[Enhanced Switch][Primitive patterns] Wasteful generation of throwing default in boolean switches

[srikanth-sankaran]

Found by code inspection and white box testing. While compiling this code:

public class X<T> {
	public static void main(String[] args) {
		Boolean b = true;
		switch (b) {
			case true -> System.out.println("True");
			case false -> System.out.println("False");
		}
	}
}

we generate this block of code:

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=4, locals=2, args_size=1
         0: iconst_1
         1: invokestatic  #18                 // Method java/lang/Boolean.valueOf:(Z)Ljava/lang/Boolean;
         4: astore_1
         5: aload_1
         6: invokevirtual #24                 // Method java/lang/Boolean.booleanValue:()Z
         9: tableswitch   { // 0 to 1

                       0: 43

                       1: 32
                 default: 54
            }
        32: getstatic     #28                 // Field java/lang/System.out:Ljava/io/PrintStream;
        35: ldc           #34                 // String True
        37: invokevirtual #36                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        40: goto          64
        43: getstatic     #28                 // Field java/lang/System.out:Ljava/io/PrintStream;
        46: ldc           #42                 // String False
        48: invokevirtual #36                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        51: goto          64
        54: new           #44                 // class java/lang/MatchException
        57: dup
        58: aconst_null
        59: aconst_null
        60: invokespecial #46                 // Method java/lang/MatchException."<init>":(Ljava/lang/String;Ljava/lang/Throwable;)V
        63: athrow
        64: return

BCI 54-64 is dead code as the switch is exhaustive and there can be no further evolution that can cause it to cease to be exhaustive.

Javac has the same defect. Plus javac is also unable to generate the code it generates as of JDK23 (Bootstrap method init error)

[srikanth-sankaran]

I'll include a fix in #3460

[srikanth-sankaran]

This comment which has been deleted from master may need addressing in the process of fixing this:

/* a default case is not needed for an exhaustive switch expression * we need to handle the default case to throw an error in order to make the stack map consistent. * All cases will return a value on the stack except the missing default case. * There is no returned value for the default case so we handle it with an exception thrown. */