Optimise Switch code generation on booleans.

stedolan · stedolan · commit 26997ef33646 · 2019-05-15T12:21:39.000+01:00
If the value being matched is known to be 0 or 1, we need not do
a further comparison with 0 before dispatching.
diff --git a/Changes b/Changes
@@ -20,6 +20,11 @@ Working version
   [Flambda_middle_end].  Run [Un_anf] from the middle end, not [Cmmgen].
   (Mark Shinwell, review by Pierre Chambart)
 
+### Code generation and optimizations:
+
+- #8672: Optimise Switch code generation on booleans.
+  (Stephen Dolan, review by ??)
+
 ### Runtime system:
 
 - #8619: Ensure Gc.minor_words remains accurate after a GC.
diff --git a/lambda/switch.ml b/lambda/switch.ml
@@ -186,6 +186,9 @@ let prerr_inter i = Printf.fprintf stderr
   and get_low cases i =
     let r,_,_ = cases.(i) in
     r
+  and get_high cases i =
+    let _,r,_ = cases.(i) in
+    r
 
   type ctests = {
     mutable n : int ;
@@ -659,9 +662,14 @@ let rec pkey chan  = function
           and right = {s with cases=right} in
 
           if i=1 && (lim+ctx.off)=1 && get_low cases 0+ctx.off=0 then
-            make_if_ne
-              ctx.arg 0
-              (c_test ctx right) (c_test ctx left)
+            if lcases = 2 && get_high cases 1+ctx.off = 1 then
+              Arg.make_if
+                ctx.arg
+                (c_test ctx right) (c_test ctx left)
+            else
+              make_if_ne
+                ctx.arg 0
+                (c_test ctx right) (c_test ctx left)
           else if less_tests cright cleft then
             make_if_lt
               ctx.arg (lim+ctx.off)
