JDK-8208664: JDK8,JDK9-breakpoint on a continue doesnt work on jdb and intellij

[Jan Lahoda]

Hi,

There is a small issue in some cases when trying to put breakpoints on 
break/continue (or stepping through code that has break/continue). 
Consider this code:
---
    static void test(boolean a, boolean b) {
        for (int i = 0; i < 10; i++) {
            if (a || b) {
                continue; //set breakpoint here
            }
            System.out.println();
        }
    }
---

This gets compiled into:
---
          0: iconst_0
          1: istore_2
          2: iload_2
          3: bipush        10
          5: if_icmpge     28
          8: iload_0
          9: ifne          22
         12: iload_1
         13: ifeq          19
         16: goto          22
         19: invokestatic  #3                  // Method nop:()V
         22: iinc          2, 1
         25: goto          2
         28: return
---


The instruction that corresponds to the "continue" is the "goto" on 
bytecode index 16. But, if "a" is true, that instruction is never 
executed, because the test on bytecode index 9 will jump directly to the 
"continue" target. We cannot generate LineNumberTable saying that 
bytecode index 9 corresponds to the "continue", as it really belongs to 
the if, and debuggers would in some cases stop on the wrong line instead 
of stopping on the continue. So, the issue is that if "a" is true, then 
there is never a real instruction executed that corresponds to the 
"continue". What we can do is to change the target if the "ifne" on 
bytecode index 9 to "16", so that the goto is always executed, and 
everything would work OK in debuggers, except that one additional jump 
is performed (and, in some rarer cases, generated, as the actual "goto" 
can sometimes be skipped).

I tried to compile the JDK with and without the proposed patch, and 
about 1100 out of 31000 classfiles are changed, so this affects roughly 
3.5% of the JDK classfiles.

The proposed patch is only enabling this behavior when LineNumberTable 
is being generated, but that is fairly common, and so it is not a big 
help. One of the alternatives would be to require a special "-g" flag, 
like "-g:debuglines".

JBS: https://bugs.openjdk.java.net/browse/JDK-8208664
Webrev: http://cr.openjdk.java.net/~jlahoda/8208664/webrev.00/

Any opinions?

Thanks!

Jan