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* [Bug c++/102932] New: Wrong implementation of abs with O3
@ 2021-10-25 16:26 rajpal.gusain at gmail dot com
2021-10-25 16:38 ` [Bug c++/102932] " xtkoba at gmail dot com
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: rajpal.gusain at gmail dot com @ 2021-10-25 16:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102932
Bug ID: 102932
Summary: Wrong implementation of abs with O3
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: rajpal.gusain at gmail dot com
Target Milestone: ---
Looks like while optimizing abs, optimizer trips and produces wrong result
Test case:
#include <cstdlib>
#include <iostream>
using namespace std;
int main()
{
int left , right ;
cout << "Enter left and right\n";
cin >> left >> right;
unsigned int rangeSize = abs(left-right)+1;
cout << rangeSize << endl;
return 0;
}
Inputs: 0, 2147483647 (2**31-1)
Compile command:
g++ a.cpp -O3
Compiler version:
g++ --version
g++ (GCC) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Output:
18446744071562067968
Workaround:
1. Non-optimized compilation works as expected.
2. Passing fno-builtin-abs to compiler works.
g++ a.cpp -O3 -fno-builtin-abs
Looking at generated assembly, it looks like implementation of abs is flawed
and needs to be fixed.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/102932] Wrong implementation of abs with O3
2021-10-25 16:26 [Bug c++/102932] New: Wrong implementation of abs with O3 rajpal.gusain at gmail dot com
@ 2021-10-25 16:38 ` xtkoba at gmail dot com
2021-10-25 16:41 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: xtkoba at gmail dot com @ 2021-10-25 16:38 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102932
Tee KOBAYASHI <xtkoba at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |xtkoba at gmail dot com
--- Comment #1 from Tee KOBAYASHI <xtkoba at gmail dot com> ---
Signed integer overflow. You can use -fwrapv.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/102932] Wrong implementation of abs with O3
2021-10-25 16:26 [Bug c++/102932] New: Wrong implementation of abs with O3 rajpal.gusain at gmail dot com
2021-10-25 16:38 ` [Bug c++/102932] " xtkoba at gmail dot com
@ 2021-10-25 16:41 ` pinskia at gcc dot gnu.org
2021-10-25 16:46 ` rajpal.gusain at gmail dot com
2021-10-25 17:54 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-10-25 16:41 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102932
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
As mentioned signed integer overflow is undefined. You would have seen an error
at runtime if you used -fsanitize=undefined.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/102932] Wrong implementation of abs with O3
2021-10-25 16:26 [Bug c++/102932] New: Wrong implementation of abs with O3 rajpal.gusain at gmail dot com
2021-10-25 16:38 ` [Bug c++/102932] " xtkoba at gmail dot com
2021-10-25 16:41 ` pinskia at gcc dot gnu.org
@ 2021-10-25 16:46 ` rajpal.gusain at gmail dot com
2021-10-25 17:54 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: rajpal.gusain at gmail dot com @ 2021-10-25 16:46 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102932
--- Comment #3 from Rajpal Singh <rajpal.gusain at gmail dot com> ---
(In reply to Tee KOBAYASHI from comment #1)
> Signed integer overflow. You can use -fwrapv.
Thanks ! Yes, it's signed integer overflow, is there any way to catch it
statically at compile time ?
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/102932] Wrong implementation of abs with O3
2021-10-25 16:26 [Bug c++/102932] New: Wrong implementation of abs with O3 rajpal.gusain at gmail dot com
` (2 preceding siblings ...)
2021-10-25 16:46 ` rajpal.gusain at gmail dot com
@ 2021-10-25 17:54 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-10-25 17:54 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102932
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Rajpal Singh from comment #3)
> Thanks ! Yes, it's signed integer overflow, is there any way to catch it
> statically at compile time ?
Not really because there would so many false positives, it is not worth the
effort.
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2021-10-25 17:54 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
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2021-10-25 16:26 [Bug c++/102932] New: Wrong implementation of abs with O3 rajpal.gusain at gmail dot com
2021-10-25 16:38 ` [Bug c++/102932] " xtkoba at gmail dot com
2021-10-25 16:41 ` pinskia at gcc dot gnu.org
2021-10-25 16:46 ` rajpal.gusain at gmail dot com
2021-10-25 17:54 ` pinskia at gcc dot gnu.org
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