bytecode for += and field access

accessing vars generate inefficient bytecode, and non idiomatic code WRT java equiv

Not sure if this is inefficient machine code though, but at least it is increating the classfile size

comparing with java -

fields that are not private[this] are accessed via the accessors. There is no need for this if the function cannot be overridden. vars are final if overridevars is false(its not supported any removed in 2.13), its final, private, effectively final etc and not on a trait. The generated code usd the invokespecial/virtual
bytecode doesnt emit INC to increment a local variable
bytecode could use DUP rather than reloading this
fields that are private should not generate an accessor unless needed (for an inner class or companion maybe, but we know that at compile time)

object Foo {
  private[this] var data1 = 7
  private var data2 = 7
  final var data3 = 7

  data1 += 1
  data2 += 1
  data3 += 1

  def foo: Unit = {
    var data1 = 7
    data1 += 1
    this.data1 += 1
    this.data2 += 1
  }
}

generated bytecode (abbriviated) of

public final class scala/tools/nsc/classpath/Foo$ {

... private I data1 private I data2 private I data3

private data2()I ... private data2$eq(I)V ... public final data3()I ... public final data3$eq(I)V ... public foo()V L0 LINENUMBER 14 L0 BIPUSH 7 L1 ISTORE 1 L2 LINENUMBER 15 L2 ILOAD 1 ICONST_1 IADD ISTORE 1 L3 LINENUMBER 16 L3 ALOAD 0 ALOAD 0 GETFIELD scala/tools/nsc/classpath/Foo$.data1 : I ICONST_1 IADD PUTFIELD scala/tools/nsc/classpath/Foo$.data1 : I L4 LINENUMBER 17 L4 ALOAD 0 ALOAD 0 INVOKESPECIAL scala/tools/nsc/classpath/Foo$.data2 ()I ICONST1 IADD INVOKESPECIAL scala/tools/nsc/classpath/Foo$.data2$eq (I)V L5 RETURN L6 ... private ()V L0 LINENUMBER 153 L0 ALOAD 0 INVOKESPECIAL java/lang/Object. ()V ALOAD 0 PUTSTATIC scala/tools/nsc/classpath/Foo$.MODULE$ : Lscala/tools/nsc/classpath/Foo$; L1 LINENUMBER 5 L1 ALOAD 0 BIPUSH 7 PUTFIELD scala/tools/nsc/classpath/Foo$.data1 : I L2 LINENUMBER 6 L2 ALOAD 0 BIPUSH 7 PUTFIELD scala/tools/nsc/classpath/Foo$.data2 : I L3 LINENUMBER 7 L3 ALOAD 0 BIPUSH 7 PUTFIELD scala/tools/nsc/classpath/Foo$.data3 : I L4 LINENUMBER 9 L4 ALOAD 0 ALOAD 0 GETFIELD scala/tools/nsc/classpath/Foo$.data1 : I ICONST_1 IADD PUTFIELD scala/tools/nsc/classpath/Foo$.data1 : I L5 LINENUMBER 10 L5 ALOAD 0 ALOAD 0 INVOKESPECIAL scala/tools/nsc/classpath/Foo$.data2 ()I ICONST1 IADD **INVOKESPECIAL scala/tools/nsc/classpath/Foo$.data2$eq (I)V L6 LINENUMBER 11 L6 ALOAD 0 ALOAD 0 INVOKEVIRTUAL scala/tools/nsc/classpath/Foo$.data3 ()I ICONST_1 IADD INVOKEVIRTUAL scala/tools/nsc/classpath/Foo$.data3_$eq (I)V** L7 LINENUMBER 4 L7 RETURN }

compare this with java

public class XX {
    int data1 = 7;

    XX() {
        this.data1 += 1;
    }

    void foo() {
        int data1 = 7;
        data1 += 1;
        this.data1 += 1;
    }
}

generates

I data1

()V ... ALOAD 0 **DUP** **GETFIELD scala/tools/nsc/classpath/XX.data1 : I** ICONST_1 IADD **PUTFIELD scala/tools/nsc/classpath/XX.data1 : I** L3 LINENUMBER 7 L3 RETURN L4 LOCALVARIABLE this Lscala/tools/nsc/classpath/XX; L0 L4 0 MAXSTACK = 3 MAXLOCALS = 1 // access flags 0x0 foo()V L0 LINENUMBER 10 L0 BIPUSH 7 ISTORE 1 L1 LINENUMBER 11 L1 **IINC 1 1** L2 LINENUMBER 12 L2 ALOAD 0 **DUP GETFIELD scala/tools/nsc/classpath/XX.data1 : I ICONST_1 IADD PUTFIELD scala/tools/nsc/classpath/XX.data1 : I** L3 LINENUMBER 13 L3 RETURN } ```

rorygraves / scalac_perf

bytecode for += and field access #20