Optimize lub algorithm (#20034)

Replace mergeIfSuper by a different algorithm that is more efficient.
We drop or-summands in both arguments of a lub that are subsumed by the
other.
This avoids expensive recursive calls to lub or expensive comparisons
with union types on the right.

I tested the previous performance regression #19907 with the new
algorithm, and without the changes in #19995 that avoid a slow lub.
Where previously it took minutes it now compiles fast.

Specifically, we get for i19907_slow_1000_3.scala: 2.9s with the
optimizations in #19995, 3.3s with just this PR. And for
i19907_slow_1000_4.scala: 3.9s with the optimizations in #19995, 4.5s
with just this PR. So the optimizations in #19995 are much less critical
now since lubs are much faster. Still, it's probably worthwhile to leave
them in in case there is a humongous program that stresses lubs even
more.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -2352,8 +2352,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
   }
 
   /** The greatest lower bound of two types */
-  def glb(tp1: Type, tp2: Type): Type = /*>|>*/ trace(s"glb(${tp1.show}, ${tp2.show})", subtyping, show = true) /*<|<*/ {
-    if (tp1 eq tp2) tp1
+  def glb(tp1: Type, tp2: Type): Type = // trace(s"glb(${tp1.show}, ${tp2.show})", subtyping, show = true):
+    if tp1 eq tp2 then tp1
     else if !tp1.exists || (tp1 eq WildcardType) then tp2
     else if !tp2.exists || (tp2 eq WildcardType) then tp1
     else if tp1.isAny && !tp2.isLambdaSub || tp1.isAnyKind || isBottom(tp2) then tp2
@@ -2366,12 +2366,12 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
           val tp2a = dropIfSuper(tp2, tp1)
           if tp2a ne tp2 then glb(tp1, tp2a)
           else tp2 match // normalize to disjunctive normal form if possible.
-            case tp2 @ OrType(tp21, tp22) =>
-              lub(tp1 & tp21, tp1 & tp22, isSoft = tp2.isSoft)
+            case tp2 @ OrType(tp2L, tp2R) =>
+              lub(tp1 & tp2L, tp1 & tp2R, isSoft = tp2.isSoft)
             case _ =>
               tp1 match
-                case tp1 @ OrType(tp11, tp12) =>
-                  lub(tp11 & tp2, tp12 & tp2, isSoft = tp1.isSoft)
+                case tp1 @ OrType(tp1L, tp1R) =>
+                  lub(tp1L & tp2, tp1R & tp2, isSoft = tp1.isSoft)
                 case tp1: ConstantType =>
                   tp2 match
                     case tp2: ConstantType =>
@@ -2386,8 +2386,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                       NothingType
                     case _ => andType(tp1, tp2)
                 case _ => andType(tp1, tp2)
+      end mergedGlb
+
       mergedGlb(dropExpr(tp1.stripLazyRef), dropExpr(tp2.stripLazyRef))
-  }
+  end glb
 
   def widenInUnions(using Context): Boolean =
     migrateTo3 || ctx.erasedTypes
@@ -2396,14 +2398,23 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
    *  @param canConstrain  If true, new constraints might be added to simplify the lub.
    *  @param isSoft        If the lub is a union, this determines whether it's a soft union.
    */
-  def lub(tp1: Type, tp2: Type, canConstrain: Boolean = false, isSoft: Boolean = true): Type = /*>|>*/ trace(s"lub(${tp1.show}, ${tp2.show}, canConstrain=$canConstrain, isSoft=$isSoft)", subtyping, show = true) /*<|<*/ {
-    if (tp1 eq tp2) tp1
+  def lub(tp1: Type, tp2: Type, canConstrain: Boolean = false, isSoft: Boolean = true): Type = // trace(s"lub(${tp1.show}, ${tp2.show}, canConstrain=$canConstrain, isSoft=$isSoft)", subtyping, show = true):
+    if tp1 eq tp2 then tp1
     else if !tp1.exists || (tp2 eq WildcardType) then tp1
     else if !tp2.exists || (tp1 eq WildcardType) then tp2
     else if tp1.isAny && !tp2.isLambdaSub || tp1.isAnyKind || isBottom(tp2) then tp1
     else if tp2.isAny && !tp1.isLambdaSub || tp2.isAnyKind || isBottom(tp1) then tp2
     else
-      def mergedLub(tp1: Type, tp2: Type): Type = {
+      def mergedLub(tp1: Type, tp2: Type): Type =
+        // First, if tp1 and tp2 are the same singleton type, return one of them.
+        if tp1.isSingleton && isSubType(tp1, tp2, whenFrozen = !canConstrain) then
+          return tp2
+        if tp2.isSingleton && isSubType(tp2, tp1, whenFrozen = !canConstrain) then
+          return tp1
+
+        // Second, handle special cases when tp1 and tp2 are disjunctions of
+        // singleton types. This saves time otherwise spent in
+        // costly subtype comparisons performed in dropIfSub below.
         tp1.atoms match
           case Atoms.Range(lo1, hi1) if !widenInUnions =>
             tp2.atoms match
@@ -2413,20 +2424,24 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                 if (hi1 & hi2).isEmpty then return orType(tp1, tp2, isSoft = isSoft)
               case none =>
           case none =>
-        val t1 = mergeIfSuper(tp1, tp2, canConstrain)
-        if (t1.exists) return t1
 
-        val t2 = mergeIfSuper(tp2, tp1, canConstrain)
-        if (t2.exists) return t2
-
-        def widen(tp: Type) = if (widenInUnions) tp.widen else tp.widenIfUnstable
+        // Third, try to simplify after widening as follows:
+        //  1. Drop all or-factors in tp2 that are subtypes of an or-factor
+        //     in tp1, yielding tp2Final.
+        //  2. Drop all or-factors in tp1 that are subtypes of an or-factor
+        //     in tp2Final, yielding tp1Final.
+        //  3. Combine the two final types in an OrType
+        def widen(tp: Type) =
+          if widenInUnions then tp.widen else tp.widenIfUnstable
         val tp1w = widen(tp1)
         val tp2w = widen(tp2)
-        if ((tp1 ne tp1w) || (tp2 ne tp2w)) lub(tp1w, tp2w, canConstrain = canConstrain, isSoft = isSoft)
-        else orType(tp1w, tp2w, isSoft = isSoft) // no need to check subtypes again
-      }
+        val tp2Final = dropIfSub(tp2w, tp1w, canConstrain)
+        val tp1Final = dropIfSub(tp1w, tp2Final, canConstrain)
+        recombine(tp1Final, tp2Final, orType(_, _, isSoft = isSoft))
+      end mergedLub
+
       mergedLub(dropExpr(tp1.stripLazyRef), dropExpr(tp2.stripLazyRef))
-  }
+  end lub
 
   /** Try to produce joint arguments for a lub `A[T_1, ..., T_n] | A[T_1', ..., T_n']` using
    *  the following strategies:
@@ -2488,60 +2503,48 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         Nil
     }
 
-  private def recombineAnd(tp: AndType, tp1: Type, tp2: Type) =
-    if (!tp1.exists) tp2
-    else if (!tp2.exists) tp1
-    else tp.derivedAndType(tp1, tp2)
+  private def recombine(tp1: Type, tp2: Type, rebuild: (Type, Type) => Type): Type =
+    if !tp1.exists then tp2
+    else if !tp2.exists then tp1
+    else rebuild(tp1, tp2)
+
+  private def recombine(tp1: Type, tp2: Type, tp: AndOrType): Type =
+    recombine(tp1, tp2, tp.derivedAndOrType)
 
   /** If some (&-operand of) `tp` is a supertype of `sub` replace it with `NoType`.
    */
   private def dropIfSuper(tp: Type, sub: Type): Type =
-    if (isSubTypeWhenFrozen(sub, tp)) NoType
-    else tp match {
+
+    def isSuperOf(sub: Type): Boolean = sub match
+      case AndType(sub1, sub2) => isSuperOf(sub1) || isSuperOf(sub2)
+      case sub: TypeVar if sub.isInstantiated => isSuperOf(sub.inst)
+      case _ => isSubTypeWhenFrozen(sub, tp)
+
+    tp match
       case tp @ AndType(tp1, tp2) =>
-        recombineAnd(tp, dropIfSuper(tp1, sub), dropIfSuper(tp2, sub))
+        recombine(dropIfSuper(tp1, sub), dropIfSuper(tp2, sub), tp)
+      case tp: TypeVar if tp.isInstantiated =>
+        dropIfSuper(tp.inst, sub)
       case _ =>
-        tp
-    }
+        if isSuperOf(sub) then NoType else tp
+  end dropIfSuper
 
-  /** Merge `t1` into `tp2` if t1 is a subtype of some &-summand of tp2.
-   */
-  private def mergeIfSub(tp1: Type, tp2: Type): Type =
-    if (isSubTypeWhenFrozen(tp1, tp2)) tp1
-    else tp2 match {
-      case tp2 @ AndType(tp21, tp22) =>
-        val lower1 = mergeIfSub(tp1, tp21)
-        if (lower1 eq tp21) tp2
-        else if (lower1.exists) lower1 & tp22
-        else {
-          val lower2 = mergeIfSub(tp1, tp22)
-          if (lower2 eq tp22) tp2
-          else if (lower2.exists) tp21 & lower2
-          else NoType
-        }
-      case _ =>
-        NoType
-    }
+  /** If some (|-operand of) `tp` is a subtype of `sup` replace it with `NoType`. */
+  private def dropIfSub(tp: Type, sup: Type, canConstrain: Boolean): Type =
 
-  /** Merge `tp1` into `tp2` if tp1 is a supertype of some |-summand of tp2.
-   *  @param canConstrain  If true, new constraints might be added to make the merge possible.
-   */
-  private def mergeIfSuper(tp1: Type, tp2: Type, canConstrain: Boolean): Type =
-    if (isSubType(tp2, tp1, whenFrozen = !canConstrain)) tp1
-    else tp2 match {
-      case tp2 @ OrType(tp21, tp22) =>
-        val higher1 = mergeIfSuper(tp1, tp21, canConstrain)
-        if (higher1 eq tp21) tp2
-        else if (higher1.exists) lub(higher1, tp22, isSoft = tp2.isSoft)
-        else {
-          val higher2 = mergeIfSuper(tp1, tp22, canConstrain)
-          if (higher2 eq tp22) tp2
-          else if (higher2.exists) lub(tp21, higher2, isSoft = tp2.isSoft)
-          else NoType
-        }
+    def isSubOf(sup: Type): Boolean = sup match
+      case OrType(sup1, sup2) => isSubOf(sup1) || isSubOf(sup2)
+      case sup: TypeVar if sup.isInstantiated => isSubOf(sup.inst)
+      case _ => isSubType(tp, sup, whenFrozen = !canConstrain)
+
+    tp match
+      case tp @ OrType(tp1, tp2) =>
+        recombine(dropIfSub(tp1, sup, canConstrain), dropIfSub(tp2, sup, canConstrain), tp)
+      case tp: TypeVar if tp.isInstantiated =>
+        dropIfSub(tp.inst, sup, canConstrain)
       case _ =>
-        NoType
-    }
+        if isSubOf(sup) then NoType else tp
+  end dropIfSub
 
   /** There's a window of vulnerability between ElimByName and Erasure where some
    *  ExprTypes `=> T` that appear as parameters of function types are not yet converted

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -157,15 +157,8 @@ object TypeOps:
         tp.derivedAlias(simplify(tp.alias, theMap))
       case AndType(l, r) if !ctx.mode.is(Mode.Type) =>
         simplify(l, theMap) & simplify(r, theMap)
-      case tp @ OrType(l, r)
-      if !ctx.mode.is(Mode.Type)
-         && (tp.isSoft || l.isBottomType || r.isBottomType) =>
-        // Normalize A | Null and Null | A to A even if the union is hard (i.e.
-        // explicitly declared), but not if -Yexplicit-nulls is set. The reason is
-        // that in this case the normal asSeenFrom machinery is not prepared to deal
-        // with Nulls (which have no base classes). Under -Yexplicit-nulls, we take
-        // corrective steps, so no widening is wanted.
-        simplify(l, theMap) | simplify(r, theMap)
+      case tp @ OrType(l, r) if !ctx.mode.is(Mode.Type) =>
+        TypeComparer.lub(simplify(l, theMap), simplify(r, theMap), isSoft = tp.isSoft)
       case tp @ CapturingType(parent, refs) =>
         if !ctx.mode.is(Mode.Type)
             && refs.subCaptures(parent.captureSet, frozen = true).isOK

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3606,12 +3606,11 @@ object Types extends TypeUtils {
 
     override def widenUnionWithoutNull(using Context): Type =
       if myUnionPeriod != ctx.period then
-        myUnion =
-          if isSoft then
-            TypeComparer.lub(tp1.widenUnionWithoutNull, tp2.widenUnionWithoutNull, canConstrain = true, isSoft = isSoft) match
-              case union: OrType => union.join
-              case res => res
-          else derivedOrType(tp1.widenUnionWithoutNull, tp2.widenUnionWithoutNull, soft = isSoft)
+        val union = TypeComparer.lub(
+          tp1.widenUnionWithoutNull, tp2.widenUnionWithoutNull, canConstrain = isSoft, isSoft = isSoft)
+        myUnion = union match
+          case union: OrType if isSoft => union.join
+          case _ => union
         if !isProvisional then myUnionPeriod = ctx.period
       myUnion
 

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1961,7 +1961,8 @@ class Namer { typer: Typer =>
       else
         // don't strip @uncheckedVariance annot for default getters
         TypeOps.simplify(tp.widenTermRefExpr,
-            if defaultTp.exists then TypeOps.SimplifyKeepUnchecked() else null) match
+            if defaultTp.exists then TypeOps.SimplifyKeepUnchecked() else null)
+        match
           case ctp: ConstantType if sym.isInlineVal => ctp
           case tp => TypeComparer.widenInferred(tp, pt, widenUnions = true)
 

tests/pos/i10693.scala

@@ -0,0 +1,8 @@
+def test[A, B](a: A, b: B): A | B = a
+val v0 = test("string", 1)
+val v1 = test(1, "string")
+val v2 = test(v0, v1)
+val v3 = test(v1, v0)
+val v4 = test(v2, v3)
+val v5 = test(v3, v2)
+val v6 = test(v4, v5)

tests/semanticdb/metac.expect

@@ -2020,7 +2020,7 @@ Symbols:
 example/InstrumentTyper# => class InstrumentTyper extends Object { self: AnyRef & InstrumentTyper => +5 decls }
 example/InstrumentTyper#AnnotatedType# => type AnnotatedType  = Int @param
 example/InstrumentTyper#`<init>`(). => primary ctor <init> (): InstrumentTyper
-example/InstrumentTyper#all(). => method all => List[Float | Double | List[Nothing] | Boolean | Unit | Char | String | LinkOption | Int | Long | Class[Option[Int]]]
+example/InstrumentTyper#all(). => method all => List[Char | String | LinkOption | Int | Long | Class[Option[Int]] | Float | Double | Boolean | Unit | List[Nothing]]
 example/InstrumentTyper#clazzOf. => final val method clazzOf Option[Int]
 example/InstrumentTyper#singletonType(). => method singletonType (param x: Predef.type): Nothing
 example/InstrumentTyper#singletonType().(x) => param x: Predef.type
@@ -2082,7 +2082,7 @@ Occurrences:
 [24:37..24:40): Int -> scala/Int#
 
 Synthetics:
-[8:12..8:16):List => *.apply[Float | Double | List[Nothing] | Boolean | Unit | Char | String | LinkOption | Int | Long | Class[Option[Int]]]
+[8:12..8:16):List => *.apply[Char | String | LinkOption | Int | Long | Class[Option[Int]] | Float | Double | Boolean | Unit | List[Nothing]]
 [20:4..20:8):List => *.apply[Nothing]
 
 expect/InventedNames.scala