ruby: Fix object cache lookups on 32-bit platforms (#13494) #13580
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#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 Closes #13494 COPYBARA_INTEGRATE_REVIEW=#13494 from stanhu:sh-fix-ruby-protobuf-32bit d63122a PiperOrigin-RevId: 557211768
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Aug 17, 2023
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#13204 refactored the Ruby object cache to use a key of
LL2NUM(key_val)instead ofLL2NUM(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:
FIXNUM_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
Closes #13494
COPYBARA_INTEGRATE_REVIEW=#13494 from stanhu:sh-fix-ruby-protobuf-32bit d63122a PiperOrigin-RevId: 557211768