/*===-- int128_builtins.cpp - Implement __muloti4 --------------------------===
 *
 *                     The LLVM Compiler Infrastructure
 *
 * This file is dual licensed under the MIT and the University of Illinois Open
 * Source Licenses. See LICENSE.TXT for details.
 *
 * ===----------------------------------------------------------------------===
 *
 * This file implements __muloti4, and is stolen from the compiler_rt library.
 *
 * FIXME: we steal and re-compile it into filesystem, which uses __int128_t,
 * and requires this builtin when sanitized. See llvm.org/PR30643
 *
 * ===----------------------------------------------------------------------===
 */
#include "__config"
#include "climits"

#ifndef _LIBCPP_HAS_NO_INT128

__attribute__((no_sanitize("undefined"))) extern "C" __int128_t
__muloti4(__int128_t a, __int128_t b, int* overflow) {
  const int N = (int)(sizeof(__int128_t) * CHAR_BIT);
  const __int128_t MIN = (__int128_t)1 << (N - 1);
  const __int128_t MAX = ~MIN;
  *overflow = 0;
  __int128_t result = a * b;
  if (a == MIN) {
    if (b != 0 && b != 1)
      *overflow = 1;
    return result;
  }
  if (b == MIN) {
    if (a != 0 && a != 1)
      *overflow = 1;
    return result;
  }
  __int128_t sa = a >> (N - 1);
  __int128_t abs_a = (a ^ sa) - sa;
  __int128_t sb = b >> (N - 1);
  __int128_t abs_b = (b ^ sb) - sb;
  if (abs_a < 2 || abs_b < 2)
    return result;
  if (sa == sb) {
    if (abs_a > MAX / abs_b)
      *overflow = 1;
  } else {
    if (abs_a > MIN / -abs_b)
      *overflow = 1;
  }
  return result;
}

#endif

Add hack to allow ubsan to work w/o compiler-rt (__muloti4 is undefined)
[libc++] Add hack to allow ubsan to work w/o compiler-rt (__muloti4 is undefined)

Summary:
Using int128_t with UBSAN causes link errors unless compiler-rt is providing the runtime library.
Specifically ubsan generates calls to __muloti4 but libgcc doesn't provide a definition.

In order to avoid this, and allow users to continue using sanitized versions of libc++, this patch introduces a hack.
It adds a cribbed version of  the compiler-rt builtin to the libc++ filesystem sources.

I don't think this approach will work in the long run, but it seems OK for now.

Also see:

https://bugs.llvm.org/show_bug.cgi?id=30643
https://bugs.llvm.org/show_bug.cgi?id=16404


Reviewers: mclow.lists, ldionne, rsmith, jyknight, echristo

Reviewed By: echristo

Subscribers: dberris, cfe-commits

Differential Revision: https://reviews.llvm.org/D49828

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@337990 91177308-0d34-0410-b5e6-96231b3b80d8
