[Git][ghc/ghc][wip/romes/9557] deriving Ord compare and <= only

[Rodrigo Mesquita (@alt-romes)]

Rodrigo Mesquita pushed to branch wip/romes/9557 at Glasgow Haskell Compiler / GHC


Commits:
4ed10080 by Rodrigo Mesquita at 2024-10-26T10:05:27+01:00
deriving Ord compare and <= only

Since the implementation of CLC proposal #24, the default
implementations of Ord's `<`, `>`, and `>=` are given in terms of `<=`.

This means we no longer need to generate implementations for these
methods when stock deriving `Ord`. Rather, just derive the
implementation of `compare` and `<=`, and rely on the default
implementations for the others.

Progress towards #9557

Using the same sample module from #9557, this commit takes compilation
on my machine from being:
* 4.3s to 3.3s with -O0
* 6.9s to 5.2s with -O1

- - - - -


1 changed file:

- compiler/GHC/Tc/Deriv/Generate.hs


Changes:

=====================================
compiler/GHC/Tc/Deriv/Generate.hs
=====================================
@@ -339,7 +339,7 @@ Several special cases:
   See function unliftedOrdOp
 
 Note [Game plan for deriving Ord]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 It's a bad idea to define only 'compare', and build the other binary
 comparisons on top of it; see #2130, #4019.  Reason: we don't
 want to laboriously make a three-way comparison, only to extract a
@@ -350,16 +350,22 @@ binary result, something like this:
                                        True  -> False
                                        False -> True
 
-This being said, we can get away with generating full code only for
-'compare' and '<' thus saving us generation of other three operators.
-Other operators can be cheaply expressed through '<':
-a <= b = not $ b < a
-a > b = b < a
-a >= b = not $ a < b
-
 So for sufficiently small types (few constructors, or all nullary)
 we generate all methods; for large ones we just use 'compare'.
 
+This being said, we can get away with generating full code only for
+'compare' and '<=' thus saving us generation of other three operators.
+Other operators can be cheaply expressed through '<=' -- indeed, that's what
+the default implementations of >, <, and >= do.
+
+Historically, derived instances defined '<' and the remaining operators as
+cheap expressions in function of it:
+  a <= b = not $ b < a
+  a > b = b < a
+  a >= b = not $ a < b
+but since the CLC proposal #24 (see 8f174e06185143674d6cbfee75c30e68805d85b8),
+it suffices to derive '<=' and rely on the
+default implementation for the others.
 -}
 
 data OrdOp = OrdCompare | OrdLT | OrdLE | OrdGE | OrdGT
@@ -417,19 +423,10 @@ gen_Ord_binds loc dit@(DerivInstTys{ dit_rep_tc = tycon
     other_ops
       | (last_tag - first_tag) <= 2     -- 1-3 constructors
         || null non_nullary_cons        -- Or it's an enumeration
-      = [mkOrdOp OrdLT, lE, gT, gE]
+      = [mkOrdOp OrdLE]
       | otherwise
       = []
 
-    negate_expr = nlHsApp (nlHsVar not_RDR)
-    pats = noLocA [a_Pat, b_Pat]
-    lE = mkSimpleGeneratedFunBind loc le_RDR pats $
-        negate_expr (nlHsApp (nlHsApp (nlHsVar lt_RDR) b_Expr) a_Expr)
-    gT = mkSimpleGeneratedFunBind loc gt_RDR pats $
-        nlHsApp (nlHsApp (nlHsVar lt_RDR) b_Expr) a_Expr
-    gE = mkSimpleGeneratedFunBind loc ge_RDR pats $
-        negate_expr (nlHsApp (nlHsApp (nlHsVar lt_RDR) a_Expr) b_Expr)
-
     get_tag con = dataConTag con - fIRST_TAG
         -- We want *zero-based* tags, because that's what
         -- con2Tag returns (generated by untag_Expr)!



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/4ed1008080b97ce69c26ac873ba8e8f99f63881b

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/4ed1008080b97ce69c26ac873ba8e8f99f63881b
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