std::sort triggers negate-overflow ubsan

[kongy]

Change 0fb76ba introduced a regression in std::sort.

Building

#include <algorithm>
#include <vector>

int main() {
        std::vector<int> other_ops;
        for (int i = 0; i < 1024; ++i) {
                other_ops.push_back(i);
        }

        std::sort(other_ops.begin(), other_ops.end(), std::greater<int>());
}

with

~/llvm/build/bin/clang++ -O2 -fsanitize=integer ubsan_test.cpp -stdlib=libc++ -static-libstdc++ -l:libc++abi.a

crashes with the following error:

include/c++/v1/__bit/blsr.h:25:23: runtime error: negation of 18446744073709551615 cannot be represented in type 'unsigned long'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior include/c++/v1/__bit/blsr.h:25:23

[kongy]

@philnik777

[frederick-vs-ja]

I strongly believe that __x ^ (__x & -__x) should be a valid strategy when __x is of type unsigned long, and UBSan is being a bit insane here.

[philnik777]

Currently over 200 tests are failing with -fsanitize=integer enabled, so I wouldn't consider this a regression, but rather a feature request. If someone is willing to clean this up I'm sure we're happy to support -fsanitize=integer.