Mixed Blocks: Storing More Fields Flat

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" summary

Extends OCaml’s runtime to allow mixed blocks where some fields are stored flat (numeric payloads inline) while the GC scans a prefix only, encoded by stealing one header bit to record scan-prefix length (supports up to 255 scanned words). Implemented in Jane Street’s flambda-backend branch, it keeps object size unchanged, supports int64/int32/float/nativeint, and yields modest real-world speedups (~1–2%). Trade-offs: unscanned fields must be a suffix and it disables generic comparison.

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" chapters (15)

1. 0:00Introduction
2. 1:50Memory layout vs performance
3. 2:31OCaml value representation
4. 4:40Mixed blocks concept
5. 6:05Limitations of current garbage collector tags
6. 7:08Mixed blocks design
7. 8:34Header word encoding
8. 10:21Scan prefix advantages
9. 12:30Mixed blocks limitations
10. 13:54Comparison & generic ops
11. 14:37Explicit mixed blocks syntax
12. 16:25Microbenchmarking
13. 17:06Indirection impact
14. 17:27Non-scanned fields
15. 18:10Q&A