[debug] code gen for WHILE loop unfriendly for debugging

[liufengyun]

Given the code:

    var a = 1 + 2

    while (a * 8 < 100) { // first jump to end
      a += 1
    }

The compilation generates following code, which has an unnecessary jump at the beginning:

       9: goto          37                                         <---- unnecessary jump
      12: iload_2
      13: bipush        8
      15: imul
      16: bipush        100
      18: if_icmplt     24
      21: goto          34
      24: iload_2
      25: iconst_1
      26: iadd
      27: istore_2
      28: goto          12
      31: nop
      32: nop
      33: athrow
      34: goto          40
      37: goto          12

[DarkDimius]

My guess is that Closure tree has wrong position.

On 30 August 2016 19:49:01 liu fengyun notifications@github.com wrote:

Given the code:

    var a = 1 + 2

    while (a * 8 < 100) { // first jump to end
      a += 1
    }

The compilation generates following code, which has an unnecessary jump at
the beginning:

       9: goto          37                                         <---- 
       unnecessary jump
      12: iload_2
      13: bipush        8
      15: imul
      16: bipush        100
      18: if_icmplt     24
      21: goto          34
      24: iload_2
      25: iconst_1
      26: iadd
      27: istore_2
      28: goto          12
      31: nop
      32: nop
      33: athrow
      34: goto          40
      37: goto          12

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#1484

[liufengyun]

It seems to me it's related to how while is compiled. The code snippet above compiles to something like follows:

      {
        def while$(): Unit =
          if a.*(8).<(100) then {
            a = a.+(1)
            while$()
          } else ()
        while$()
      }

With such a translation, it seems natural for the backend to generate the byte code above.

[DarkDimius]

@liufengyun, is the problem that you can see this forward jump in the debugger? if yes, this may be because while$() tree has wrong position. AFAIK this can be fixed without changing the compilation scheme of labels.

[liufengyun]

@DarkDimius Yes, when I step through the code, it first goes to the end of WHILE and then goes back to the beginning. I'll investigate as you hinted, thanks a lot.