What is the best pointer tagging method?
Pointer tagging methods show varying performance based on architecture and use case, with nan boxing being efficient for floating-point languages. Overall performance is more influenced by memory access patterns and cache efficiency.
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pointer tagging methods yield similar performance, with specific advantages depending on the architecture and use case. The article discusses various pointer tagging techniques, including lower bits, lower byte, upper byte, upper bits, and nan boxing, each utilizing different parts of the pointer to store metadata. Benchmarks reveal that while untagged pointers generally perform better, certain tagged methods can outperform in specific scenarios, particularly when compiler optimizations are applied. The performance of these methods can vary significantly between architectures, such as ARM and x86, due to differences in instruction sets. Notably, nan boxing is highlighted for its efficiency in languages that primarily use floating-point numbers, as it allows for direct embedding of pointers into floating-point values. Ultimately, the article concludes that while pointer tagging can optimize certain operations, the overall system performance is more heavily influenced by memory access patterns and cache efficiency than by the choice of pointer tagging method.
- Pointer tagging encodes metadata into pointers for compact representation.
- Performance varies by architecture; ARM and x86 show different efficiencies.
- Nan boxing is beneficial for floating-point-heavy languages.
- Untagged pointers generally outperform tagged ones, but specific scenarios favor tagging.
- Memory access patterns and cache efficiency are critical for overall system performance.
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10 commentsis not true. Intel and AMD both have variants of TBI on their chips, called Linear Address Masking and Upper Address Ignore respectively. It's a bit of a mess, unfortunately, with both masking off different bits from the top of the address (and different bits than ARM TBI does), but it does exist.
That's not the only benefit. The main benefit is arguably that you don't have to allocate floats on the heap and garbage collect them. Numerical code allocates lots of numbers, so having these all be inline rather than heap-allocated saves lots of space and time.
https://en.wikipedia.org/wiki/Classic_Mac_OS_memory_manageme...
The other approach is CompressedOops, where instead of wasting pointer bits (and maybe using them for tags), Java's HotSpot VM chooses to only store a 32-bit offset for an eight-aligned heap object if the entire heap is known to fit within 2^(32+3) which is 32 GB from its base address.
https://news.ycombinator.com/item?id=22398251
And didn't somebody write about creating a large aligned arena for each type and essentially grabbing the base address of the arena as a (non-unique) type tag for its objects? Then the moving GC would use these arenas as semispaces.
If you can reduce your tag to a single bit (object vs primitive), a single byte of tag data can cover 8 variables, and a bigger integer can cover a whole 64 variables, plenty for most functions.
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