String encodeUTF8 latin1 with negatives

[Roger Riggs]

Hi Brett,

Extra care is needed if the input array might be modified concurrently 
with the method execution.
When control flow decisions are made based on array contents, the 
integrity of the result depends on reading each byte of the array 
exactly once.

Regards, Roger



On 7/27/25 4:45 PM, Brett Okken wrote:
> In String.encodeUTF8, when the coder is latin1, there is a call to 
> StringCoding.hasNegatives to determine if any special handling is 
> needed. If not, a clone of the val is returned.
> If there are negative values, it then loops, from the beginning, 
> through all the values to handle any individual negative values.
>
> Would it be better to call StringCoding.countPositives? If the result 
> equals the length, the clone can still be returned. But if it does 
> not, all the values which are positive can be simply copied to the 
> target byte[] and only values beyond that point need to be checked again.
>
> https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/String.java#L1287-L1300
>
>         if (!StringCoding.hasNegatives(val, 0, val.length)) {
>             return val.clone();
>         }
>
>         int dp = 0;
>         byte[] dst = StringUTF16.newBytesFor(val.length);
>         for (byte c : val) {
>             if (c < 0) {
>                 dst[dp++] = (byte) (0xc0 | ((c & 0xff) >> 6));
>                 dst[dp++] = (byte) (0x80 | (c & 0x3f));
>             } else {
>                 dst[dp++] = c;
>             }
>         }
>
>
> Can be changed to look like:
>
>         int positives = StringCoding.countPositives(val, 0, val.length);
>         if (positives == val.length) {
>             return val.clone();
>         }
>
> int dp = positives;
>         byte[] dst = StringUTF16.newBytesFor(val.length);
>         if (positives > 0) {
>             System.arraycopy(val, 0, dst, 0, positives);
>         }
>         for (int i=dp; i<val.length; ++i) {
>             byte c = val[i];
>             if (c < 0) {
>                 dst[dp++] = (byte) (0xc0 | ((c & 0xff) >> 6));
>                 dst[dp++] = (byte) (0x80 | (c & 0x3f));
>             } else {
>                 dst[dp++] = c;
>             }
>         }
>
>
>
> I have done a bit of testing with the StringEncode jmh benchmark on my 
> local windows device.
>
> encodeLatin1LongEnd speeds up significantly (~70%)
> encodeLatin1LongStart slows down (~20%)
> encodeLatin1Mixed speeds up by ~30%
>
> The remaining tests do not show much difference either way.
>
> Brett
>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/core-libs-dev/attachments/20250728/9fc269c8/attachment-0001.htm>