fix: compute field offsets per-class instead of globally#1683
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derklaro
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### Motivation Currently our unsafe replacement implementation computes field offsets using a global long counter (which should be fine as the documentation in `sun.misc.Unsafe` is not specifying how field offsets are resolved). However, there are cases where libraries depend on the fact that the returned offset is rather small and computed based on the class tree. This is for example the case in the google protobuf implementation, which allow a max field offset value of `(1 << 20) - 1` (1,048.575) as they do some optimizations using bit packing. ### Modification Compute field offsets based on the class hierachy instead of globally and provide values that are much closer to the values that are actually returned by `sun.misc.Unsafe`. ### Result No more breaking library code because of bad assumptions about the return value of field offset getter methods. ##### Other context Code that packs the field offset and breaks: https://github.com/protocolbuffers/protobuf/blob/960e79087b332583c80537c949621108a85aa442/java/core/src/main/java/com/google/protobuf/MessageSchema.java#L568-L575 (cherry picked from commit cf4e1ec)
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Motivation
Currently our unsafe replacement implementation computes field offsets using a global long counter (which should be fine as the documentation in
sun.misc.Unsafeis not specifying how field offsets are resolved). However, there are cases where libraries depend on the fact that the returned offset is rather small and computed based on the class tree. This is for example the case in the google protobuf implementation, which allow a max field offset value of(1 << 20) - 1(1,048.575) as they do some optimizations using bit packing.Modification
Compute field offsets based on the class hierachy instead of globally and provide values that are much closer to the values that are actually returned by
sun.misc.Unsafe.Result
No more breaking library code because of bad assumptions about the return value of field offset getter methods.
Other context
Code that packs the field offset and breaks: https://github.com/protocolbuffers/protobuf/blob/960e79087b332583c80537c949621108a85aa442/java/core/src/main/java/com/google/protobuf/MessageSchema.java#L568-L575