Commit 0380c5f1 authored by's avatar
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Add comments about primop rules

parent bd45ccd3
......@@ -54,6 +54,25 @@ import Data.Word ( Word64 )
Note [Constant folding]
primOpRules generates the rewrite rules for each primop
These rules do what is often called "constant folding"
E.g. the rules for +# might say
4 +# 5 = 9
Well, of course you'd need a lot of rules if you did it
like that, so we use a BuiltinRule instead, so that we
can match in any two literal values. So the rule is really
more like
(Lit 4) +# (Lit y) = Lit (x+#y)
where the (+#) on the rhs is done at compile time
That is why these rules are built in here. Other rules
which don't need to be built in are in GHC.Base. For
x +# 0 = x
primOpRules :: PrimOp -> Name -> [CoreRule]
primOpRules op op_name = primop_rule op
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