diff options
Diffstat (limited to 'docs/users_guide')
| -rw-r--r-- | docs/users_guide/9.4.1-notes.rst | 27 | ||||
| -rw-r--r-- | docs/users_guide/editing-guide.rst | 33 | ||||
| -rw-r--r-- | docs/users_guide/using-optimisation.rst | 56 |
3 files changed, 97 insertions, 19 deletions
diff --git a/docs/users_guide/9.4.1-notes.rst b/docs/users_guide/9.4.1-notes.rst index 31f09fa6bc..5cb2736547 100644 --- a/docs/users_guide/9.4.1-notes.rst +++ b/docs/users_guide/9.4.1-notes.rst @@ -38,6 +38,33 @@ Compiler enabled the :extension:`UnboxedTuples` extension but didn't explicitly enable :extension:`UnboxedSums` will continue to work without changes. +- Constructed Product Result analysis (c.f. :ghc-flag:`-fcpr-anal`) has been + overhauled and will now unbox nestedly, if termination properties of the + function permit. This allows unboxing of constructed results returned by + ``IO`` actions. E.g.:: + + sumIO :: [Int] -> IO Int + sumIO [] = return 0 + sumIO (x:xs) = do + r <- sumIO xs + return $! x + r + + Note the use of ``$!``: Without it, GHC would be unable to see that evaluation + of ``r`` and ``x`` terminates (and rapidly, at that). An alternative would be to + evaluate both with a bang pattern or a ``seq``, but the ``return $! <res>`` + idiom should work more reliably and needs less thinking. + +- Demand analysis (cf. :ghc-flag:`-fstrictness`) now integrates a + Boxity Analysis that tracks whether a function needs a parameter boxed. If + that is the case, the worker/wrapper transformation (cf. + :ghc-flag:`-fworker-wrapper`) will not unbox that parameter, leading to less + reboxing in many cases. + + For reasons of backwards-compatible performance, you may find that the new + mechanism is too aggressive in a few cases (e.g., still unboxing a parameter + that is used boxed in a hot path). Do post a bug report with your example! + Then wrap the uses of the parameter in ``GHC.Exts.lazy`` for a short-term fix. + ``base`` library ~~~~~~~~~~~~~~~~ diff --git a/docs/users_guide/editing-guide.rst b/docs/users_guide/editing-guide.rst index 737abd3b2e..b4720e1a71 100644 --- a/docs/users_guide/editing-guide.rst +++ b/docs/users_guide/editing-guide.rst @@ -6,15 +6,15 @@ for the Glasgow Haskell Compiler. Even more than this, it at times serves (for better or for worse) as a de-facto language standard, being the sole non-academic reference for many widely used language extensions. -Since GHC 8.0, the User's Guide is authored in `ReStructuredText -<https://en.wikipedia.org/wiki/ReStructuredText>`__ (or ReST or RST, for short) +Since GHC 8.0, the User's Guide is authored in `reStructuredText +<https://en.wikipedia.org/wiki/ReStructuredText>`__ (or reST or RST, for short) a rich but light-weight mark-up language aimed at producing documentation. The -`Sphinx <http://sphinx-doc.org/>`__ tool is used to produce the final PDF and +`Sphinx <https://www.sphinx-doc.org/>`__ tool is used to produce the final PDF and HTML documentation. -This document (also written in ReST) serves as a brief introduction to ReST and to +This document (also written in reST) serves as a brief introduction to reST and to document the conventions used in the User's Guide. This document is *not* intended -to be a thorough guide to ReST. For this see the resources referenced +to be a thorough guide to reST. For this see the resources referenced `below <#references>`__. Basics @@ -91,7 +91,7 @@ The above would be rendered as, Headings ~~~~~~~~ -While ReST can accommodate a wide range of heading styles, we have standardized +While reST can accommodate a wide range of heading styles, we have standardized on this convention in the User's Guide, .. code-block:: rest @@ -141,7 +141,7 @@ Other languages Double-colon blocks are syntax-highlighted as Haskell by default. To avoid this use a ``.. code-block`` `directive -<http://sphinx-doc.org/markup/code.html#directive-code-block>`__ with explicit +<https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-code-block>`__ with explicit language designation, .. code-block:: rest @@ -314,7 +314,7 @@ There are a number of admonitions types, * warning -.. _Admonitions: http://docutils.sourceforge.net/docs/ref/rst/directives.html#admonitions +.. _Admonitions: https://docutils.sourceforge.io/docs/ref/rst/directives.html#admonitions Documenting command-line options and GHCi commands -------------------------------------------------- @@ -420,16 +420,17 @@ user-substitutable tokens. In this document we use the convention, ``⟨subst⟩ (note that these are angle brackets, ``U+27E8`` and ``U+27E9``, not less-than/greater-than signs). +.. _references: -ReST reference materials +reST reference materials ------------------------ -* `Sphinx ReST Primer`_: A great place to start. -* `Sphinx extensions`_: How Sphinx extends ReST -* `ReST reference`_: When you really need the details. +* `Sphinx reST Primer`_: A great place to start. +* `Sphinx extensions`_: How Sphinx extends reST +* `reST reference`_: When you really need the details. * `Directives reference`_ -.. _Sphinx ReST Primer: http://sphinx-doc.org/rest.html -.. _ReST reference: http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html -.. _Sphinx extensions: http://sphinx-doc.org/markup/index.html -.. _Directives reference: http://docutils.sourceforge.net/docs/ref/rst/directives.html#code +.. _Sphinx reST Primer: https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html +.. _reST reference: https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html +.. _Sphinx extensions: https://www.sphinx-doc.org/en/master/usage/extensions/index.html +.. _Directives reference: https://docutils.sourceforge.io/docs/ref/rst/directives.html#code diff --git a/docs/users_guide/using-optimisation.rst b/docs/users_guide/using-optimisation.rst index 37e4a4aae6..139031ad1c 100644 --- a/docs/users_guide/using-optimisation.rst +++ b/docs/users_guide/using-optimisation.rst @@ -247,7 +247,7 @@ by saying ``-fno-wombat``. :default: on Enables some control flow optimisations in the Cmm code - generator, merging basic blocks and avoiding jumps right after jumps. + generator, merging basic blocks and avoiding jumps right after jumps. .. ghc-flag:: -fasm-shortcutting :shortdesc: Enable shortcutting on assembly. Implied by :ghc-flag:`-O2`. @@ -319,14 +319,64 @@ by saying ``-fno-wombat``. block layout behaves the same as in 8.6 and earlier. .. ghc-flag:: -fcpr-anal - :shortdesc: Turn on CPR analysis in the demand analyser. Implied by :ghc-flag:`-O`. + :shortdesc: Turn on Constructed Product Result analysis. Implied by :ghc-flag:`-O`. :type: dynamic :reverse: -fno-cpr-anal :category: :default: on - Turn on CPR analysis in the demand analyser. + Turn on CPR analysis, which enables the worker/wrapper transformation (cf. + :ghc-flag:`-fworker-wrapper`) to unbox the result of a function, such as :: + + sum :: [Int] -> Int + sum [] = 0 + sum (x:xs) = x + sum xs + + CPR analysis will see that each code path produces a *constructed product* + such as ``I# 0#`` in the first branch (where ``GHC.Exts.I#`` is the data + constructor of ``Int``, boxing up the the primitive integer literal ``0#`` + of type ``Int#``) and optimise to :: + + sum xs = I# ($wsum xs) + $wsum [] = 0# + $wsum (I# x:xs) = x# +# $wsum xs + + and then ``sum`` can inline to potentially cancel away the ``I#`` box. + + Here's an example of the function that *does not* return a constructed product: :: + + f :: [Int] -> (Int -> Int) -> Int + f [] g = g 0 + f (x:xs) g = x + f xs g + + The expression ``g 0`` is not a constructed product, because we don't know + anything about ``g``. + + CPR analysis also works *nestedly*, for example :: + + sumIO :: [Int] -> IO Int + sumIO [] = return 0 + sumIO (x:xs) = do + r <- sumIO xs + return $! x + r + + Note the use of ``$!``: Without it, GHC would be unable to see that evaluation + of ``r`` and ``x`` terminates (and rapidly, at that). An alternative would be to + evaluate both with a bang pattern or a ``seq``, but the ``return $! <res>`` + idiom should work more reliably and needs less thinking. The above example + will be optimised to :: + + sumIO :: [Int] -> IO Int + sumIO xs = IO $ \s -> case $wsum xs s of + (# s', r #) -> (# s', I# r #) + $wsumIO :: [Int] -> (# RealWorld#, Int# #) + $wsumIO [] s = (# s, 0# #) + $wsumIO (I# x:xs) s = case $wsumIO xs of + (# s', r #) -> (# s', x +# r#) + + And the latter can inline ``sumIO`` and cancel away the ``I#`` constructor. + Unboxing the result of a ``State`` action should work similarly. .. ghc-flag:: -fcse :shortdesc: Enable common sub-expression elimination. Implied by :ghc-flag:`-O`. |