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.
