https://github.com/protocolbuffers/protobuf/pull/13204 refactored the Ruby object cache to use a key of `LL2NUM(key_val)` instead of `LL2NUM(key_val >> 2)`. On 32-bit systems, `LL2NUM(key_val)` returns inconsistent results because a large value has to be stored as a Bignum on the heap. This causes cache lookups to fail.
This commit restores the previous behavior of using `ObjectCache_GetKey`, which discards the lower 2 bits, which are zero. This enables a key to be stored as a Fixnum on both 32 and 64-bit platforms.
As https://patshaughnessy.net/2014/1/9/how-big-is-a-bignum describes, a Fixnum uses:
* 1 bit for the `FIXNUM_FLAG`.
* 1 bit for the sign flag.
Therefore the largest possible Fixnum value on a 64-bit value is 4611686018427387903 (2^62 - 1). On a 32-bit system, the largest value is 1073741823 (2^30 - 1).
For example, a possible VALUE pointer address on a 32-bit system:
0xff5b4af8 => 4284173048
Dropping the lower 2 bits makes up for the loss of range to these flags. In the example above, we see that shifting by 2 turns the value into a 30-bit number, which can be represented as a Fixnum:
(0xff5b4af8 >> 2) => 1071043262
This bug can also manifest on a 64-bit system if the upper bits are 0xff.
Closes#13481
Stan Hu
055c3782f6