Revamp derived Eq instance code generation (#17240)

This patch improves code generation for derived Eq instances.
The idea is to use 'dataToTag' to evaluate both arguments.
This allows to 'short-circuit' when tags do not match.
Unfortunately, inner evals are still present when we branch
on tags. This is due to the way 'dataToTag#' primop
evaluates its argument in the code generator. #21207 was
created to explore further optimizations.

Metric Decrease:
    LargeRecord

4 files changed, 50 insertions, 2 deletions

diff --git a/testsuite/tests/deriving/should_compile/T17240.hs b/testsuite/tests/deriving/should_compile/T17240.hs
new file mode 100644
index 0000000000..6b847c578a
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T17240.hs
@@ -0,0 +1,5 @@
+module T17240 where
+
+data T = A | B Int | C Char | D Int deriving Eq
+
+data Nullary = X | Y | Z deriving Eq
diff --git a/testsuite/tests/deriving/should_compile/T17240.stderr b/testsuite/tests/deriving/should_compile/T17240.stderr
new file mode 100644
index 0000000000..cce538b59d
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T17240.stderr
@@ -0,0 +1,42 @@
+
+==================== Derived instances ====================
+Derived class instances:
+  instance GHC.Classes.Eq T17240.Nullary where
+    (GHC.Classes.==) a b
+      = case (GHC.Prim.dataToTag# a) of
+          a#
+            -> case (GHC.Prim.dataToTag# b) of
+                 b# -> (GHC.Prim.tagToEnum# (a# GHC.Prim.==# b#))
+  
+  instance GHC.Classes.Eq T17240.T where
+    (GHC.Classes.==) a b
+      = case
+            (case (GHC.Prim.dataToTag# a) of
+               a# -> case (GHC.Prim.dataToTag# b) of b# -> a# GHC.Prim./=# b#)
+        of
+          1# -> GHC.Types.False
+          _ -> case a of
+                 (T17240.B a1)
+                   -> case b of (T17240.B b1) -> ((a1 GHC.Classes.== b1))
+                 (T17240.C a1)
+                   -> case b of (T17240.C b1) -> ((a1 GHC.Classes.== b1))
+                 (T17240.D a1)
+                   -> case b of (T17240.D b1) -> ((a1 GHC.Classes.== b1))
+                 _ -> GHC.Types.True
+  
+
+Derived type family instances:
+
+
+
+==================== Filling in method body ====================
+GHC.Classes.Eq [T17240.Nullary]
+  GHC.Classes./= = GHC.Classes.$dm/= @(T17240.Nullary)
+
+
+
+==================== Filling in method body ====================
+GHC.Classes.Eq [T17240.T]
+  GHC.Classes./= = GHC.Classes.$dm/= @(T17240.T)
+
+
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index a33cb364c3..05f9e87dcb 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -122,6 +122,7 @@ test('T15831', normal, compile, [''])
 test('T16179', normal, compile, [''])
 test('T16341', normal, compile, [''])
 test('T16518', normal, compile, [''])
+test('T17240', normal, compile, ['-ddump-deriv -dsuppress-uniques'])
 test('T17324', normal, compile, [''])
 test('T17339', normal, compile,
      ['-ddump-simpl -dsuppress-idinfo -dno-typeable-binds'])
diff --git a/testsuite/tests/deriving/should_fail/drvfail011.stderr b/testsuite/tests/deriving/should_fail/drvfail011.stderr
index d439bd03eb..5b26f5b575 100644
--- a/testsuite/tests/deriving/should_fail/drvfail011.stderr
+++ b/testsuite/tests/deriving/should_fail/drvfail011.stderr
@@ -3,8 +3,8 @@ drvfail011.hs:8:1: error:
     • No instance for (Eq a) arising from a use of ‘==’
       Possible fix: add (Eq a) to the context of the instance declaration
     • In the expression: a1 == b1
-      In an equation for ‘==’: (==) (T1 a1) (T1 b1) = ((a1 == b1))
+      In a case alternative: (T1 b1) -> ((a1 == b1))
+      In the expression: case b of (T1 b1) -> ((a1 == b1))
       When typechecking the code for ‘==’
         in a derived instance for ‘Eq (T a)’:
         To see the code I am typechecking, use -ddump-deriv
-      In the instance declaration for ‘Eq (T a)’