[llvm-bugs] [Bug 34663] New: UBSan goes off on uniform_int_distribution

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Mon Sep 18 20:53:38 PDT 2017


            Bug ID: 34663
           Summary: UBSan goes off on uniform_int_distribution
           Product: libc++
           Version: 5.0
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: mclow.lists at gmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

ON x86_64 Linux, UBSan complains when the following program is run:

#include <array>
#include <random>
#include <cassert>

uint32_t array[1024 * 1024];

int main () {
  std::mt19937 randEngine;
  std::uniform_int_distribution<uint32_t> dist;

  for (auto &i : array)
    i = dist(randEngine);

  std::sort(std::begin(array), std::end(array));
  assert(std::is_sorted(std::begin(array), std::end(array)));

in particular, it doesn't like the line <algorithm>:3025.

This does no occur on Mac OS. BUT, if you change the `randEngine` declaration
    std::mersenne_twister_engine<unsigned long, 32ul, 624ul, 397ul, 31ul,
2567483615ul, 11ul, 4294967295ul, 7ul, 2636928640ul, 15ul, 4022730752ul, 18ul,
1812433253ul> randEngine;

then UBSan will fire on Mac OS as well.

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