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Diffstat (limited to 'compiler/GHC/Core/Op/Simplify/Utils.hs')
| -rw-r--r-- | compiler/GHC/Core/Op/Simplify/Utils.hs | 18 |
1 files changed, 11 insertions, 7 deletions
diff --git a/compiler/GHC/Core/Op/Simplify/Utils.hs b/compiler/GHC/Core/Op/Simplify/Utils.hs index 4b85bff280..1b8c21f81b 100644 --- a/compiler/GHC/Core/Op/Simplify/Utils.hs +++ b/compiler/GHC/Core/Op/Simplify/Utils.hs @@ -1872,22 +1872,26 @@ Historical note: if you use let-bindings instead of a substitution, beware of th prepareAlts tries these things: -1. Eliminate alternatives that cannot match, including the - DEFAULT alternative. +1. filterAlts: eliminate alternatives that cannot match, including + the DEFAULT alternative. Here "cannot match" includes knowledge + from GADTs -2. If the DEFAULT alternative can match only one possible constructor, - then make that constructor explicit. +2. refineDefaultAlt: if the DEFAULT alternative can match only one + possible constructor, then make that constructor explicit. e.g. case e of x { DEFAULT -> rhs } ===> case e of x { (a,b) -> rhs } where the type is a single constructor type. This gives better code when rhs also scrutinises x or e. + See CoreUtils Note [Refine DEFAULT case alternatives] -3. Returns a list of the constructors that cannot holds in the - DEFAULT alternative (if there is one) +3. combineIdenticalAlts: combine identical alternatives into a DEFAULT. + See CoreUtils Note [Combine identical alternatives], which also + says why we do this on InAlts not on OutAlts -Here "cannot match" includes knowledge from GADTs +4. Returns a list of the constructors that cannot holds in the + DEFAULT alternative (if there is one) It's a good idea to do this stuff before simplifying the alternatives, to avoid simplifying alternatives we know can't happen, and to come up with |