Commit f3ecd3c3 authored by simonm's avatar simonm
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[project @ 1999-04-06 09:48:01 by simonm]

Document -funbox-strict-fields.
parent d1bc23d5
......@@ -17,4 +17,7 @@ computations on small integers. The performance of <tt/Integer/ is now
only slightly slower than <tt/Int/ for values between <tt/minBound :: Int/
and <tt/maxBound :: Int/.
<item> Added @-funbox-strict-fields@ for unboxing/unpacking strict
constructor fields.
......@@ -1000,6 +1000,52 @@ arbitrary; they are of the ``seem to be OK'' variety. The `8' is the
more critical one; it's what determines how eager GHC is about
expanding unfoldings.
<nidx>-funbox-strict-fields option</nidx>
<nidx>strict constructor fields</nidx>
<nidx>constructor fields, strict</nidx>
This option causes all constructor fields which are marked strict
(i.e. ``!'') to be unboxed or unpacked if possible. For example:
data T = T !Float !Float
will create a constructor @T@ containing two unboxed floats if the
@-funbox-strict-fields@ flag is given. This may not always be an
optimisation: if the @T@ constructor is scrutinised and the floats
passed to a non-strict function for example, they will have to be
reboxed (this is done automatically by the compiler).
This option should only be used in conjunction with @-O@, in order to
expose unfoldings to the compiler so the reboxing can be removed as
often as possible. For example:
f :: T -> Float
f (T f1 f2) = f1 + f2
The compiler will avoid reboxing @f1@ and @f2@ by inlining @+@ on
floats, but only when @-O@ is on.
Any single-constructor data is eligible for unpacking; for example
data T = T !(Int,Int)
will store the two @Int@s directly in the @T@ constructor, by flattening
the pair. Multi-level unpacking is also supported:
data T = T !S
data S = S !Int !Int
will store two unboxed @Int#@s directly in the @T@ constructor.
This option (which <em>does not work</em> with the native-code generator)
tells the compiler to add extra code to test for already-evaluated
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