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Introduce the flag `-fwrite-if-compression=<n>` which allows to configure the compression level of writing .hi files. The motivation is that some deduplication operations are too expensive for the average use case. Hence, we introduce multiple compression levels with variable impact on performance, but still reduce the memory residency and `.hi` file size on disk considerably. We introduce three compression levels: * `1`: `Normal` mode. This is the least amount of compression. It deduplicates only `Name` and `FastString`s, and is naturally the fastest compression mode. * `2`: `Safe` mode. It has a noticeable impact on .hi file size and is marginally slower than `Normal` mode. In general, it should be safe to always use `Safe` mode. * `3`: `Full` deduplication mode. Deduplicate as much as we can, resulting in minimal .hi files, but at the cost of additional compilation time. Reading .hi files doesn't need to know the initial compression level, and can always deserialise a `ModIface`, as we write out a byte that indicates the next value has been deduplicated. This allows users to experiment with different compression levels for packages, without recompilation of dependencies. Note, the deduplication also has an additional side effect of reduced memory consumption to implicit sharing of deduplicated elements. See #24540 for example where that matters. ------------------------- Metric Decrease: MultiLayerModulesDefsGhciWithCore T16875 T21839c T24471 hard_hole_fits libdir -------------------------36aa7cf1
