Cubic root of zero gives wrong result with clang 64-bit

Hi,

I am using Valgrind 3.17.0 and noticed a strange behavior while running a
code compiled with clang:

#include <cmath>
#include <cstdio>

int main() {
    long double x = std::cbrtl(0.0L);
    printf("%Lf", x);

    return 0;
}

This example gives complete wrong output:

clang++ cbrtl.cpp -o cbrtl && valgrind ./cbrtl
==338929== Memcheck, a memory error detector
==338929== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==338929== Using Valgrind-3.17.0 and LibVEX; rerun with -h for copyright
info
==338929== Command: ./cbrtl
==338929==
-0.178840
==338929==
==338929== HEAP SUMMARY:
==338929==     in use at exit: 0 bytes in 0 blocks
==338929==   total heap usage: 2 allocs, 2 frees, 73,728 bytes allocated
==338929==
==338929== All heap blocks were freed -- no leaks are possible
==338929==
==338929== For lists of detected and suppressed errors, rerun with: -s
==338929== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

However, using -m32 option for clang or gcc compiler (both 32 and 64-bit)
prints 0.0 as expected. I know that valgrind does not support 80-bit long
double and converts it to 64-bit.
I think it might cause this issue as the original code is compiled for
80-bit long doubles. It also might be an issue with clang.

Regards,
Vlad Yaglamunov

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