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Summary: Reimplement global FastString table using a concurrent hashatable with fixed size segments and dynamically growing buckets instead of fixed size buckets. This addresses the problem that `mkFastString` was not linear when the total number of entries was large. Test Plan: ./validate ``` inplace/bin/ghc-stage2 --interactive -dfaststring-stats < /dev/null GHCi, version 8.7.20181005: http://www.haskell.org/ghc/ :? for help Prelude> Leaving GHCi. FastString stats: segments: 256 buckets: 16384 entries: 7117 largest segment: 64 smallest segment: 64 longest bucket: 5 has z-encoding: 0% ``` Also comapre the two implementation using {P187} The new implementation is on a par with the old version with different conbination of parameters and perform better when the number of FastString's are large. {P188} Reviewers: simonmar, bgamari, niteria Reviewed By: simonmar, bgamari Subscribers: rwbarton, carter GHC Trac Issues: #14854 Differential Revision: https://phabricator.haskell.org/D5211
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