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* [Bug other/96447] New: False positive -Wstringop-overflow with -O3
@ 2020-08-03 20:51 daniel at constexpr dot org
2020-08-03 22:13 ` [Bug tree-optimization/96447] False positive -Wstringop-overflow with -O3 due to loop unrolling msebor at gcc dot gnu.org
2020-08-04 17:38 ` daniel at constexpr dot org
0 siblings, 2 replies; 3+ messages in thread
From: daniel at constexpr dot org @ 2020-08-03 20:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96447
Bug ID: 96447
Summary: False positive -Wstringop-overflow with -O3
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: daniel at constexpr dot org
Target Milestone: ---
Created attachment 48990
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48990&action=edit
test.c
The attached reduced test case results in the following false positive warning
at -03 (but not -O2) without any additional compiler flags with both GCC 10.2.0
and GCC 11.0.0 (compiled from git today):
test.c: In function ‘load’:
test.c:23:25: warning: writing 1 byte into a region of size 0
[-Wstringop-overflow=]
23 | data[i] = get8u(s);
| ~~~~~~~~^~~~~~~~~~
test.c:21:23: note: at offset 4 to object ‘data’ with size 4 declared here
21 | unsigned char data[4];
| ^~~~
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/96447] False positive -Wstringop-overflow with -O3 due to loop unrolling
2020-08-03 20:51 [Bug other/96447] New: False positive -Wstringop-overflow with -O3 daniel at constexpr dot org
@ 2020-08-03 22:13 ` msebor at gcc dot gnu.org
2020-08-04 17:38 ` daniel at constexpr dot org
1 sibling, 0 replies; 3+ messages in thread
From: msebor at gcc dot gnu.org @ 2020-08-03 22:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96447
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Keywords| |diagnostic,
| |missed-optimization
Component|other |tree-optimization
CC| |msebor at gcc dot gnu.org
Summary|False positive |False positive
|-Wstringop-overflow with |-Wstringop-overflow with
|-O3 |-O3 due to loop unrolling
Last reconfirmed| |2020-08-03
Ever confirmed|0 |1
--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning works as designed. The false positive is caused by the assignment
below in the unrolled loop with the index in RANGE [4, 5]. GCC doesn't
understand that the constraint on bpp implies that i < 4, likely because the
range of bpp is discontiguous.
;; basic block 19, loop depth 0, count 0 (precise), probably never executed
;; prev block 18, next block 20, flags: (NEW, REACHABLE, VISITED)
;; pred: 18 [80.0% (guessed)] count:0 (precise)
(TRUE_VALUE,EXECUTABLE)
# .MEM_3 = VDEF <.MEM_50>
dataD.1941[i_49] = 0;
# RANGE [4, 5] NONZERO 5
i_2 = i_49 + 1;
# RANGE [32, 32] NONZERO 32
_1 = _6 + 8;
if (bpp_25 > 32)
goto <bb 20>; [80.00%]
else
goto <bb 16>; [20.00%]
;; succ: 20 [80.0% (guessed)] count:0 (precise)
(TRUE_VALUE,EXECUTABLE)
;; 16 [20.0% (guessed)] count:0 (precise)
(FALSE_VALUE,EXECUTABLE)
;; basic block 20, loop depth 0, count 0 (precise), probably never executed
;; prev block 19, next block 21, flags: (NEW, REACHABLE, VISITED)
;; pred: 19 [80.0% (guessed)] count:0 (precise)
(TRUE_VALUE,EXECUTABLE)
# .MEM_45 = VDEF <.MEM_3>
dataD.1941[i_2] = 0; <<< warning
The warning can be avoided by constraining the range of bpp to [1, 4] instead
(by shifting it to the right by 3 bits) and changing the loop test to i < bpp.
That also leads to GCC emitting better code.
int load(input * s)
{
unsigned char bpp = get8u (s);
if(bpp != 8 && bpp != 16 && bpp != 24 && bpp != 32) {
return 0;
}
bpp >>= 3;
unsigned char data[4];
for(int i = 0; i < bpp; ++i) {
data[i] = get8u (s);
}
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/96447] False positive -Wstringop-overflow with -O3 due to loop unrolling
2020-08-03 20:51 [Bug other/96447] New: False positive -Wstringop-overflow with -O3 daniel at constexpr dot org
2020-08-03 22:13 ` [Bug tree-optimization/96447] False positive -Wstringop-overflow with -O3 due to loop unrolling msebor at gcc dot gnu.org
@ 2020-08-04 17:38 ` daniel at constexpr dot org
1 sibling, 0 replies; 3+ messages in thread
From: daniel at constexpr dot org @ 2020-08-04 17:38 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96447
--- Comment #2 from Daniel Scharrer <daniel at constexpr dot org> ---
For warnings enabled by something like -fanalyzer this might be reasonable but
for a warning I enabled by default (not even requiring -Wall) the bar should
ideally be a bit higher.
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2020-08-04 17:38 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2020-08-03 20:51 [Bug other/96447] New: False positive -Wstringop-overflow with -O3 daniel at constexpr dot org
2020-08-03 22:13 ` [Bug tree-optimization/96447] False positive -Wstringop-overflow with -O3 due to loop unrolling msebor at gcc dot gnu.org
2020-08-04 17:38 ` daniel at constexpr dot org
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