A simple optimization proposal

[Martin Grajcar]

Hi Kris,

I guess that without the line

if (a.length == 0) throw new Exception();

it looks the same (just throwing another Exception), right?

Regards, Martin.


On Thu, Feb 13, 2014 at 9:18 AM, Krystal Mok <rednaxelafx at gmail.com> wrote:

> Hi John,
>
> Nice to hear from you!
>
> I ran a couple of tests on the patch, one of them is:
>
> public class ArrayRangeCheck {
>   private static Object foo(Object[] a, int x) throws Exception {
>     if (a.length == 0) throw new Exception();
>     return a[x & (a.length - 1)];
>   }
>
>   public static void main(String[] args) throws Exception {
>     Object[] a = new Object[8];
>     foo(a, 6);
>     foo(a, 6);
>     System.in.read();
>   }
> }
>
> Ran with:
>
> $ java -XX:CompileCommand="compileonly ArrayRangeCheck foo"
> -XX:-TieredCompilation -XX:CICompilerCount=1 -XX:CompileThreshold=1
> -XX:PrintIdealGraphLevel=4 -XX:PrintIdealGraphFile=ideal.xml
> -XX:+PrintCompilation -XX:+PrintAssembly ArrayRangeCheck
>
> And confirmed that in this case the range check is indeed elided: there's
> only one a.length != 0 check dominating the actual element load, and the
> else branch goes to the exception throwing code.
>
> Thanks,
> Kris
>
>
> On Wed, Feb 12, 2014 at 9:47 PM, John Rose <john.r.rose at oracle.com> wrote:
>
>> One point behind the bug report is to give a little reward to Java coders
>> who write dominating tests that exclude a.length==0, as:
>>
>>  if (a.length == 0) goAway();
>>  else return a[i & a.length-1];
>>
>> Kris, can your patch do this?  The logic in IfNode can probably elide the
>> duplicate dominating test, if the right normalizations occur.  ��
>>
>> – John
>>
>> On Feb 12, 2014, at 6:05 PM, Martin Grajcar <maaartinus at gmail.com> wrote:
>>
>> That's what I've meant with goAway. My point was the jump using already
>> computed flags
>>
>>
>
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