review request for 6798511/6860431: Include functionality of Surrogate in Character

[Xueming Shen]

Martin Buchholz wrote:
> On Tue, Mar 16, 2010 at 13:06, Xueming Shen <Xueming.Shen at sun.com> wrote:
>   
>> Martin Buchholz wrote:
>>     
>>> Therefore the existing implementation
>>>       
>>>>>  return codePoint>= MIN_SUPPLEMENTARY_CODE_POINT
>>>>>            &&  codePoint<= MAX_CODE_POINT;
>>>>>
>>>>> will almost always perform just one comparison against a constant,
>>>>> which is hard to beat.
>>>>>
>>>>>
>>>>>           
>>>> 1. Wondering: I think there are TWO comparisons.
>>>> 2. Those comparisons need to load 32 bit values from machine code,
>>>> against
>>>> only 8 bit values in my case.
>>>>
>>>>         
>>> It's a good point.  In the machine code, shifts are likely to use
>>> immediate values, and so will be a small win.
>>>
>>> int x = codePoint >>> 16;
>>> return x != 0 && x < 0x11;
>>>
>>> (On modern hardware, these optimizations
>>> are less valuable than they used to be;
>>> ordinary integer arithmetic is almost free)
>>>
>>>
>>>       
>> I'm not convinced if the proposed code is really better...a "small win".
>>     
>
> The primary theory here is that branches are expensive,
> and we are reducing them by one.
>
>   

There are still two branches in new impl, if you count the "ifeq" and 
"if_icmpge"(?)

We are trying to "optimize" this piece of code with the assumption that 
the new impl MIGHT help certain vm (hotspot?)
to optimize certain use scenario (some consecutive usages), if the 
compiler and/or the vm are both smart enough at certain
point, with no supporting benchmark data?

My concern is that the reality might be that this optimization might 
even hurt the BMP use
case (the majority of the possible real world use scenarios) with a 10% 
bigger bytecode size.

-Sherman



public class Character extends java.lang.Object {
  public static final int MIN_SUPPLEMENTARY_CODE_POINT = 65536;

  public static final int MAX_CODE_POINT = 1114111;

  public Character();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        

  public static boolean isSupplementaryCodePoint(int);
    Code:
       0: iload_0       
       1: ldc           #2                  // int 65536
       3: if_icmplt     16
       6: iload_0       
       7: ldc           #3                  // int 1114111
       9: if_icmpgt     16
      12: iconst_1      
      13: goto          17
      16: iconst_0      
      17: ireturn       

  public static boolean isSupplementaryCodePoint_new(int);
    Code:
       0: iload_0       
       1: bipush        16
       3: iushr         
       4: istore_1
       5: iload_1       
       6: ifeq          19
       9: iload_1       
      10: bipush        17
      12: if_icmpge     19
      15: iconst_1      
      16: goto          20
      19: iconst_0      
      20: ireturn       
}