A DESCRIPTION OF THE REQUEST :
1.) Default hash to non-zero constant value except for String.length() == 0.
2.) Use positive constant in 8-bit range, ideally less than 5.
3.) Drop comparison of strings length.
4.) Only increment 1 variable in loop.
5.) Use '!=' instead of '<' to determine end of loop.



JUSTIFICATION :
1.) Eliminates need of changes from Bug ID 6921374
2.) Minimizes byte code and compiled machine code footprint + should perform better.
3.) Makes hashCode() as fast as before for all cases.
4.) In a small micro-benchmark I have proven a speed gain about ~15 %.
5.) Could be faster on some CPU code because no hidden subtraction must be performed.

--> In compiled mode, actually needs only 21 bytes against 40 bytes for inner loop.



EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -

Inner loop compiles to (needs only 21 bytes of machine code):

  0x00b8729c: sub %edx,%ebx
  0x00b8729e: mov %ebx,%edx ;*imul
                                        ; - java.lang.String::hashCode7@40 (line 1824)
  0x00b872a0: movzwl 0xc(%esi,%edi,2),%eax
  0x00b872a5: add %eax,%edx ;*iadd
                                        ; - java.lang.String::hashCode7@47 (line 1824)
  0x00b872a7: inc %edi ;*iinc
                                        ; - java.lang.String::hashCode7@43 (line 1824)
  0x00b872a8: mov %edx,%ebx
  0x00b872aa: shl $0x5,%ebx ;*imul
                                        ; - java.lang.String::hashCode7@40 (line 1824)
  0x00b872ad: cmp %ebp,%edi
  0x00b872af: jl 0x00b8729c
  0x00b872b1:


ACTUAL -

Inner loop compiles to (needs *40* bytes of machine code):

  0x00b86f1a: movzwl 0xc(%ebp),%ebp
  0x00b86f1e: sub %edi,%ebx
  0x00b86f20: add %ebp,%ebx ;*iadd
                                        ; - java.lang.String::hashCode@45 (line 1727)
  0x00b86f22: mov %eax,%esi
  0x00b86f24: inc %esi ;*iinc
                                        ; - java.lang.String::hashCode@47 (line 1726)
  0x00b86f25: cmp %edx,%esi
  0x00b86f27: jl 0x00b86f2d
  0x00b86f29: mov %ebx,%eax
  0x00b86f2b: jmp 0x00b86eea
  0x00b86f2d: mov 0x4(%esp),%edi
  0x00b86f31: lea 0x2(%edi,%eax,2),%ebp ;*caload
                                        ; - java.lang.String::hashCode@44 (line 1727)
  0x00b86f35: mov %ebx,%eax
  0x00b86f37: shl $0x5,%eax ;*imul
                                        ; - java.lang.String::hashCode@38 (line 1727)
  0x00b86f3a: mov %ebx,%edi
  0x00b86f3c: mov %eax,%ebx
  0x00b86f3e: mov %esi,%eax
  0x00b86f40: jmp 0x00b86f1a
  0x00b86f42:



---------- BEGIN SOURCE ----------
public class String {

    private static final int UNKNOWN_HASH = 1;
// private static final int UNKNOWN_HASH = Integer.MIN_VALUE; // alternative

    public String(xxx) {
        ...;
        hash = count == 0 ? 0 : UNKNOWN_HASH;
    }

    public int hashCode() {
        int h = hash;
        if (h == UNKNOWN_HASH) {
            h = 0;
            char[] val = value;
            for (int i = offset, limit = i + count; i != limit; )
                h = h * 31 + val[i++];
            hash = h;
        }
        return h;
    }
}

---------- END SOURCE ----------