Ubuntu – Undefined behavior (according to clang -fsanitize=integer) on libstdc++ std::random due to negative index on Mersenne Twister engine

unsigned types have well-defined wrapping behaviour in C++. This is one reason why they get used in PRNGs and other bit-manipulation use-cases where that’s desired and expected (and necessary for the algorithm), not an error.

GCC devs are right: it’s unreasonable to treat all unsigned wrapping as a problem. It’s even more unreasonable to print out that it’s "undefined behaviour", rather than a possible problem. If clang’s ubsan had told you in the first place that it was well-defined in C++ and perhaps intended, you wouldn’t have had to bother the GCC devs with a bug report that wasn’t useful to them. Or you could have phrased it as a feature request after understanding the issue.

But you’re also right: with library functions in headers where they become part of your own code, that makes it very hard to disentangle library code (such as this PRNG) from your own code, when it inlines into the same compilation unit. And ubsan options are per-file.

libc++’s implementation of mt19937 disables that ubsan check where necessary. It’s a more recent clean-slate implementation of the C++ standard library developed as part of LLVM and mostly used with clang. If any header-library was going to cater to this sanitizer that flags some valid C++ operations as problems, it would be libc++. https://godbolt.org/z/aeY5Yn9c6 shows that adding -stdlib=libc++ to the compile options on Godbolt lets your test case run cleanly. You’d have to get it installed locally to actually use it.

libc++ defines a preprocessor macro _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK as __attribute__((__no_sanitize__("unsigned-integer-overflow"))) (if supported), so it can disable that on a per-function basis. See for example libcxx’s <utility> header where various functions use that tag, and mersenne_twister_engine<...>::seed() in <random>. But interestingly, it doesn’t use it everywhere, so you can still get the benefit of overflow checking.

Or you could write a wrapper function around random number generation and put that in a separate .cpp you compile without sanitize=integer. In a release build with -flto, it can still get inlined. Or if you don’t need as high quality randomness, use libc random(3); it is separately compiled, not an inline header. Linux’s random() is non-terrible, although not great either. Other PRNGs like xorshift / xoroshiro are fast and good, but would also use unsigned types and rely on their wrapping for multiply and/or add/sub, unless they only use shifts and xor like an LFSR.

There’s no way to mark only some unsigned operations as wrapping-expected in ISO C++.

At least one language, Rust, does solve this problem: overflow of the value-range is always an error for plain +, -, *, /, etc. for any integral type including signed and unsigned. You can use x.wrapping_sub(y) to do signed or unsigned subtraction with well-defined wrap-around. Similarly for add/mul/div/rem/shift/pow. And there’s saturating_sub/add/etc, and overflowing_… that returns the wrapped result and a boolean, or checked_add/sub/etc that returns a type that can be None instead of holding an integer. So if you want to fuss over integer overflows, Rust might be the language for you.

(I wouldn’t be surprised if LLVM’s back-end checking for unsigned overflow was partially motivated by Rust, and someone thought it might sometimes be useful to expose that for use by C++. But expect false positives in code not written with that checker in mind.)

GNU C integer wrapping overflow extensions

GCC/Clang and other compilers that understand the GNU dialect of C and C++ have integer-overflow builtins. That includes both signed and unsigned wrapping add/sub/mul. But only for (unsigned) int/long/long long; you’d have to figure out which one to use for size_t in libstdc++. (e.g. on Windows x64, size_t has to be long long, but on x86-64 System V it’s long)

unsigned long wrapping_sub(unsigned long x, unsigned long y)
{
    // return x - y;       // ISO C++ without working around sanitize=integer

    unsigned long res;
    bool borrow = __builtin_usubl_overflow(x, y, &res);
    return res;
}

A test case on Godbolt shows that __builtin_usubl_overflow does safely do a wrapping subtract of 1UL, 2UL. (Making asm that doesn’t even try to detect wrapping, because we’ve told the compiler that’s not an error on this one operation.) Uncommenting return x-y; does trap the overflow.

It would be very cumbersome to use this for every unsigned operation in standard library code where wrapping isn’t an error, which is why libc++ disables the unsigned-wrapping sanitizer on a per-function basis instead.

Since unsigned math is well-defined as wrapping, the normal reason for using the unsigned versions of these GNU C builtins is to capture the carry/borrow output, so you know if they wrapped. Instead of using clang’s sanitize=integer, you could use these functions on your own unsigned operations, and assert() that the bool result is false (no wrapping overflow).