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Reimplement unsafe coercions in terms of unsafe equality proofs

Ömer Sinan Ağacan requested to merge wip/osa1/T16893 into master

See Note [Implementing unsafeCoerce] for how unsafe equality proofs and the new unsafeCoerce# are implemented.

New notes added:

  • [Checking for levity polymorphism] in CoreLint.hs
  • [Implementing unsafeCoerce] in base/Unsafe/Coerce.hs
  • [Patching magic definitions] in Desugar.hs
  • [Wiring in unsafeCoerce#] in Desugar.hs

Only breaking change in this patch is unsafeCoerce# is not exported from GHC.Exts, instead of GHC.Prim.

Fixes #17443 (closed)

Fixes #16893 (closed)

NoFib
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        Program           Size    Allocs    Instrs     Reads    Writes
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            CSD          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
             FS          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
              S          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
             VS          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
            VSD          -0.1%      0.0%     -0.0%     -0.0%     -0.1%
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           bspt          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
      cacheprof          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
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            scc          -0.1%      0.0%     -0.1%     -0.1%     -0.1%
          sched          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
            scs          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
         simple          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
          solid          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
        sorting          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
  spectral-norm          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
         sphere          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
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       treejoin          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
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           wang          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
      wave4main          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
   wheel-sieve1          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
   wheel-sieve2          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
           x2n1          -0.1%      0.0%     -0.0%     -0.0%     -0.0%
--------------------------------------------------------------------------------
            Min          -0.1%      0.0%     -0.5%     -0.3%     -0.4%
            Max          -0.0%      0.0%     +0.0%     +0.0%     +0.0%
 Geometric Mean          -0.1%     -0.0%     -0.0%     -0.0%     -0.0%

Test changes

  • break006 is marked as broken, see #17833
  • The compiler allocates less when building T14683 (an unsafeCoerce#- heavy happy-generated code) on 64-platforms. Allocates more on 32-bit platforms.
Edited by Ömer Sinan Ağacan

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