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Prefer packed representation for CompiledByteCode

Hannes Siebenhandl requested to merge fendor/ghc:wip/fendor/array-bcos into master

As there are many 'CompiledByteCode' objects alive during a GHCi session, representing its element in a more packed manner improves space behaviour at a minimal cost.

When running GHCi on the agda codebase, we find around 380 live 'CompiledByteCode' objects. Packing their respective 'UnlinkedByteCode' can save quite some pointers.

In experiments on the agda codebase, the number of live bytes was reduced by around 15MB, which is roughly 2% of the overall live bytes. These numbers have been obtained using -hT -i1 -l.

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