Unconditionally reserve the top header byte in runtime 4#2397
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ncik-roberts merged 1 commit intooxcaml:reserve-header-bits-in-runtime-5from Apr 3, 2024
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Builds on #2389 to also unconditionally reserve the top header byte in runtime 4. This is a prerequisite for the rollout of mixed blocks.
A priori, you'd expect this change to be a no-op for existing code on 64 bit platforms. The top header byte, if not reserved, is interpreted as part of the size of the block. And you'd need to create a comically big value to require stealing bits from the top byte. Napkin math to support this claim: with the top header byte reserved, there are 46 remaining bits used to encode the size. To require more than 46 size bits, you'd need to create a value that's 2^46 words = 512 TiB large.
Internal testing supports this claim.