[Git][ghc/ghc][wip/T23146] Update Note [Core letrec invariant]

[Rodrigo Mesquita (@alt-romes)]

Rodrigo Mesquita pushed to branch wip/T23146 at Glasgow Haskell Compiler / GHC


Commits:
b7784859 by Rodrigo Mesquita at 2023-05-01T11:28:15+01:00
Update Note [Core letrec invariant]

Authored by @simonpj

- - - - -


1 changed file:

- compiler/GHC/Core.hs


Changes:

=====================================
compiler/GHC/Core.hs
=====================================
@@ -368,18 +368,36 @@ Note [Core letrec invariant]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 The Core letrec invariant:
 
-    The right hand sides of all
-      /top-level/ or /recursive/
-    bindings must be of lifted type
-
-    There is one exception to this rule, top-level @let at s are
-    allowed to bind primitive string literals: see
-    Note [Core top-level string literals].
+  The right hand sides of all /top-level/ or /recursive/
+  bindings must be of lifted type
 
 See "Type#type_classification" in GHC.Core.Type
-for the meaning of "lifted" vs. "unlifted").
-
-For the non-top-level, non-recursive case see Note [Core let-can-float invariant].
+for the meaning of "lifted" vs. "unlifted".
+
+For the non-top-level, non-recursive case see
+Note [Core let-can-float invariant].
+
+At top level, however, there are two exceptions to this rule:
+
+(TL1) A top-level binding is allowed to bind primitive string literal,
+      (which is unlifted).  See Note [Core top-level string literals].
+
+(TL2) In CorePrep, we generate a top-level binding for every data constructor
+      e.g.   data T = MkT Int
+      we generate
+             MkT :: Int -> T
+             MkT = \x. MkT x
+      (This binding looks recursive, but isn't; it defines a top-level, curried
+      function whose body just allocates and returns the data constructor.)
+
+      But if (a) the data contructor is nullary and (b) the data type is unlifted,
+      this binding is unlifted.
+      e.g.   data S :: UnliftedType where { S1 :: S, S2 :: S -> S }
+      we generate
+             S1 :: S   -- A top-level unlifted binding
+             S1 = S1
+      We allow this top-level unlifted binding to exist, after CorePrep
+      only.
 
 Note [Core let-can-float invariant]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b7784859d190e75b8fe52df002c0b615c88f0365

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b7784859d190e75b8fe52df002c0b615c88f0365
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