compiler/coreprep: Turn off dictionary speculation by default

Speculative evaluation can cause performance regressions,
therefore we turn it off by default. It can be enabled again
with the -fspec-eval-dictfun flag

See #25284

Author: luite

compiler/GHC/CoreToStg/Prep.hs

@@ -2054,12 +2054,27 @@ floats and how far they go.
 Note [Controlling Speculative Evaluation]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Most of the time, speculative evaluation has a positive effect on performance,
-but we have found a case where speculative evaluation of dictionary functions
-leads to a performance regression #25284.
-
-Therefore we have some flags to control it. See the optimization section in
-the User's Guide for the description of these flags and when to use them.
+Most of the time, speculative evaluation in the coreprep phase has a positive
+effect on performance, however we have found that some forms of speculative
+evaluation can lead to large performance regressions. See #25284.
+
+Therefore we have some flags to control which types of speculative evaluation
+are done:
+
+  -fspec-eval
+     Globally enable/disable speculative evaluation ( -fno-spec-eval also turns
+     off all other speculative evaluation). On by default for all
+     optimization levels. Turning on this flag by itself should never cause
+     a performance regression. Please open a ticket if you find any.
+
+  -fspec-eval-dictfun
+     Enable speculative evaluation for dictionary functions. Off by default
+     since it can cause an increase in allocations (#24284). We have no
+     examples that show a large performance improvement when turning on this
+     flag. Please open a ticket if you find any.
+
+Also see the optimization section in the User's Guide for the description of
+these flags and when to use them.
 
 Note [Floats and FloatDecision]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

compiler/GHC/Driver/DynFlags.hs

@@ -1288,7 +1288,7 @@ optLevelFlags -- see Note [Documenting optimisation flags]
 --  , ([2],     Opt_StaticArgumentTransformation)
 --   Static Argument Transformation needs investigation. See #9374
     , ([0,1,2], Opt_SpecEval)
-    , ([0,1,2], Opt_SpecEvalDictFun)
+    , ([],      Opt_SpecEvalDictFun)
     ]
 
 

docs/users_guide/using-optimisation.rst

@@ -428,7 +428,7 @@ as such you shouldn't need to set any of them explicitly. A flag
     :category:
     :reverse: -fno-spec-eval-dictfun
 
-    :default: on
+    :default: off
     :since: 9.14.1
 
     Enables speculative (strict) evaluation of dictionary functions.

testsuite/tests/rts/ipe/T24005/t24005.hs

@@ -32,5 +32,5 @@ instance Show a => Show (B a) where
 main :: IO ()
 main = do
     -- Should both result in InfoProvs with correct source locations
-    (\(Box d) -> print =<< whereFrom d) $ mkBox @(Show A)
-    (\(Box d) -> print =<< whereFrom d) $ mkBox @(Show (B A))
+    (\(Box !d) -> print =<< whereFrom d) $ mkBox @(Show A)
+    (\(Box !d) -> print =<< whereFrom d) $ mkBox @(Show (B A))