Emit efficient code for switch over strings

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

@@ -3,6 +3,7 @@ package backend
 package jvm
 
 import scala.annotation.switch
+import scala.collection.mutable.SortedMap
 
 import scala.tools.asm
 import scala.tools.asm.{Handle, Label, Opcodes}
@@ -840,61 +841,170 @@ trait BCodeBodyBuilder extends BCodeSkelBuilder {
       generatedType
     }
 
-    /*
-     * A Match node contains one or more case clauses,
-     * each case clause lists one or more Int values to use as keys, and a code block.
-     * Except the "default" case clause which (if it exists) doesn't list any Int key.
-     *
-     * On a first pass over the case clauses, we flatten the keys and their targets (the latter represented with asm.Labels).
-     * That representation allows JCodeMethodV to emit a lookupswitch or a tableswitch.
-     *
-     * On a second pass, we emit the switch blocks, one for each different target.
+    /* A Match node contains one or more case clauses, each case clause lists one or more
+     * Int/String values to use as keys, and a code block. The exception is the "default" case
+     * clause which doesn't list any key (there is exactly one of these per match).
      */
     private def genMatch(tree: Match): BType = tree match {
       case Match(selector, cases) =>
       lineNumber(tree)
-      genLoad(selector, INT)
       val generatedType = tpeTK(tree)
+      val postMatch = new asm.Label
 
-      var flatKeys: List[Int]       = Nil
-      var targets:  List[asm.Label] = Nil
-      var default:  asm.Label       = null
-      var switchBlocks: List[(asm.Label, Tree)] = Nil
-
-      // collect switch blocks and their keys, but don't emit yet any switch-block.
-      for (caze @ CaseDef(pat, guard, body) <- cases) {
-        assert(guard == tpd.EmptyTree, guard)
-        val switchBlockPoint = new asm.Label
-        switchBlocks ::= (switchBlockPoint, body)
-        pat match {
-          case Literal(value) =>
-            flatKeys ::= value.intValue
-            targets  ::= switchBlockPoint
-          case Ident(nme.WILDCARD) =>
-            assert(default == null, s"multiple default targets in a Match node, at ${tree.span}")
-            default = switchBlockPoint
-          case Alternative(alts) =>
-            alts foreach {
-              case Literal(value) =>
-                flatKeys ::= value.intValue
-                targets  ::= switchBlockPoint
-              case _ =>
-                abort(s"Invalid alternative in alternative pattern in Match node: $tree at: ${tree.span}")
-            }
-          case _ =>
-            abort(s"Invalid pattern in Match node: $tree at: ${tree.span}")
+      // Only two possible selector types exist in `Match` trees at this point: Int and String
+      if (tpeTK(selector) == INT) {
+
+        /* On a first pass over the case clauses, we flatten the keys and their
+         * targets (the latter represented with asm.Labels). That representation
+         * allows JCodeMethodV to emit a lookupswitch or a tableswitch.
+         *
+         * On a second pass, we emit the switch blocks, one for each different target.
+         */
+
+        var flatKeys: List[Int]       = Nil
+        var targets:  List[asm.Label] = Nil
+        var default:  asm.Label       = null
+        var switchBlocks: List[(asm.Label, Tree)] = Nil
+
+        genLoad(selector, INT)
+
+        // collect switch blocks and their keys, but don't emit yet any switch-block.
+        for (caze @ CaseDef(pat, guard, body) <- cases) {
+          assert(guard == tpd.EmptyTree, guard)
+          val switchBlockPoint = new asm.Label
+          switchBlocks ::= (switchBlockPoint, body)
+          pat match {
+            case Literal(value) =>
+              flatKeys ::= value.intValue
+              targets  ::= switchBlockPoint
+            case Ident(nme.WILDCARD) =>
+              assert(default == null, s"multiple default targets in a Match node, at ${tree.span}")
+              default = switchBlockPoint
+            case Alternative(alts) =>
+              alts foreach {
+                case Literal(value) =>
+                  flatKeys ::= value.intValue
+                  targets  ::= switchBlockPoint
+                case _ =>
+                  abort(s"Invalid alternative in alternative pattern in Match node: $tree at: ${tree.span}")
+              }
+            case _ =>
+              abort(s"Invalid pattern in Match node: $tree at: ${tree.span}")
+          }
         }
-      }
 
-      bc.emitSWITCH(mkArrayReverse(flatKeys), mkArrayL(targets.reverse), default, MIN_SWITCH_DENSITY)
+        bc.emitSWITCH(mkArrayReverse(flatKeys), mkArrayL(targets.reverse), default, MIN_SWITCH_DENSITY)
 
-      // emit switch-blocks.
-      val postMatch = new asm.Label
-      for (sb <- switchBlocks.reverse) {
-        val (caseLabel, caseBody) = sb
-        markProgramPoint(caseLabel)
-        genLoad(caseBody, generatedType)
-        bc goTo postMatch
+        // emit switch-blocks.
+        for (sb <- switchBlocks.reverse) {
+          val (caseLabel, caseBody) = sb
+          markProgramPoint(caseLabel)
+          genLoad(caseBody, generatedType)
+          bc goTo postMatch
+        }
+      } else {
+
+        /* Since the JVM doesn't have a way to switch on a string, we  switch
+         * on the `hashCode` of the string then do an `equals` check (with a
+         * possible second set of jumps if blocks can be reach from multiple
+         * string alternatives).
+         *
+         * This mirrors the way that Java compiles `switch` on Strings.
+         */
+
+        var default:  asm.Label       = null
+        var indirectBlocks: List[(asm.Label, Tree)] = Nil
+
+        import scala.collection.mutable
+
+        // Cases grouped by their hashCode
+        val casesByHash = SortedMap.empty[Int, List[(String, Either[asm.Label, Tree])]]
+        var caseFallback: Tree = null
+
+        for (caze @ CaseDef(pat, guard, body) <- cases) {
+          assert(guard == tpd.EmptyTree, guard)
+          pat match {
+            case Literal(value) =>
+              val strValue = value.stringValue
+              casesByHash.updateWith(strValue.##) { existingCasesOpt =>
+                val newCase = (strValue, Right(body))
+                Some(newCase :: existingCasesOpt.getOrElse(Nil))
+              }
+            case Ident(nme.WILDCARD) =>
+              assert(default == null, s"multiple default targets in a Match node, at ${tree.span}")
+              default = new asm.Label
+              indirectBlocks ::= (default, body)
+            case Alternative(alts) =>
+              // We need an extra basic block since multiple strings can lead to this code
+              val indirectCaseGroupLabel = new asm.Label
+              indirectBlocks ::= (indirectCaseGroupLabel, body)
+              alts foreach {
+                case Literal(value) =>
+                  val strValue = value.stringValue
+                  casesByHash.updateWith(strValue.##) { existingCasesOpt =>
+                    val newCase = (strValue, Left(indirectCaseGroupLabel))
+                    Some(newCase :: existingCasesOpt.getOrElse(Nil))
+                  }
+                case _ =>
+                  abort(s"Invalid alternative in alternative pattern in Match node: $tree at: ${tree.span}")
+              }
+
+            case _ =>
+              abort(s"Invalid pattern in Match node: $tree at: ${tree.span}")
+          }
+        }
+
+        // Organize the hashCode options into switch cases
+        var flatKeys: List[Int]       = Nil
+        var targets:  List[asm.Label] = Nil
+        var hashBlocks: List[(asm.Label, List[(String, Either[asm.Label, Tree])])] = Nil
+        for ((hashValue, hashCases) <- casesByHash) {
+          val switchBlockPoint = new asm.Label
+          hashBlocks ::= (switchBlockPoint, hashCases)
+          flatKeys ::= hashValue
+          targets  ::= switchBlockPoint
+        }
+
+        // Push the hashCode of the string (or `0` it is `null`) onto the stack and switch on it
+        genLoadIf(
+          If(
+            tree.selector.select(defn.Any_==).appliedTo(nullLiteral),
+            Literal(Constant(0)),
+            tree.selector.select(defn.Any_hashCode).appliedToNone
+          ),
+          INT
+        )
+        bc.emitSWITCH(mkArrayReverse(flatKeys), mkArrayL(targets.reverse), default, MIN_SWITCH_DENSITY)
+
+        // emit blocks for each hash case
+        for ((hashLabel, caseAlternatives) <- hashBlocks.reverse) {
+          markProgramPoint(hashLabel)
+          for ((caseString, indirectLblOrBody) <- caseAlternatives) {
+            val comparison = if (caseString == null) defn.Any_== else defn.Any_equals
+            val condp = Literal(Constant(caseString)).select(defn.Any_==).appliedTo(tree.selector)
+            val keepGoing = new asm.Label
+            indirectLblOrBody match {
+              case Left(jump) =>
+                genCond(condp, jump, keepGoing, targetIfNoJump = keepGoing)
+
+              case Right(caseBody) =>
+                val thisCaseMatches = new asm.Label
+                genCond(condp, thisCaseMatches, keepGoing, targetIfNoJump = thisCaseMatches)
+                markProgramPoint(thisCaseMatches)
+                genLoad(caseBody, generatedType)
+                bc goTo postMatch
+            }
+            markProgramPoint(keepGoing)
+          }
+          bc goTo default
+        }
+
+        // emit blocks for common patterns
+        for ((caseLabel, caseBody) <- indirectBlocks.reverse) {
+          markProgramPoint(caseLabel)
+          genLoad(caseBody, generatedType)
+          bc goTo postMatch
+        }
       }
 
       markProgramPoint(postMatch)

compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala

@@ -2872,12 +2872,6 @@ class JSCodeGen()(using genCtx: Context) {
     def abortMatch(msg: String): Nothing =
       throw new FatalError(s"$msg in switch-like pattern match at ${tree.span}: $tree")
 
-    /* Although GenBCode adapts the scrutinee and the cases to `int`, only
-     * true `int`s can reach the back-end, as asserted by the String-switch
-     * transformation in `cleanup`. Therefore, we do not adapt, preserving
-     * the `string`s and `null`s that come out of the pattern matching in
-     * Scala 2.13.2+.
-     */
     val genSelector = genExpr(selector)
 
     // Sanity check: we can handle Ints and Strings (including `null`s), but nothing else
@@ -2934,11 +2928,6 @@ class JSCodeGen()(using genCtx: Context) {
      * When no optimization applies, and any of the case values is not a
      * literal int, we emit a series of `if..else` instead of a `js.Match`.
      * This became necessary in 2.13.2 with strings and nulls.
-     *
-     * Note that dotc has not adopted String-switch-Matches yet, so these code
-     * paths are dead code at the moment. However, they already existed in the
-     * scalac, so were ported, to be immediately available and working when
-     * dotc starts emitting switch-Matches on Strings.
      */
     def isInt(tree: js.Tree): Boolean = tree.tpe == jstpe.IntType
 

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -20,7 +20,7 @@ import util.Property._
 
 /** The pattern matching transform.
  *  After this phase, the only Match nodes remaining in the code are simple switches
- *  where every pattern is an integer constant
+ *  where every pattern is an integer or string constant
  */
 class PatternMatcher extends MiniPhase {
   import ast.tpd._
@@ -768,13 +768,15 @@ object PatternMatcher {
         (tpe isRef defn.IntClass) ||
         (tpe isRef defn.ByteClass) ||
         (tpe isRef defn.ShortClass) ||
-        (tpe isRef defn.CharClass)
+        (tpe isRef defn.CharClass) ||
+        (tpe isRef defn.StringClass)
 
-      val seen = mutable.Set[Int]()
+      val seen = mutable.Set[Any]()
 
-      def isNewIntConst(tree: Tree) = tree match {
-        case Literal(const) if const.isIntRange && !seen.contains(const.intValue) =>
-          seen += const.intValue
+      def isNewSwitchableConst(tree: Tree) = tree match {
+        case Literal(const)
+        if (const.isIntRange || const.tag == Constants.StringTag) && !seen.contains(const.value) =>
+          seen += const.value
           true
         case _ =>
           false
@@ -789,7 +791,7 @@ object PatternMatcher {
               val alts = List.newBuilder[Tree]
               def rec(innerPlan: Plan): Boolean = innerPlan match {
                 case SeqPlan(TestPlan(EqualTest(tree), scrut, _, ReturnPlan(`innerLabel`)), tail)
-                if scrut === scrutinee && isNewIntConst(tree) =>
+                if scrut === scrutinee && isNewSwitchableConst(tree) =>
                   alts += tree
                   rec(tail)
                 case ReturnPlan(`outerLabel`) =>
@@ -809,7 +811,7 @@ object PatternMatcher {
 
       def recur(plan: Plan): List[(List[Tree], Plan)] = plan match {
         case SeqPlan(testPlan @ TestPlan(EqualTest(tree), scrut, _, ons), tail)
-        if scrut === scrutinee && !canFallThrough(ons) && isNewIntConst(tree) =>
+        if scrut === scrutinee && !canFallThrough(ons) && isNewSwitchableConst(tree) =>
           (tree :: Nil, ons) :: recur(tail)
         case SeqPlan(AlternativesPlan(alts, ons), tail) =>
           (alts, ons) :: recur(tail)
@@ -832,29 +834,32 @@ object PatternMatcher {
 
     /** Emit a switch-match */
     private def emitSwitchMatch(scrutinee: Tree, cases: List[(List[Tree], Plan)]): Match = {
-      /* Make sure to adapt the scrutinee to Int, as well as all the alternatives
-       * of all cases, so that only Matches on pritimive Ints survive this phase.
+      /* Make sure to adapt the scrutinee to Int or String, as well as all the
+       * alternatives, so that only Matches on pritimive Ints or Strings survive
+       * this phase.
        */
 
-      val intScrutinee =
-        if (scrutinee.tpe.widen.isRef(defn.IntClass)) scrutinee
-        else scrutinee.select(nme.toInt)
+      val (primScrutinee, scrutineeTpe) =
+        if (scrutinee.tpe.widen.isRef(defn.IntClass)) (scrutinee, defn.IntType)
+        else if (scrutinee.tpe.widen.isRef(defn.StringClass)) (scrutinee, defn.StringType)
+        else (scrutinee.select(nme.toInt), defn.IntType)
 
-      def intLiteral(lit: Tree): Tree =
+      def primLiteral(lit: Tree): Tree =
         val Literal(constant) = lit
         if (constant.tag == Constants.IntTag) lit
+        else if (constant.tag == Constants.StringTag) lit
         else cpy.Literal(lit)(Constant(constant.intValue))
 
       val caseDefs = cases.map { (alts, ons) =>
         val pat = alts match {
-          case alt :: Nil => intLiteral(alt)
-          case Nil => Underscore(defn.IntType) // default case
-          case _ => Alternative(alts.map(intLiteral))
+          case alt :: Nil => primLiteral(alt)
+          case Nil => Underscore(scrutineeTpe) // default case
+          case _ => Alternative(alts.map(primLiteral))
         }
         CaseDef(pat, EmptyTree, emit(ons))
       }
 
-      Match(intScrutinee, caseDefs)
+      Match(primScrutinee, caseDefs)
     }
 
     /** If selfCheck is `true`, used to check whether a tree gets generated twice */

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

@@ -118,6 +118,28 @@ class TestBCode extends DottyBytecodeTest {
     }
   }
 
+  @Test def switchOnStrings = {
+    val source =
+      """
+        |object Foo {
+        |  import scala.annotation.switch
+        |  def foo(s: String) = s match {
+        |    case "AaAa" => println(3)
+        |    case "BBBB" | "c" => println(2)
+        |    case "D" | "E" => println(1)
+        |    case _ => println(0)
+        |  }
+        |}
+      """.stripMargin
+
+    checkBCode(source) { dir =>
+      val moduleIn   = dir.lookupName("Foo$.class", directory = false)
+      val moduleNode = loadClassNode(moduleIn.input)
+      val methodNode = getMethod(moduleNode, "foo")
+      assert(verifySwitch(methodNode))
+    }
+  }
+
   @Test def matchWithDefaultNoThrowMatchError = {
     val source =
       """class Test {

tests/run/string-switch-defaults-null.check

@@ -0,0 +1,2 @@
+2
+-1

tests/run/string-switch-defaults-null.scala

@@ -0,0 +1,16 @@
+import annotation.switch
+
+object Test {
+  def test(s: String): Int = {
+    (s : @switch) match {
+      case "1" => 0
+      case null => -1
+      case _ => s.toInt
+    }
+  }
+
+  def main(args: Array[String]): Unit = {
+    println(test("2"))
+    println(test(null))
+  }
+}