC
14.0.1 on Fedora 40 x86-64:
test.c: In function ‘test’:
test.c:22:15: warning: dangling pointer ‘ptr’ to an unnamed temporary may
be used [-Wdangling-pointer=]
22 | return ptr->c;
| ~~~^~~
test.c:15:37: note: unnamed temporary defined here
15 | ptr = &(const struct mytype){ 43 };
| ^
Though one thing to note -- (in addition to the above warning) it still
generates that "strange" -Wuninitialized warning if and only if there are one
or fewer calls to foo():
In function ‘test’,
inlined from ‘main’ at test.c:27:12:
test.c:22:15: warning: ‘<Ufe10>.c’ is used uninitialized [-Wuninitialized]
22 | return ptr->c;
| ~~~^~~
^ If foo() is called 2+ times then that warning disappears.
It's not a bug per se as the "real" -Wdangling-pointer warning still gets
displayed to the user (Ufe10 is probably GCC's representation of the variable
that was never initialized).
But I find it very peculiar that the threshold for silencing that
-Wuninitialized warning is 2 function calls and not say 1 or 10.
This actually has gotten me curious. Would you have an idea/explanation behind
that 2 function call threshold @Andrew Pinski?
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/89990] request warning: Use of out of scope compound literals
[not found] <bug-89990-4@http.gcc.gnu.org/bugzilla/>
` (3 preceding siblings ...)
2024-05-07 2:42 ` modchipv12 at gmail dot com
@ 2024-05-07 4:55 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-05-07 4:55 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89990
--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew D'Addesio from comment #7)
>
> This actually has gotten me curious. Would you have an idea/explanation
> behind that 2 function call threshold @Andrew Pinski?
Most likely it is due to jump threading optimization not happening if there are
2 calls while it is happening with 1. That is the growth of copying 1 call is
reasonable while 2 is not when removing the extra jump.
That is transforming:
```
int test(int x)
{
const unsigned char buf[32];
const struct mytype *ptr = &d;
if (x != 0)
ptr = &(const struct mytype){ 43 };
foo(buf);
#ifdef CALL_FOO_TWICE
foo(buf);
#endif
return ptr->c;
}
```
into something like:
```
int test(int x)
{
const unsigned char buf[32];
const struct mytype *ptr;
if (x != 0) goto a; else goto b;
a:
{
ptr = &(const struct mytype){ 43 };
}
foo(buf);
#ifdef CALL_FOO_TWICE
foo(buf);
#endif
return ptr->c;
b:
foo(buf);
#ifdef CALL_FOO_TWICE
foo(buf);
#endif
return d.c;
}
```
Where doing 2 copies of 2 calls is too expensive to be done.
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2024-05-07 4:55 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
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[not found] <bug-89990-4@http.gcc.gnu.org/bugzilla/>
2020-03-31 2:34 ` [Bug c/89990] request warning: Use of out of scope compound literals modchipv12 at gmail dot com
2024-04-09 7:32 ` pinskia at gcc dot gnu.org
2024-04-09 7:33 ` pinskia at gcc dot gnu.org
2024-05-07 2:42 ` modchipv12 at gmail dot com
2024-05-07 4:55 ` pinskia at gcc dot gnu.org
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