Handle Block in BCodeBodyBuilder.genCond.

For `Block`s, we can push `genCond` inside the final `expr`,
reducing the number of cases where we need to go through
`loadAndTestBoolean`.

This is particularly effective for the shape of `do..while` loops,
as shown in the changed test.

Author: sjrd

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

@@ -1052,15 +1052,21 @@ trait BCodeBodyBuilder extends BCodeSkelBuilder {
       varsInScope = Nil
       stats foreach genStat
       genLoadTo(expr, expectedType, dest)
-      val end = currProgramPoint()
+      emitLocalVarScopes()
+      varsInScope = savedScope
+    }
+
+    /** Add entries to the `LocalVariableTable` JVM attribute for all the vars in
+     *  `varsInScope`, ending at the current program point.
+     */
+    def emitLocalVarScopes(): Unit =
       if (emitVars) {
-        // add entries to LocalVariableTable JVM attribute
+        val end = currProgramPoint()
         for ((sym, start) <- varsInScope.reverse) {
           emitLocalVarScope(sym, start, end)
         }
       }
-      varsInScope = savedScope
-    }
+    end emitLocalVarScopes
 
     def adapt(from: BType, to: BType): Unit = {
       if (!from.conformsTo(to)) {
@@ -1553,6 +1559,17 @@ trait BCodeBodyBuilder extends BCodeSkelBuilder {
                 loadAndTestBoolean()
           }
 
+        case Block(stats, expr) =>
+          /* Push the decision further down the `expr`.
+           * This is particularly effective for the shape of do..while loops.
+           */
+          val savedScope = varsInScope
+          varsInScope = Nil
+          stats foreach genStat
+          genCond(expr, success, failure, targetIfNoJump)
+          emitLocalVarScopes()
+          varsInScope = savedScope
+
         case _ => loadAndTestBoolean()
       }
 

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

@@ -1355,13 +1355,7 @@ class TestBCode extends DottyBytecodeTest {
         Invoke(INVOKEVIRTUAL, "java/io/Writer", "write", "(I)V", false),
         VarOp(ILOAD, 2),
         Op(ICONST_0),
-        Jump(IF_ICMPEQ, Label(35)),
-        Op(ICONST_1),
-        Jump(GOTO, Label(38)),
-        Label(35),
-        Op(ICONST_0),
-        Label(38),
-        Jump(IFNE, Label(2)),
+        Jump(IF_ICMPNE, Label(2)),
         Op(RETURN),
       ))
     }