Remove unnecessary store when generating "=="

Author: guillaumecarraux

compiler/src/dotty/tools/backend/jvm/BCodeBodyBuilder.scala

@@ -1713,26 +1713,23 @@ trait BCodeBodyBuilder extends BCodeSkelBuilder {
           genCZJUMP(success, failure, Primitives.NE, BOOL, targetIfNoJump)
         } else {
           // l == r -> if (l eq null) r eq null else l.equals(r)
-          val eqEqTempLocal = locals.makeLocal(ObjectRef, nme.EQEQ_LOCAL_VAR.mangledString, defn.ObjectType, r.span)
           val lNull    = new asm.Label
           val lNonNull = new asm.Label
 
-          genLoad(l, ObjectRef)
+          genLoad(r, ObjectRef) //  load rhs --> (r)
           stack.push(ObjectRef)
-          genLoad(r, ObjectRef)
+          genLoad(l, ObjectRef) // load lhs --> (l,r)
           stack.pop()
-          locals.store(eqEqTempLocal)
-          bc dup ObjectRef
-          genCZJUMP(lNull, lNonNull, Primitives.EQ, ObjectRef, targetIfNoJump = lNull)
+          bc dup ObjectRef // duplicate top stack variable --> (l,l,r)
+          genCZJUMP(lNull, lNonNull, Primitives.EQ, ObjectRef, targetIfNoJump = lNull) // compare lhs with NULL --> (l,r)
 
           markProgramPoint(lNull)
-          bc drop ObjectRef
-          locals.load(eqEqTempLocal)
-          genCZJUMP(success, failure, Primitives.EQ, ObjectRef, targetIfNoJump = lNonNull)
+          bc drop ObjectRef // drop top stack variable --> (r)
+          genCZJUMP(success, failure, Primitives.EQ, ObjectRef, targetIfNoJump = lNonNull) // --> (-)
 
           markProgramPoint(lNonNull)
-          locals.load(eqEqTempLocal)
-          genCallMethod(defn.Any_equals, InvokeStyle.Virtual)
+          emit(asm.Opcodes.SWAP) //swap l and r for correct .equals ordering --> (r,l)
+          genCallMethod(defn.Any_equals, InvokeStyle.Virtual) // --> (-)
           genCZJUMP(success, failure, Primitives.NE, BOOL, targetIfNoJump)
         }
       }

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -1168,7 +1168,7 @@ class DottyBytecodeTests extends DottyBytecodeTest {
 
       val m1Meth = getMethod(fooClass, "m1")
 
-      assertSameCode(m1Meth, List(
+      assertSameCode(m1Meth,List(
         VarOp(ALOAD, 1),
         VarOp(ASTORE, 2),
         VarOp(ALOAD, 2),
@@ -1189,29 +1189,27 @@ class DottyBytecodeTests extends DottyBytecodeTest {
         VarOp(ILOAD, 5),
         Op(IRETURN),
         Label(19),
+        VarOp(ALOAD, 2),
         Field(GETSTATIC, "scala/package$", "MODULE$", "Lscala/package$;"),
         Invoke(INVOKEVIRTUAL, "scala/package$", "Nil", "()Lscala/collection/immutable/Nil$;", false),
-        VarOp(ALOAD, 2),
-        VarOp(ASTORE, 7),
         Op(DUP),
-        Jump(IFNONNULL, Label(31)),
+        Jump(IFNONNULL, Label(29)),
         Op(POP),
-        VarOp(ALOAD, 7),
-        Jump(IFNULL, Label(36)),
-        Jump(GOTO, Label(40)),
-        Label(31),
-        VarOp(ALOAD, 7),
+        Jump(IFNULL, Label(34)),
+        Jump(GOTO, Label(38)),
+        Label(29),
+        Op(SWAP),
         Invoke(INVOKEVIRTUAL, "java/lang/Object", "equals", "(Ljava/lang/Object;)Z", false),
-        Jump(IFEQ, Label(40)),
-        Label(36),
+        Jump(IFEQ, Label(38)),
+        Label(34),
         IntOp(BIPUSH, 20),
         Op(IRETURN),
-        Label(40),
+        Label(38),
         TypeOp(NEW, "scala/MatchError"),
         Op(DUP),
         VarOp(ALOAD, 2),
         Invoke(INVOKESPECIAL, "scala/MatchError", "<init>", "(Ljava/lang/Object;)V", false),
-        Op(ATHROW),
+        Op(ATHROW)
       ))
 
       // ---------------