[LLVMbugs] [Bug 23011] New: miscompile of % in loop

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23011

            Bug ID: 23011
           Summary: miscompile of % in loop
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: nlewycky at google.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

$ clang++ -v
clang version 3.7.0 (trunk 233044)
Target: x86_64-unknown-linux-gnu

Testcase:
#include <stdio.h>
#include <stdlib.h>
#include <string>

using namespace std;

int main(int argc, char **argv) {
  int r = 2;
  bool ok = true;
  while (ok) {
    string ab;
    for (int i = 0; i < r % 3; i++) {
      ab += "ab";
    }
    printf("%d %s\n", r, ab.c_str());
    r++;
    ok = (r < 3);
  }
}

nlewycky at ducttape:~$ clang++ -O2 a.cc -o a
nlewycky at ducttape:~$ ./a
2 ab
nlewycky at ducttape:~$ clang++ a.cc -o a
nlewycky at ducttape:~$ ./a
2 abab

The -O0 result is correct.

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