perf(parser): reduce Token size from 16 to 12 bytes
#2010
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Counter-intuitively, it seems that *increasing* the size of `Token` improves performance slightly. This appears to be because when `Token` is 16 bytes, copying `Token` is a single 16-byte load/store. At present, it's 12 bytes which requires an 8-byte load/store + a 4-byte load/store. https://godbolt.org/z/KPYsn3ab7 This suggests that either: 1. #2010 could be reverted at no cost, and the overhead of the hash table removed. or: 2. We need to get `Token` down to 8 bytes! I have an idea how to *maybe* do (2), so I'd suggest leaving it as is for now until I've been able to research that. NB I also tried putting `#[repr(align(16))]` on `Token` so that copying uses aligned loads/stores. That [hurt the benchmarks very slightly](https://codspeed.io/overlookmotel/oxc/branches/lexer-pad-token), though it might produce a gain on architectures where unaligned loads are more expensive (ARM64 I think?). But I can't test that theory, so have left it out.
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I also had to change how the string for private identifiers are built, otherwise they will always be allocated.
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Counter-intuitively, it seems that *increasing* the size of `Token` improves performance slightly. This appears to be because when `Token` is 16 bytes, copying `Token` is a single 16-byte load/store. At present, it's 12 bytes which requires an 8-byte load/store + a 4-byte load/store. https://godbolt.org/z/KPYsn3ab7 This suggests that either: 1. oxc-project#2010 could be reverted at no cost, and the overhead of the hash table removed. or: 2. We need to get `Token` down to 8 bytes! I have an idea how to *maybe* do (2), so I'd suggest leaving it as is for now until I've been able to research that. NB I also tried putting `#[repr(align(16))]` on `Token` so that copying uses aligned loads/stores. That [hurt the benchmarks very slightly](https://codspeed.io/overlookmotel/oxc/branches/lexer-pad-token), though it might produce a gain on architectures where unaligned loads are more expensive (ARM64 I think?). But I can't test that theory, so have left it out.
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I also had to change how the string for private identifiers are built, otherwise they will always be allocated.