Fix #9246 and #6800: Remove pure statements

Author: nicolasstucki

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -94,7 +94,8 @@ class Compiler {
          new LetOverApply,            // Lift blocks from receivers of applications
          new ArrayConstructors) ::   // Intercept creation of (non-generic) arrays and intrinsify.
     List(new Erasure) ::             // Rewrite types to JVM model, erasing all type parameters, abstract types and refinements.
-    List(new ElimErasedValueType,    // Expand erased value types to their underlying implmementation types
+    List(new PureStats,              // Remove pure stats from blocks
+         new ElimErasedValueType,    // Expand erased value types to their underlying implmementation types
          new VCElideAllocations,     // Peep-hole optimization to eliminate unnecessary value class allocations
          new ArrayApply,             // Optimize `scala.Array.apply([....])` and `scala.Array.apply(..., [....])` into `[...]`
          new ElimPolyFunction,       // Rewrite PolyFunction subclasses to FunctionN subclasses

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

@@ -0,0 +1,30 @@
+package dotty.tools.dotc
+package transform
+
+import ast.{Trees, tpd}
+import core._, core.Decorators._
+import MegaPhase._
+import Types._, Contexts._, Flags._, DenotTransformers._
+import Symbols._, StdNames._, Trees._
+
+object PureStats {
+  val name: String = "pureStats"
+}
+
+/** Remove pure statements in blocks */
+class PureStats extends MiniPhase {
+
+  import tpd._
+
+  override def phaseName: String = PureStats.name
+
+  override def runsAfter: Set[String] = Set(Erasure.name)
+
+  override def transformBlock(tree: Block)(implicit ctx: Context): Tree =
+    val stats = tree.stats.flatMap {
+      case Typed(Block(stats, expr), _) if isPureExpr(expr) => stats
+      case stat => if !stat.symbol.isConstructor && isPureExpr(stat) then Nil else List(stat)
+    }
+    cpy.Block(tree)(stats, tree.expr)
+
+}

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

@@ -82,10 +82,7 @@ class InlineBytecodeTests extends DottyBytecodeTest {
          |    a
          |  }
          |
-         |  def meth2: Unit = {
-         |    val a = 5
-         |    a
-         |  }
+         |  def meth2: Unit = ()
          |}
          """.stripMargin
 
@@ -418,4 +415,70 @@ class InlineBytecodeTests extends DottyBytecodeTest {
 
     }
   }
+
+  @Test def i6800a = {
+    val source = """class Foo:
+                   |  inline def inlined(f: => Unit): Unit = f
+                   |  def test: Unit = inlined { println("") }
+                 """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn      = dir.lookupName("Foo.class", directory = false).input
+      val clsNode    = loadClassNode(clsIn)
+
+      val fun = getMethod(clsNode, "test")
+      val instructions = instructionsFromMethod(fun)
+      val expected = List(Invoke(INVOKESTATIC, "Foo", "f$1", "()V", false), Op(RETURN))
+      assert(instructions == expected,
+        "`inlined` was not properly inlined in `test`\n" + diffInstructions(instructions, expected))
+
+    }
+  }
+
+  @Test def i6800b = {
+    val source = """class Foo:
+                   |  inline def printIfZero(x: Int): Unit = inline x match
+                   |    case 0 => println("zero")
+                   |    case _ => ()
+                   |  def test: Unit = printIfZero(0)
+                 """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn      = dir.lookupName("Foo.class", directory = false).input
+      val clsNode    = loadClassNode(clsIn)
+
+      val fun = getMethod(clsNode, "test")
+      val instructions = instructionsFromMethod(fun)
+      val expected = List(
+        Field(GETSTATIC, "scala/Predef$", "MODULE$", "Lscala/Predef$;"),
+        Ldc(LDC, "zero"),
+        Invoke(INVOKEVIRTUAL, "scala/Predef$", "println", "(Ljava/lang/Object;)V", false),
+        Op(RETURN)
+      )
+      assert(instructions == expected,
+        "`printIfZero` was not properly inlined in `test`\n" + diffInstructions(instructions, expected))
+    }
+  }
+
+
+  @Test def i9246 = {
+    val source = """class Foo:
+                   |  inline def check(v:Double): Unit = if(v==0) throw new Exception()
+                   |  inline def divide(v: Double, d: Double): Double = { check(d); v / d }
+                   |  def test =  divide(10,2)
+                 """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn      = dir.lookupName("Foo.class", directory = false).input
+      val clsNode    = loadClassNode(clsIn)
+
+      val fun = getMethod(clsNode, "test")
+      val instructions = instructionsFromMethod(fun)
+      val expected = List(Ldc(LDC, 5.0), Op(DRETURN))
+      assert(instructions == expected,
+        "`divide` was not properly inlined in `test`\n" + diffInstructions(instructions, expected))
+
+    }
+  }
+
 }