Fix Trac #3736: do not preInlineUnconditionally with INLINE

preInlineUnconditionally was, in effect, nuking an INLINE pragma, with
very bad effect on runtime in this program.  Fortunately the fix is
very simple.

See Note [InlineRule and preInlineUnconditionally] in SimplUtils.

diff --git a/compiler/simplCore/SimplUtils.lhs b/compiler/simplCore/SimplUtils.lhs
index 1c08d6b..500f286 100644 (file)
--- a/compiler/simplCore/SimplUtils.lhs
+++ b/compiler/simplCore/SimplUtils.lhs
@@ -697,6 +697,27 @@ let-float if you inline windowToViewport
 However, as usual for Gentle mode, do not inline things that are
 inactive in the intial stages.  See Note [Gentle mode].
 
 However, as usual for Gentle mode, do not inline things that are
 inactive in the intial stages.  See Note [Gentle mode].
 

+Note [InlineRule and preInlineUnconditionally]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Surprisingly, do not pre-inline-unconditionally Ids with INLINE pragmas!
+Example
+
+   {-# INLINE f #-}
+   f :: Eq a => a -> a
+   f x = ...
+   
+   fInt :: Int -> Int
+   fInt = f Int dEqInt
+
+   ...fInt...fInt...fInt...
+
+Here f occurs just once, in the RHS of f1. But if we inline it there
+we'll lose the opportunity to inline at each of fInt's call sites.
+The INLINE pragma will only inline when the application is saturated
+for exactly this reason; and we don't want PreInlineUnconditionally
+to second-guess it.  A live example is Trac #3736.
+    c.f. Note [InlineRule and postInlineUnconditionally]
+

 Note [Top-level botomming Ids]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Don't inline top-level Ids that are bottoming, even if they are used just
 Note [Top-level botomming Ids]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Don't inline top-level Ids that are bottoming, even if they are used just

@@ -707,6 +728,7 @@ Inlining them won't make the program run faster!
 preInlineUnconditionally :: SimplEnv -> TopLevelFlag -> InId -> InExpr -> Bool
 preInlineUnconditionally env top_lvl bndr rhs
   | not active                                      = False
 preInlineUnconditionally :: SimplEnv -> TopLevelFlag -> InId -> InExpr -> Bool
 preInlineUnconditionally env top_lvl bndr rhs
   | not active                                      = False

+  | isStableUnfolding (idUnfolding bndr)     = False    -- Note [InlineRule and preInlineUnconditionally]

   | isTopLevel top_lvl && isBottomingId bndr = False   -- Note [Top-level bottoming Ids]
   | opt_SimplNoPreInlining                   = False
   | otherwise = case idOccInfo bndr of
   | isTopLevel top_lvl && isBottomingId bndr = False   -- Note [Top-level bottoming Ids]
   | opt_SimplNoPreInlining                   = False
   | otherwise = case idOccInfo bndr of

@@ -971,6 +993,8 @@ Then there's a danger we'll optimise to
 and now postInlineUnconditionally, losing the InlineRule on f.  Now f'
 won't inline because 'e' is too big.
 
 and now postInlineUnconditionally, losing the InlineRule on f.  Now f'
 won't inline because 'e' is too big.
 

+    c.f. Note [InlineRule and preInlineUnconditionally]
+

 
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