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Ömer Sinan Ağacan authored
Given two unboxed sum terms: (# 1 | #) :: (# Int | Int# #) (# 1 | #) :: (# Int | Int #) These two terms are not equal as they unarise to different unboxed tuples. However StgCse was thinking that these are equal, and replacing one of these with a binder to the other. To not deal with unboxed sums in StgCse we now do it after unarise. For StgCse to maintain post-unarise invariants we factor-out case binder in-scopeness check to `stgCaseBndrInScope` and use it in StgCse. Also did some refactoring in SimplStg. Another way to fix this would be adding a special case in StgCse to not bring unboxed sum binders in scope: diff --git a/compiler/simplStg/StgCse.hs b/compiler/simplStg/StgCse.hs index 6c740ca4cb..93a0f8f6ad 100644 --- a/compiler/simplStg/StgCse.hs +++ b/compiler/simplStg/StgCse.hs @@ -332,7 +332,11 @@ stgCseExpr env (StgLetNoEscape binds body) stgCseAlt :: CseEnv -> OutId -> InStgAlt -> OutStgAlt stgCseAlt env case_bndr (DataAlt dataCon, args, rhs) = let (env1, args') = substBndrs env args - env2 = addDataCon case_bndr dataCon (map StgVarArg args') env1 + env2 + | isUnboxedSumCon dataCon + = env1 + | otherwise + = addDataCon case_bndr dataCon (map StgVarArg args') env1 -- see note [Case 2: CSEing case binders] rhs' = stgCseExpr env2 rhs in (DataAlt dataCon, args', rhs') I think this patch seems better in that it doesn't add a special case to StgCse. Test Plan: Validate. I tried to come up with a minimal example but failed. I thought a simple program like data T = T (# Int | Int #) (# Int# | Int #) case T (# 1 | #) (# 1 | #) of ... should be enough to trigger this bug, but for some reason StgCse doesn't do anything on this program. Reviewers: simonpj, bgamari Reviewed By: simonpj Subscribers: rwbarton, thomie, carter GHC Trac Issues: #15300 Differential Revision: https://phabricator.haskell.org/D4962 (cherry picked from commit 3c311e50)
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