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

[Xueming Shen]

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".

Without seeing the real native machine code generated, I'm not sure
if

       0: iload_0 
       1: bipush        16
       3: iushr         
       4: istore_1      
       5: iload_1       
       6: ifeq          19

is really better than

       0: iload_0 
       1: ldc           #2                  // int 65536
       3: if_icmplt     16


for bmp character case, especially given the existing code has better 
readability and yes, shorter....

Yes, shift might be able to use the immediate values, but it still needs 
to handle the "operands"
and it is an extra operation. The only chance the new one might be 
better is that the "ifeq" is
faster than "if_icmplt", but have not worked on the instruction set 
level for too long, so I can't
tell (kinda remember you have to check the "circles" of each operation 
to see which one is
"faster" during my old gcc compiler day)

OK, convince me:-)

-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       
}