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* [Bug tree-optimization/107274] New: Incorrect -Wuninitialized on indexed initialization
@ 2022-10-15 22:15 keegan at undefinedbehaviour dot org
2022-10-15 22:19 ` [Bug tree-optimization/107274] " pinskia at gcc dot gnu.org
2022-10-17 8:00 ` rguenth at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: keegan at undefinedbehaviour dot org @ 2022-10-15 22:15 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107274
Bug ID: 107274
Summary: Incorrect -Wuninitialized on indexed initialization
Product: gcc
Version: 12.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: keegan at undefinedbehaviour dot org
Target Milestone: ---
Given:
void
example(unsigned long val)
{
unsigned char b[4];
b[0] = val;
b[1] = val >> 8;
b[2] = val >> 16;
b[3] = val >> 24;
val = *(unsigned long *)b;
}
GCC 12.2 issues the following (incorrect) diagnostic:
issue.c: In function 'example':
issue.c:11:13: warning: 'b' is used uninitialized [-Wuninitialized]
11 | val = *(unsigned long *)b;
| ~~~~^~~~~~~~~~~~~~~~~~~~~
issue.c:4:23: note: 'b' declared here
4 | unsigned char b[4];
| ^
Instead this code is correct in that it initializes all fields of "b". It
appears the cast indirection is what is causing the issue, because I cannot get
the diagnostic to be issued for other uses.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/107274] Incorrect -Wuninitialized on indexed initialization
2022-10-15 22:15 [Bug tree-optimization/107274] New: Incorrect -Wuninitialized on indexed initialization keegan at undefinedbehaviour dot org
@ 2022-10-15 22:19 ` pinskia at gcc dot gnu.org
2022-10-17 8:00 ` rguenth at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-10-15 22:19 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107274
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Strict aliasing issue. Store is via char but load is via long to a char array.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/107274] Incorrect -Wuninitialized on indexed initialization
2022-10-15 22:15 [Bug tree-optimization/107274] New: Incorrect -Wuninitialized on indexed initialization keegan at undefinedbehaviour dot org
2022-10-15 22:19 ` [Bug tree-optimization/107274] " pinskia at gcc dot gnu.org
@ 2022-10-17 8:00 ` rguenth at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu.org @ 2022-10-17 8:00 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107274
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Blocks| |24639
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
No, the issue is that you are accessing 8 bytes on the read and thus the
padding you access that way is not initialized. Use *(unsigned int)b and the
diagnostic goes away.
[internally GCC doesn't consider an indirect load of size 8 ever aliasing
a store to a 4 byte object - otherwise we wouldn't diagnose a partial
uninit value -- like
void
example(unsigned long val)
{
char b[8];
b[0] = val;
b[1] = val >> 8;
b[2] = val >> 16;
b[3] = val >> 24;
val = *(unsigned long *)b;
}
which isn't diagnosed]
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24639
[Bug 24639] [meta-bug] bug to track all Wuninitialized issues
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2022-10-17 8:00 UTC | newest]
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2022-10-15 22:15 [Bug tree-optimization/107274] New: Incorrect -Wuninitialized on indexed initialization keegan at undefinedbehaviour dot org
2022-10-15 22:19 ` [Bug tree-optimization/107274] " pinskia at gcc dot gnu.org
2022-10-17 8:00 ` rguenth at gcc dot gnu.org
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