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| author | simonmar <unknown> | 2003-11-27 13:27:11 +0000 |
|---|---|---|
| committer | simonmar <unknown> | 2003-11-27 13:27:11 +0000 |
| commit | fdac0d5f9911b64fa1080fbb4758d1fa0dd0070c (patch) | |
| tree | 92da0ca88f051bdcd0a8d7164e2938d4778fdc2c /ghc/docs/users_guide/using.sgml | |
| parent | e9e72bd1cd7776609aa7d7f5e71edc4c2edced4d (diff) | |
| download | haskell-fdac0d5f9911b64fa1080fbb4758d1fa0dd0070c.tar.gz | |
[project @ 2003-11-27 13:27:11 by simonmar]
Document UNPACK pragma
Diffstat (limited to 'ghc/docs/users_guide/using.sgml')
| -rw-r--r-- | ghc/docs/users_guide/using.sgml | 57 |
1 files changed, 9 insertions, 48 deletions
diff --git a/ghc/docs/users_guide/using.sgml b/ghc/docs/users_guide/using.sgml index 0abae40ba1..fb5f6f6644 100644 --- a/ghc/docs/users_guide/using.sgml +++ b/ghc/docs/users_guide/using.sgml @@ -1185,54 +1185,15 @@ f "2" = 2 <para>This option causes all constructor fields which are marked strict (i.e. “!”) to be unboxed or - unpacked if possible. For example:</para> - -<ProgramListing> -data T = T !Float !Float -</ProgramListing> - - <para>will create a constructor <literal>T</literal> - containing two unboxed floats if the - <option>-funbox-strict-fields</option> flag is given. - This may not always be an optimisation: if the - <Function>T</Function> 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).</para> - - <para>This option should only be used in conjunction with - <option>-O</option>, in order to expose unfoldings to the - compiler so the reboxing can be removed as often as - possible. For example:</para> - -<ProgramListing> -f :: T -> Float -f (T f1 f2) = f1 + f2 -</ProgramListing> - - <para>The compiler will avoid reboxing - <Function>f1</Function> and <Function>f2</Function> by - inlining <Function>+</Function> on floats, but only when - <option>-O</option> is on.</para> - - <para>Any single-constructor data is eligible for - unpacking; for example</para> - -<ProgramListing> -data T = T !(Int,Int) -</ProgramListing> - - <para>will store the two <literal>Int</literal>s directly - in the <Function>T</Function> constructor, by flattening - the pair. Multi-level unpacking is also supported:</para> - -<ProgramListing> -data T = T !S -data S = S !Int !Int -</ProgramListing> - - <para>will store two unboxed <literal>Int#</literal>s - directly in the <Function>T</Function> constructor.</para> + unpacked if possible. It is equivalent to adding an + <literal>UNPACK</literal> pragma to every strict + constructor field (see <xref + linkend="unpack-pragma">).</para> + + <para>This option is a bit of a sledgehammer: it might + sometimes make things worse. Selectively unboxing fields + by using <literal>UNPACK</literal> pragmas might be + better.</para> </listitem> </varlistentry> |