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Simon Peyton Jones pushed to branch wip/T26722 at Glasgow Haskell Compiler / GHC

Commits:

2a7e8721

by Simon Peyton Jones at 2026-01-09T11:42:28+00:00

Try using `uf_expandable` in `interestingArg`

See hmm in https://gitlab.haskell.org/ghc/ghc/-/issues/26722#note_652486

2 changed files:

compiler/GHC/Core/Opt/Simplify/Utils.hs
compiler/GHC/Core/Utils.hs

Changes:

compiler/GHC/Core/Opt/Simplify/Utils.hs

@@ -1066,18 +1066,18 @@ interestingArg env e = go env 0 e
             DFunUnfolding {} -> ValueArg   -- We konw that idArity=0
             CoreUnfolding{ uf_cache = cache }
               | uf_is_conlike cache -> ValueArg    -- Includes constructor applications
 -             | uf_is_value cache   -> NonTrivArg  -- Things like partial applications
 +             | uf_expandable cache -> NonTrivArg  -- Things like partial applications,
 +                                                  -- and strict data constructors
               | otherwise           -> TrivArg
             BootUnfolding           -> TrivArg
             NoUnfolding             -> TrivArg
 -************************************************************************
 +{- *********************************************************************
  *                                                                      *
                    SimplMode
  *                                                                      *
 -************************************************************************
 --}
 +********************************************************************* -}
  updModeForStableUnfoldings :: ActivationGhc -> SimplMode -> SimplMode
  -- See Note [The environments of the Simplify pass]

compiler/GHC/Core/Utils.hs

@@ -1509,6 +1509,22 @@ going to put up with this, because the previous more aggressive inlining
  (which treated 'noFactor' as work-free) was duplicating primops, which
  in turn was making inner loops of array calculations runs slow (#5623)
 +Wrinkles
++
 +(WF1) Strict constructor fields.  We regard (K x) as work-free even if
 +  K is a strict data constructor (see Note [Strict fields in Core])
 +      data T a = K !a
 +  If we have
 +    let t = K x in ...(case t of K y -> blah)...
 +  we want to treat t's binding as expandable so that `exprIsConApp_maybe`
 +  will look through its unfolding. (NB: exprIsWorkFree implies
 +  exprIsExpandable.)
++
 +  Because K is strict, after inlining we'll get a leftover eval on x, which may
 +  or may not disappear
 +    let t = K x in ...(case x of y -> blah)...
 +  In effect we count duplicating the eval as work-free
++
  Note [Case expressions are work-free]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Are case-expressions work-free?  Consider
@@ -1650,7 +1666,8 @@ isWorkFreeApp fn n_val_args
    = True
    | otherwise
    = case idDetails fn of
 -      DataConWorkId {} -> True
 +      DataConWorkId {} -> True  -- Even if the data constructor is strict
 +                                -- See (WF1) in Note [exprIsWorkFree]
        PrimOpId op _    -> primOpIsWorkFree op
        _                -> False
@@ -1751,6 +1768,8 @@ expansion.  Specifically:
    duplicate the (a +# b) primop, which we should not do lightly.
    (It's quite hard to trigger this bug, but T13155 does so for GHC 8.0.)
 +NB: exprIsWorkFree implies exprIsExpandable.
++
  Note [isExpandableApp: bottoming functions]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  It's important that isExpandableApp does not respond True to bottoming