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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/using-optimisation.rst | 35 |
1 files changed, 35 insertions, 0 deletions
diff --git a/docs/users_guide/using-optimisation.rst b/docs/users_guide/using-optimisation.rst index ee5b1de95e..92fd62cde7 100644 --- a/docs/users_guide/using-optimisation.rst +++ b/docs/users_guide/using-optimisation.rst @@ -306,6 +306,41 @@ by saying ``-fno-wombat``. Turn on CPR analysis in the demand analyser. +.. ghc-flag:: -fcase-binder-cpr-depth + :shortdesc: Maximum depth at which case binders have the CPR property. + :type: dynamic + :category: + + :default: 1 + + Normally, case binders get the CPR property if their scrutinee had it. + But depending on whether the case binder occurs on a cold path, it may make sense + to give it the CPR property unconditionally. + + This flag controls how deep inside a constructor application we still + consider CPR binders to have th CPR property. The default is 1, so the + following function will have the CPR property: :: + + f :: Bool -> Int -> Int + f False _ = 1 + f _ x@2 = x + f _ _ = 3 + + Note that ``x`` did not occur nested inside a constructor, so depth 1. + + On the other hand, the following function will *not* have the Nested CPR + property: :: + + g :: Bool -> Int -> (Int, Int) + g False _ = (1, 1) + g _ x@2 = (x, x) + g _ _ = (3, 3) + + Because ``x`` occurs nested inside a pair, so at depth 2. + + Depth 0 will never give any CPR binder the CPR property, unless the + scrutinee had it to begin with. + .. ghc-flag:: -fcse :shortdesc: Enable common sub-expression elimination. Implied by :ghc-flag:`-O`. :type: dynamic |