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From: "rajpal.gusain at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/102932] New: Wrong implementation of abs with O3
Date: Mon, 25 Oct 2021 16:26:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102932

            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 =3D 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.=20
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.=