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Alp Mestanogullari authored
Previously, as reported in #15938, resuming a build "in the middle", e.g when building _build/stage1/libraries/base/, hadrian would take up to a whole minute to get started doing actual work, building code. This was mostly due to a big enumeration that we do in Rules.hs, to generate all the possible patterns for object files for 1) all ways, 2) all packages and 3) all stages. Since rule enumeration is always performed, whatever the target, we were always paying this cost, which seemed to grow bigger the farther in the build we stopped and were resuming from. Instead, this patch borrows the approach that we took for Rules.Library in https://github.com/snowleopard/hadrian/pull/571, which exposes all the relevant object files under as few catch-all rules as possible (8 here), and parses all the information we need out of the object's path. The concrete effect of this patch that I have observed is to reduce the 45-60 seconds pause to <5 seconds. Along with the Shake performance improvements that Neil mentions in #15938, most of the pause should effectively disappear. Reviewers: snowleopard, bgamari, goldfire Reviewed By: snowleopard Subscribers: rwbarton, carter GHC Trac Issues: #15938 Differential Revision: https://phabricator.haskell.org/D5412
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