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[Bug c++/65273] New: Incorrect output in SIMPLE program compiled from high version gcc with O2
- From: "codeeply at foxmail dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Mon, 02 Mar 2015 04:08:49 +0000
- Subject: [Bug c++/65273] New: Incorrect output in SIMPLE program compiled from high version gcc with O2
- Auto-submitted: auto-generated
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65273
Bug ID: 65273
Summary: Incorrect output in SIMPLE program compiled from high
version gcc with O2
Product: gcc
Version: 4.8.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: codeeply at foxmail dot com
#include <iostream>
#include <cstring>
using namespace std;
const int N = 2000017;
int hash(const char *s) {
volatile int ret = 1;
int i, len = strlen(s);
for (i = 0; i < len; ++i) {
ret = ret * 131 + s[i];
}
ret %= N;
if (ret < 0) ret += N;
return ret;
}
int hash1(const char *s) {
int ret = 1;
int i, len = strlen(s);
for (i = 0; i < len; ++i) {
ret = ret * 131 + s[i];
}
ret %= N;
if (ret < 0) ret += N;
return ret;
}
int main() {
cout << hash("sfihsifef") << endl;
cout << hash1("sfihsifef") << endl;
return 0;
}
---------------------------------------------------------------------
:~/test>g++ -O2 g.cpp
:~/test>./a.out
602373
-1397644
---------------------------------------------------------------------
It's found that "if (ret < 0) ret += N;" was "optimized" without volatile.
And this bug has been seen in 4.7.2 and 4.8.2, but not in 4.1.2.