[aarch64] assert(is_power_of_2(x)) failed: x must be a power of 2: 0xffffffff80000000

[Andrew Haley]

On 2/24/20 4:27 PM, Schmidt, Lutz wrote:
>
    >     > A possible fix would include:
>     >  - make is_power_of_2() and exact_log_2() sign-aware.
>     >  - call is_power_of_2() and exact_log_2() with arguments of proper
>     > type (juint in this case).
>
>     Or fix exact_log2(jint) by specializing it appropriately.
>
> Yes, by treating the jint argument as unsigned.

OK, I'll look at that. However, studying the code, we have at least
one more problem, and it's a dangerous one.

template <typename T>
bool is_power_of_2_t(T x) {
  return (x != T(0)) && ((x & (x - 1)) == T(0));
}

// long version of is_power_of_2
inline bool is_power_of_2_long(jlong x) {
  return ((x != NoLongBits) && (mask_long_bits(x, x - 1) == NoLongBits));
}

These overflow when x is the largest negative integer. This is
Undefined Behaviour, so we have to fix it somehow. We'd do much better
converting to an unsigned type and then performing the calculation on
that. Maybe we should provide just one definition of this function,
casting its argument to uint64_t.

UB problems keep occurring. It'd be good to test HotSpot changes with
-fsanitize=undefined, but we still have so much UB in HotSpot that
it's impractical to do so because there's so much noise.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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