Merge pull request #1315 from nicolasstucki/optimize-try-cases

Fix #856: Handle try/catch cases as catch cases if possible.

3 files changed, 103 insertions, 75 deletions

diff --git a/src/dotty/tools/dotc/Compiler.scala b/src/dotty/tools/dotc/Compiler.scala
index 3844f42a7..ce9280d82 100644
--- a/src/dotty/tools/dotc/Compiler.scala
+++ b/src/dotty/tools/dotc/Compiler.scala
@@ -57,7 +57,8 @@ class Compiler {
            new TailRec,             // Rewrite tail recursion to loops
            new LiftTry,             // Put try expressions that might execute on non-empty stacks into their own methods
            new ClassOf),            // Expand `Predef.classOf` calls.
-      List(new PatternMatcher,      // Compile pattern matches
+      List(new TryCatchPatterns,    // Compile cases in try/catch
+           new PatternMatcher,      // Compile pattern matches
            new ExplicitOuter,       // Add accessors to outer classes from nested ones.
            new ExplicitSelf,        // Make references to non-trivial self types explicit as casts
            new CrossCastAnd,        // Normalize selections involving intersection types.
diff --git a/src/dotty/tools/dotc/transform/PatternMatcher.scala b/src/dotty/tools/dotc/transform/PatternMatcher.scala
index fd89696a8..974053769 100644
--- a/src/dotty/tools/dotc/transform/PatternMatcher.scala
+++ b/src/dotty/tools/dotc/transform/PatternMatcher.scala
@@ -1,6 +1,8 @@
 package dotty.tools.dotc
 package transform
 
+import scala.language.postfixOps
+
 import TreeTransforms._
 import core.Denotations._
 import core.SymDenotations._
@@ -53,19 +55,6 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
     translated.ensureConforms(tree.tpe)
   }
 
-
-  override def transformTry(tree: tpd.Try)(implicit ctx: Context, info: TransformerInfo): tpd.Tree = {
-    val selector =
-      ctx.newSymbol(ctx.owner, ctx.freshName("ex").toTermName, Flags.Synthetic | Flags.Case, defn.ThrowableType, coord = tree.pos)
-    val sel = Ident(selector.termRef).withPos(tree.pos)
-    val rethrow = tpd.CaseDef(EmptyTree, EmptyTree, Throw(ref(selector)))
-    val newCases = tpd.CaseDef(
-      Bind(selector, Underscore(selector.info).withPos(tree.pos)),
-      EmptyTree,
-      transformMatch(tpd.Match(sel, tree.cases ::: rethrow :: Nil)))
-    cpy.Try(tree)(tree.expr, newCases :: Nil, tree.finalizer)
-  }
-
   class Translator(implicit ctx: Context) {
 
   def translator = {
@@ -1264,27 +1253,6 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
         t
     }
 
-    /** Is this pattern node a catch-all or type-test pattern? */
-    def isCatchCase(cdef: CaseDef) = cdef match {
-      case CaseDef(Typed(Ident(nme.WILDCARD), tpt), EmptyTree, _) =>
-        isSimpleThrowable(tpt.tpe)
-      case CaseDef(Bind(_, Typed(Ident(nme.WILDCARD), tpt)), EmptyTree, _) =>
-        isSimpleThrowable(tpt.tpe)
-      case _ =>
-        isDefaultCase(cdef)
-    }
-
-    private def isSimpleThrowable(tp: Type)(implicit ctx: Context): Boolean = tp match {
-      case tp @ TypeRef(pre, _) =>
-        val sym = tp.symbol
-        (pre == NoPrefix || pre.widen.typeSymbol.isStatic) &&
-          (sym.derivesFrom(defn.ThrowableClass)) &&  /* bq */ !(sym is Flags.Trait)
-      case _ =>
-        false
-    }
-
-
-
     /** Implement a pattern match by turning its cases (including the implicit failure case)
       * into the corresponding (monadic) extractors, and combining them with the `orElse` combinator.
       *
@@ -1335,46 +1303,6 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
       Block(List(ValDef(selectorSym, sel)), combined)
     }
 
-    // return list of typed CaseDefs that are supported by the backend (typed/bind/wildcard)
-    // we don't have a global scrutinee -- the caught exception must be bound in each of the casedefs
-    // there's no need to check the scrutinee for null -- "throw null" becomes "throw new NullPointerException"
-    // try to simplify to a type-based switch, or fall back to a catch-all case that runs a normal pattern match
-    // unlike translateMatch, we type our result before returning it
-    /*def translateTry(caseDefs: List[CaseDef], pt: Type, pos: Position): List[CaseDef] =
-    // if they're already simple enough to be handled by the back-end, we're done
-      if (caseDefs forall isCatchCase) caseDefs
-      else {
-        val swatches = { // switch-catches
-        val bindersAndCases = caseDefs map { caseDef =>
-            // generate a fresh symbol for each case, hoping we'll end up emitting a type-switch (we don't have a global scrut there)
-            // if we fail to emit a fine-grained switch, have to do translateCase again with a single scrutSym (TODO: uniformize substitution on treemakers so we can avoid this)
-            val caseScrutSym = freshSym(pos, pureType(defn.ThrowableType))
-            (caseScrutSym, propagateSubstitution(translateCase(caseScrutSym, pt)(caseDef), EmptySubstitution))
-          }
-
-          for(cases <- emitTypeSwitch(bindersAndCases, pt).toList
-              if cases forall isCatchCase; // must check again, since it's not guaranteed -- TODO: can we eliminate this? e.g., a type test could test for a trait or a non-trivial prefix, which are not handled by the back-end
-              cse <- cases) yield /*fixerUpper(matchOwner, pos)*/(cse).asInstanceOf[CaseDef]
-        }
-
-        val catches = if (swatches.nonEmpty) swatches else {
-          val scrutSym = freshSym(pos, pureType(defn.ThrowableType))
-          val casesNoSubstOnly = caseDefs map { caseDef => (propagateSubstitution(translateCase(scrutSym, pt)(caseDef), EmptySubstitution))}
-
-          val exSym = freshSym(pos, pureType(defn.ThrowableType), "ex")
-
-          List(
-              CaseDef(
-                Bind(exSym, Ident(??? /*nme.WILDCARD*/)), // TODO: does this need fixing upping?
-                EmptyTree,
-                combineCasesNoSubstOnly(ref(exSym), scrutSym, casesNoSubstOnly, pt, matchOwner, Some((scrut: Symbol) => Throw(ref(exSym))))
-              )
-            )
-        }
-
-        /*typer.typedCases(*/catches/*, defn.ThrowableType, WildcardType)*/
-      }*/
-
     /**  The translation of `pat if guard => body` has two aspects:
       *     1) the substitution due to the variables bound by patterns
       *     2) the combination of the extractor calls using `flatMap`.
diff --git a/src/dotty/tools/dotc/transform/TryCatchPatterns.scala b/src/dotty/tools/dotc/transform/TryCatchPatterns.scala
new file mode 100644
index 000000000..9a6ecef51
--- /dev/null
+++ b/src/dotty/tools/dotc/transform/TryCatchPatterns.scala
@@ -0,0 +1,99 @@
+package dotty.tools.dotc
+package transform
+
+import core.Symbols._
+import core.StdNames._
+import ast.Trees._
+import core.Types._
+import dotty.tools.dotc.core.Decorators._
+import dotty.tools.dotc.core.Flags
+import dotty.tools.dotc.core.Contexts.Context
+import dotty.tools.dotc.transform.TreeTransforms.{MiniPhaseTransform, TransformerInfo}
+import dotty.tools.dotc.util.Positions.Position
+
+/** Compiles the cases that can not be handled by primitive catch cases as a common pattern match.
+ *
+ *  The following code:
+ *    ```
+ *    try { <code> }
+ *    catch {
+ *      <tryCases> // Cases that can be handled by catch
+ *      <patternMatchCases> // Cases starting with first one that can't be handled by catch
+ *    }
+ *    ```
+ *  will become:
+ *    ```
+ *    try { <code> }
+ *    catch {
+ *      <tryCases>
+ *      case e => e match {
+ *        <patternMatchCases>
+ *      }
+ *    }
+ *    ```
+ *
+ *  Cases that are not supported include:
+ *   - Applies and unapplies
+ *   - Idents
+ *   - Alternatives
+ *   - `case _: T =>` where `T` is not `Throwable`
+ *
+ */
+class TryCatchPatterns extends MiniPhaseTransform {
+  import dotty.tools.dotc.ast.tpd._
+
+  def phaseName: String = "tryCatchPatterns"
+
+  override def runsAfter = Set(classOf[ElimRepeated])
+
+  override def checkPostCondition(tree: Tree)(implicit ctx: Context): Unit = tree match {
+    case Try(_, cases, _) =>
+      cases.foreach {
+        case CaseDef(Typed(_, _), guard, _) => assert(guard.isEmpty, "Try case should not contain a guard.")
+        case CaseDef(Bind(_, _), guard, _) => assert(guard.isEmpty, "Try case should not contain a guard.")
+        case c =>
+          assert(isDefaultCase(c), "Pattern in Try should be Bind, Typed or default case.")
+      }
+    case _ =>
+  }
+
+  override def transformTry(tree: Try)(implicit ctx: Context, info: TransformerInfo): Tree = {
+    val (tryCases, patternMatchCases) = tree.cases.span(isCatchCase)
+    val fallbackCase = mkFallbackPatterMatchCase(patternMatchCases, tree.pos)
+    cpy.Try(tree)(cases = tryCases ++ fallbackCase)
+  }
+
+  /** Is this pattern node a catch-all or type-test pattern? */
+  private def isCatchCase(cdef: CaseDef)(implicit ctx: Context): Boolean = cdef match {
+    case CaseDef(Typed(Ident(nme.WILDCARD), tpt), EmptyTree, _)          => isSimpleThrowable(tpt.tpe)
+    case CaseDef(Bind(_, Typed(Ident(nme.WILDCARD), tpt)), EmptyTree, _) => isSimpleThrowable(tpt.tpe)
+    case _                                                               => isDefaultCase(cdef)
+  }
+
+  private def isSimpleThrowable(tp: Type)(implicit ctx: Context): Boolean = tp match {
+    case tp @ TypeRef(pre, _) =>
+      (pre == NoPrefix || pre.widen.typeSymbol.isStatic) && // Does not require outer class check
+      !tp.symbol.is(Flags.Trait) && // Traits not supported by JVM
+      tp.derivesFrom(defn.ThrowableClass)
+    case _ =>
+      false
+  }
+
+  private def mkFallbackPatterMatchCase(patternMatchCases: List[CaseDef], pos: Position)(
+      implicit ctx: Context, info: TransformerInfo): Option[CaseDef] = {
+    if (patternMatchCases.isEmpty) None
+    else {
+      val exName = ctx.freshName("ex").toTermName
+      val fallbackSelector =
+        ctx.newSymbol(ctx.owner, exName, Flags.Synthetic | Flags.Case, defn.ThrowableType, coord = pos)
+      val sel = Ident(fallbackSelector.termRef).withPos(pos)
+      val rethrow = CaseDef(EmptyTree, EmptyTree, Throw(ref(fallbackSelector)))
+      Some(CaseDef(
+          Bind(fallbackSelector, Underscore(fallbackSelector.info).withPos(pos)),
+          EmptyTree,
+          transformFollowing(Match(sel, patternMatchCases ::: rethrow :: Nil)))
+      )
+    }
+  }
+
+}