[Git][ghc/ghc][wip/andreask/mkTickUnsafeCoerce] mkTick: Push ticks through unsafeCoerce#.

[Andreas Klebinger (@AndreasK)]

Andreas Klebinger pushed to branch wip/andreask/mkTickUnsafeCoerce at Glasgow Haskell Compiler / GHC


Commits:
b394a664 by Andreas Klebinger at 2024-10-12T00:48:00+02:00
mkTick: Push ticks through unsafeCoerce#.

unsafeCoerce# doesn't exist at runtime so we should treat it like a Cast
for the purpose of mkTick.

This means if we have `{-# SCC foo #-} (unsafeCoerce# trivial_expr))` we
now push the scope part of the cost centre up to `trivial_expr` at which
point we can discard it completely if the expression is trivial enough.

This fixes #25212.

- - - - -


2 changed files:

- compiler/GHC/Core/Utils.hs
- compiler/GHC/Types/Tickish.hs


Changes:

=====================================
compiler/GHC/Core/Utils.hs
=====================================
@@ -318,6 +318,10 @@ mkTick t orig_expr = mkTick' id id orig_expr
           -> CoreExpr               -- current expression
           -> CoreExpr
   mkTick' top rest expr = case expr of
+    -- Float ticks into unsafe coerce.
+    Case scrut bndr ty alts@[Alt ac abs _rhs]
+      | Just rhs <- isUnsafeEqualityCase scrut bndr alts
+      -> mkTick' (\e -> Case scrut bndr ty [Alt ac abs e]) rest rhs
 
     -- Cost centre ticks should never be reordered relative to each
     -- other. Therefore we can stop whenever two collide.
@@ -1253,7 +1257,7 @@ Note [Tick trivial]
 Ticks are only trivial if they are pure annotations. If we treat
 "tick<n> x" as trivial, it will be inlined inside lambdas and the
 entry count will be skewed, for example.  Furthermore "scc<n> x" will
-turn into just "x" in mkTick.
+turn into just "x" in mkTick. At least if `x` is not a function.
 
 Note [Empty case is trivial]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~


=====================================
compiler/GHC/Types/Tickish.hs
=====================================
@@ -295,13 +295,15 @@ tickishCanSplit _  = False
 mkNoCount :: GenTickish pass -> GenTickish pass
 mkNoCount n | not (tickishCounts n)   = n
             | not (tickishCanSplit n) = panic "mkNoCount: Cannot split!"
-mkNoCount n at ProfNote{}                = n {profNoteCount = False}
+mkNoCount n at ProfNote{}                = let n' = n {profNoteCount = False}
+                                        in assert (profNoteCount n) n'
 mkNoCount _                           = panic "mkNoCount: Undefined split!"
 
 mkNoScope :: GenTickish pass -> GenTickish pass
 mkNoScope n | tickishScoped n == NoScope  = n
             | not (tickishCanSplit n)     = panic "mkNoScope: Cannot split!"
-mkNoScope n at ProfNote{}                    = n {profNoteScope = False}
+mkNoScope n at ProfNote{}                    = let n' = n {profNoteScope = False}
+                                            in assert (profNoteCount n) n'
 mkNoScope _                               = panic "mkNoScope: Undefined split!"
 
 -- | Return @True@ if this source annotation compiles to some backend



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b394a664a2b9bf7a6ea49af8f781890ae2a3863f

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b394a664a2b9bf7a6ea49af8f781890ae2a3863f
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