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# Hoopl examples
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This page gathers examples of Hoopl usage so that beginners can learn from them.
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## Dead assignment removal
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This was originally part of `CmmLive` module:
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```wiki
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removeDeadAssignments :: DynFlags -> CmmGraph
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-> UniqSM (CmmGraph, BlockEnv CmmLocalLive)
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removeDeadAssignments dflags g =
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dataflowPassBwd g [] $ analRewBwd liveLattice (xferLive dflags) rewrites
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where rewrites = mkBRewrite3 nothing middle nothing
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-- SDM: no need for deepBwdRw here, we only rewrite to empty
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-- Beware: deepBwdRw with one polymorphic function seems more
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-- reasonable here, but GHC panics while compiling, see bug
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-- #4045.
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middle :: CmmNode O O -> Fact O CmmLocalLive -> CmmReplGraph O O
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middle (CmmAssign (CmmLocal reg') _) live
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| not (reg' `elemRegSet` live)
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= return $ Just emptyGraph
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-- XXX maybe this should be somewhere else...
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middle (CmmAssign lhs (CmmReg rhs)) _ | lhs == rhs
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= return $ Just emptyGraph
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middle (CmmStore lhs (CmmLoad rhs _)) _ | lhs == rhs
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= return $ Just emptyGraph
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middle _ _ = return Nothing
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nothing :: CmmNode e x -> Fact x CmmLocalLive -> CmmReplGraph e x
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nothing _ _ = return Nothing
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```
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## CmmRewriteAssignments optimization pass
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The `CmmRewriteAssignments` pass was originally written by Edward Z. Yang to perform Cmm optimizations like inlining and sinking. However, it turned out to be too slow and was replaced with `CmmSink` pass written by Simon Marlow. `CmmSink` does almost the same things as `CmmRewriteAssignments`, the most notable difference being that the former does not handle loops. Code of `CmmRewriteAssignments` is available in [this attachment](/trac/ghc/attachment/wiki/Commentary/Compiler/Hoopl/Examples/CmmRewriteAssignments.hs)[](/trac/ghc/raw-attachment/wiki/Commentary/Compiler/Hoopl/Examples/CmmRewriteAssignments.hs). |
