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| author | Simon Peyton Jones <simon.peytonjones@gmail.com> | 2023-02-21 10:51:34 +0000 |
|---|---|---|
| committer | Marge Bot <ben+marge-bot@smart-cactus.org> | 2023-02-28 18:54:59 -0500 |
| commit | 7192ef91c855e1fae6997f75cfde76aafd0b4bcf (patch) | |
| tree | aef67a692c95e4e11b50d855ba651784eb89c109 /compiler/GHC/Core | |
| parent | 239202a2b14714740e016d7bbcd4f351356fcb00 (diff) | |
| download | haskell-7192ef91c855e1fae6997f75cfde76aafd0b4bcf.tar.gz | |
Take more care with unlifted bindings in the specialiser
As #22998 showed, we were floating an unlifted binding to top
level, which breaks a Core invariant.
The fix is easy, albeit a little bit conservative. See
Note [Care with unlifted bindings] in GHC.Core.Opt.Specialise
Diffstat (limited to 'compiler/GHC/Core')
| -rw-r--r-- | compiler/GHC/Core/Opt/Specialise.hs | 29 |
1 files changed, 27 insertions, 2 deletions
diff --git a/compiler/GHC/Core/Opt/Specialise.hs b/compiler/GHC/Core/Opt/Specialise.hs index f028bd428e..f48aeb50d7 100644 --- a/compiler/GHC/Core/Opt/Specialise.hs +++ b/compiler/GHC/Core/Opt/Specialise.hs @@ -27,7 +27,7 @@ import GHC.Core import GHC.Core.Make ( mkLitRubbish ) import GHC.Core.Unify ( tcMatchTy ) import GHC.Core.Rules -import GHC.Core.Utils ( exprIsTrivial +import GHC.Core.Utils ( exprIsTrivial, exprIsTopLevelBindable , mkCast, exprType , stripTicksTop, mkInScopeSetBndrs ) import GHC.Core.FVs @@ -1515,7 +1515,10 @@ specBind top_lvl env (NonRec fn rhs) do_body = [mkDB $ NonRec b r | (b,r) <- pairs] ++ fromOL dump_dbs - ; if float_all then + can_float_this_one = exprIsTopLevelBindable rhs (idType fn) + -- exprIsTopLevelBindable: see Note [Care with unlifted bindings] + + ; if float_all && can_float_this_one then -- Rather than discard the calls mentioning the bound variables -- we float this (dictionary) binding along with the others return ([], body', all_free_uds `snocDictBinds` final_binds) @@ -1876,6 +1879,28 @@ even in the case that @x = False@! Instead, we add a dummy 'Void#' argument to the specialisation '$sfInt' ($sfInt :: Void# -> Array# Int) in order to preserve laziness. +Note [Care with unlifted bindings] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider (#22998) + f x = let x::ByteArray# = <some literal> + n::Natural = NB x + in wombat @192827 (n |> co) +where + co :: Natural ~ KnownNat 192827 + wombat :: forall (n:Nat). KnownNat n => blah + +Left to itself, the specialiser would float the bindings for `x` and `n` to top +level, so we can specialise `wombat`. But we can't have a top-level ByteArray# +(see Note [Core letrec invariant] in GHC.Core). Boo. + +This is pretty exotic, so we take a simple way out: in specBind (the NonRec +case) do not float the binding itself unless it satisfies exprIsTopLevelBindable. +This is conservative: maybe the RHS of `x` has a free var that would stop it +floating to top level anyway; but that is hard to spot (since we don't know what +the non-top-level in-scope binders are) and rare (since the binding must satisfy +Note [Core let-can-float invariant] in GHC.Core). + + Note [Specialising Calls] ~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we have a function with a complicated type: |