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Diffstat (limited to 'compiler/GHC/Core/Utils.hs')
| -rw-r--r-- | compiler/GHC/Core/Utils.hs | 25 |
1 files changed, 21 insertions, 4 deletions
diff --git a/compiler/GHC/Core/Utils.hs b/compiler/GHC/Core/Utils.hs index c846d2ac2e..82d920a58e 100644 --- a/compiler/GHC/Core/Utils.hs +++ b/compiler/GHC/Core/Utils.hs @@ -864,10 +864,27 @@ This gave rise to a horrible sequence of cases and similarly in cascade for all the join points! -NB: it's important that all this is done in [InAlt], *before* we work -on the alternatives themselves, because Simplify.simplAlt may zap the -occurrence info on the binders in the alternatives, which in turn -defeats combineIdenticalAlts (see #7360). +Note [Combine identical alternatives: wrinkles] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +* It's important that we try to combine alternatives *before* + simplifying them, rather than after. Reason: because + Simplify.simplAlt may zap the occurrence info on the binders in the + alternatives, which in turn defeats combineIdenticalAlts use of + isDeadBinder (see #7360). + + You can see this in the call to combineIdenticalAlts in + SimplUtils.prepareAlts. Here the alternatives have type InAlt + (the "In" meaning input) rather than OutAlt. + +* combineIdenticalAlts does not work well for nullary constructors + case x of y + [] -> f [] + (_:_) -> f y + Here we won't see that [] and y are the same. Sigh! This problem + is solved in CSE, in CSE.combineAlts, which does a better version of + combineIdenticalAlts. But sadly it doesn't have the occurrence info + we have here. See Note [Combine case alts: awkward corner] in CSE). Note [Care with impossible-constructors when combining alternatives] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |