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FastString: SAT bucket_match

doyougnu requested to merge doyougnu/ghc:wip/doyougnu/faster-match-bucket into master

Working on smaller and faster FastStrings because the JS backend makes heavy use of them and I noticed that bucket_match could be SAT'd. I didn't check any core or stg, but this little patch improves MultiLayerModulesTH_OneShot test on some platforms:

Metrics: compile_time/bytes allocated
-------------------------------------
                                                    Baseline                           
                               Test    Metric          value      New value Change     
---------------------------------------------------------------------------------------
                 CoOpt_Read(normal) ghc/alloc    784,107,349    784,014,696  -0.0%     
           CoOpt_Singletons(normal) ghc/alloc    921,010,885    920,827,416  -0.0%     
           InstanceMatching(normal) ghc/alloc  5,021,488,306  5,021,564,256  +0.0%     
          InstanceMatching1(normal) ghc/alloc 27,513,195,552 27,512,182,320  -0.0%     
                LargeRecord(normal) ghc/alloc  2,945,091,925  2,945,019,736  -0.0%     
           ManyAlternatives(normal) ghc/alloc    688,812,226    688,682,472  -0.0%     
           ManyConstructors(normal) ghc/alloc  3,717,067,904  3,717,076,416  +0.0%     
      MultiComponentModules(normal) ghc/alloc  1,859,847,557  1,858,482,776  -0.1%     
MultiComponentModulesRecomp(normal) ghc/alloc    803,386,557    801,979,264  -0.2%     
          MultiLayerModules(normal) ghc/alloc  3,131,702,544  3,129,216,432  -0.1%     
    MultiLayerModulesRecomp(normal) ghc/alloc    887,582,074    885,437,552  -0.2%     
   MultiLayerModulesTH_Make(normal) ghc/alloc    620,757,056    619,758,056  -0.2%     
MultiLayerModulesTH_OneShot(normal) ghc/alloc  2,488,042,293  2,408,490,208  -3.2% GOOD
                  PmSeriesG(normal) ghc/alloc     44,297,818     44,248,704  -0.1%

I suspect this to be real because a deviation of 3% by random chance would be quite unusual.

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