RFR: 8332697: ubsan: shenandoahSimpleBitMap.inline.hpp:68:23: runtime error: signed integer overflow: -9223372036854775808 - 1 cannot be represented in type 'long int'

[Andrew Haley]

On Fri, 19 Jul 2024 22:41:11 GMT, Dean Long <dlong at openjdk.org> wrote:

>> Cast the result of `nth_bit(n)` to `uintptr_t` to prevent signed integer overflow error reported by `ubsan`. Unsigned overflow is not undefined behavior and is not checked by `ubsan`.
>
> src/hotspot/share/utilities/globalDefinitions.hpp line 1069:
> 
>> 1067: // (note: #define used only so that they can be used in enum constant definitions)
>> 1068: #define nth_bit(n)        (((n) >= BitsPerWord) ? 0 : (OneBit << (n)))
>> 1069: #define right_n_bits(n)   ((uintptr_t) nth_bit(n) - 1)
> 
> This changes the return type of right_n_bits, which could break existing code.  If we need an unsigned version, I think it should have a different name.

And this is at best a partial fix: `OneBit << 63` overflows.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20164#discussion_r1685678905