[llvm-bugs] [Bug 25421] New: Clang -O1 is incorrectly removing a comparison

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25421

            Bug ID: 25421
           Summary: Clang -O1 is incorrectly removing a comparison
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Global Analyses
          Assignee: sanjoy at playingwithpointers.com
          Reporter: rtrieu at google.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Suspected revision is r251050.  Inside the loop, the comparison (i <
result_size - 1) is being treated as true for all cases, even on the last loop
iterator where i is equal to result_size - 1

test4.cc:
#include <iostream>
#include <vector>

using std::cout;
using std::endl;
using std::vector;
int main(int argc, char** argv) {
  vector<int> vv = {1, 2};
  long int result_size = vv.back();
  for (long int i = 0; i < result_size; ++i) {
    cout << i << " < " << result_size
         << " - 1 = " << ((i < result_size - 1) ? "true" : "false") << endl;
  }

  return 0;
}

clang++ -std=c++11 test4.cc -O1 ; ./a.out
0 < 2 - 1 = true
1 < 2 - 1 = true

clang++ -std=c++11 test4.cc ; ./a.out
0 < 2 - 1 = true
1 < 2 - 1 = false

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