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Commit 4caef1c4 authored by Simon Peyton Jones's avatar Simon Peyton Jones
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Document optimisation passes, to some extent at least

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docs/users_guide/using.xml
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......@@ -1770,18 +1770,21 @@ f "2" = 2
They are normally set via the <option>-O</option> options
described above, and as such, you shouldn't need to set any of
them explicitly (indeed, doing so could lead to unexpected
results). However, there are one or two that may be of
interest:</para>
results). A flag <option>-fwombat</option> can be negated by
saying <option>-fno-wombat</option>. The flags below are off by default,
except where noted below.
</para>
<variablelist>
<varlistentry>
<term>
<option>-fno-cse</option>
<indexterm><primary><option>-fno-cse</option></primary></indexterm>
<option>-fcse</option>
<indexterm><primary><option>-fcse</option></primary></indexterm>
</term>
<listitem>
<para>Turns off the common-sub-expression elimination optimisation.
Can be useful if you have some <literal>unsafePerformIO</literal>
<para><emphasis>On by default.</emphasis>. Enables the common-sub-expression
elimination optimisation.
Switching this off can be useful if you have some <literal>unsafePerformIO</literal>
expressions that you don't want commoned-up.</para>
</listitem>
</varlistentry>
......@@ -1792,21 +1795,10 @@ f "2" = 2
<indexterm><primary><option></option></primary></indexterm>
</term>
<listitem>
<para>
<para> <emphasis>On by default.</emphasis>.
Switch on the strictness analyser. There is a very old paper about GHC's
strictness analyser, <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/simple-strictnes-analyser.ps.gz">Measuring the effectiveness of a simple strictness analyser</ulink>, but the current one is quite a bit different.
</para>
TODO: Document optimisation
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-fno-strictness</option>
<indexterm><primary><option>-fno-strictness</option></primary></indexterm>
</term>
<listitem>
<para>Turns off the strictness analyser; sometimes it eats
too many cycles.</para>
TODO: Maybe document positive cases and remove all negative cases?
</listitem>
</varlistentry>
......@@ -1840,19 +1832,21 @@ f "2" = 2
<indexterm><primary><option>-fspec-constr</option></primary></indexterm>
</term>
<listitem>
<para>Turn on call-pattern specialisation.</para>
TODO: Document optimisation
<para><emphasis>Off by default, but enabled by -O2.</emphasis>
Turn on call-pattern specialisation; see <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/spec-constr/index.htm">Call-pattern specialisation for Haskell programs</ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-fno-specialise</option>
<indexterm><primary><option>-fno-specialise</option></primary></indexterm>
<option>-fspecialise</option>
<indexterm><primary><option>-fspecialise</option></primary></indexterm>
</term>
<listitem>
<para>Turns off the automatic specialisation of overloaded functions.</para>
TODO: Document optimisation, probably change to a positive, not a -fno-* description.
<para><emphasis>On by default.</emphasis>
Specialise each type-class-overloaded function defined in this module for the types at which
it is called in this module. Also specialise imported functions that have an INLINABLE pragma
(<xref linkend="inlinable-pragma"/>) for the types at which they are called in this module.</para>
</listitem>
</varlistentry>
......@@ -1862,8 +1856,7 @@ f "2" = 2
<indexterm><primary><option>-fstatic-argument-transformation</option></primary></indexterm>
</term>
<listitem>
<para>Turn on the static argument transformation.</para>
TODO: Document optimisation
<para>Turn on the static argument transformation, which turns a recursive function into a non-recursive one with a local recursive loop. See Chapter 7 of <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/santos-thesis.ps.gz">Andre Santos's PhD thesis</ulink></para>
</listitem>
</varlistentry>
......@@ -1873,35 +1866,24 @@ f "2" = 2
<indexterm><primary><option></option></primary></indexterm>
</term>
<listitem>
<para>
<para> <emphasis>On by default.</emphasis>
Float let-bindings inwards, nearer their binding site. See <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">Let-floating: moving bindings to give faster programs (ICFP'96)</ulink>.
</para>
TODO: Document optimisation
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-fno-float-in</option>
<indexterm><primary><option>-fno-float-in</option></primary></indexterm>
<option>-ffull-laziness</option>
<indexterm><primary><option>-ffull-laziness</option></primary></indexterm>
</term>
<listitem>
<para>Turns off the float-in transformation.</para>
TODO: Maybe document positive cases and remove all negative cases?
</listitem>
</varlistentry>
<varlistentry>
<term>
<option>-fno-full-laziness</option>
<indexterm><primary><option>-fno-full-laziness</option></primary></indexterm>
</term>
<listitem>
<para>Turns off the full laziness optimisation (also known as
let-floating). Full laziness increases sharing, which can lead
<para><emphasis>On by default.</emphasis>
Run the full laziness optimisation (also known as
let-floating), which floats let-bindings outside enclosing lambdas, in the hope they will be thereby be computed less often.
See <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/float.ps.gz">Let-floating: moving bindings to give faster programs (ICFP'96)</ulink>.
Full laziness increases sharing, which can lead
to increased memory residency.</para>
TODO: Document optimisation
<para>NOTE: GHC doesn't implement complete full-laziness.
When optimisation in on, and
<option>-fno-full-laziness</option> is not given, some
......@@ -1919,10 +1901,8 @@ f "2" = 2
<option>-fdo-lambda-eta-expansion</option>
<indexterm><primary><option></option></primary></indexterm>
</term>
<listitem>
<para>
<listitem><para> <emphasis>On by default.</emphasis> Eta-expand let-bindings to increase their arity.
</para>
TODO: Document optimisation
</listitem>
</varlistentry>
......@@ -1931,10 +1911,9 @@ f "2" = 2
<option>-fdo-eta-reduction</option>
<indexterm><primary><option></option></primary></indexterm>
</term>
<listitem>
<para>
<listitem> <para> <emphasis>On by default.</emphasis>
Eta-reduce lambda expressions, if doing so gets rid of a whole group of lambdas.
</para>
TODO: Document optimisation
</listitem>
</varlistentry>
......@@ -1944,9 +1923,21 @@ f "2" = 2
<indexterm><primary><option></option></primary></indexterm>
</term>
<listitem>
<para>
<para><emphasis>On by default.</emphasis>
Merge immediately-nested case expressions that scrutinse the same variable. Example
<programlisting>
case x of
Red -> e1
_ -> case x of
Blue -> e2
Green -> e3
==>
case x of
Red -> e1
Blue -> e2
Green -> e2
</programlisting>
</para>
TODO: Document optimisation
</listitem>
</varlistentry>
......@@ -1956,8 +1947,10 @@ f "2" = 2
<indexterm><primary><option>-fliberate-case</option></primary></indexterm>
</term>
<listitem>
<para>Turn on the liberate-case transformation.</para>
TODO: Document optimisation
<para><emphasis>Off by default, but enabled by -O2.</emphasis>
Turn on the liberate-case transformation. This unrolls recursive function once in its own RHS, to
avoid repeated case analysis of free variables. It's a bit like the call-pattern specialiser
(<option>-fspec-constr</option>) but for free variables rather than arguments.</para>
</listitem>
</varlistentry>
......@@ -1967,9 +1960,8 @@ f "2" = 2
<indexterm><primary><option></option></primary></indexterm>
</term>
<listitem>
<para>
<para> A very experimental flag that makes dictionary-valued expressions seem cheap to the optimiser.
</para>
TODO: Document optimisation
</listitem>
</varlistentry>
......@@ -1979,9 +1971,9 @@ f "2" = 2
<indexterm><primary><option></option></primary></indexterm>
</term>
<listitem>
<para>
<para> Usually GHC black-holes a thunk only when it switches threads. This flag makes it do so
as soon as the thunk is entered. See <ulink url="http://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/">Haskell on a shared-memory multiprocessor</ulink>.
</para>
TODO: Document optimisation
</listitem>
</varlistentry>
......@@ -2079,11 +2071,9 @@ f "2" = 2
<para>The difference between this and
<option>-funfolding-creation-threshold</option> is that this one
determines if a function definition will be inlined at a call
site. The other option determines if a function definition will
be kept around to begin with for potential inlining.</para>
TODO: Is above para correct?
determines if a function definition will be inlined <emphasis>at a call
site</emphasis>. The other option determines if a function definition will
be kept around at all for potential inlining.</para>
</listitem>
</varlistentry>
......@@ -2093,9 +2083,8 @@ f "2" = 2
<indexterm><primary><option></option></primary></indexterm>
</term>
<listitem>
<para>
<para>An experimental flag to expose all unfoldings, even for very large or recursive functions.
</para>
TODO: Document optimisation
</listitem>
</varlistentry>
......@@ -2105,7 +2094,7 @@ f "2" = 2
<indexterm><primary><option></option></primary></indexterm>
</term>
<listitem>
<para>
<para>Data Parallel Haskell.
</para>
TODO: Document optimisation
</listitem>
......@@ -2117,7 +2106,7 @@ f "2" = 2
<indexterm><primary><option></option></primary></indexterm>
</term>
<listitem>
<para>
<para>Data Parallel Haskell.
</para>
TODO: Document optimisation
</listitem>
......
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