RFR: 8302863: Speed up String::encodeASCII using countPositives
David Schlosnagle
duke at openjdk.org
Mon Feb 20 11:55:16 UTC 2023
On Sat, 18 Feb 2023 23:26:08 GMT, Claes Redestad <redestad at openjdk.org> wrote:
> When encoding Strings to US-ASCII we can speed up the happy path significantly by using `StringCoding.countPositives` as a speculative check for whether there are any chars that needs to be replaced by `'?'`. Once a non-ASCII char is encountered we fall back to the slow loop and replace as needed.
>
> An alternative could be unrolling or using a byte array VarHandle, as show-cased by Brett Okken here: https://mail.openjdk.org/pipermail/core-libs-dev/2023-February/100573.html Having to replace chars with `?` is essentially an encoding error so it might be safe to assume this case is exceptional in practice.
src/java.base/share/classes/java/lang/String.java line 976:
> 974: private static byte[] encodeASCII(byte coder, byte[] val) {
> 975: if (coder == LATIN1) {
> 976: byte[] dst = Arrays.copyOf(val, val.length);
Given the tweaks in https://git.openjdk.org/jdk/pull/12613 should this use `val.clone()` (would skip the length check)
Suggestion:
byte[] dst = val.clone();
src/java.base/share/classes/java/lang/String.java line 982:
> 980: if (dst[i] < 0) {
> 981: dst[i] = '?';
> 982: }
I'm curious if using countPositives (and vectorization) to scan forward would be valuable for long (mostly ASCII) strings or if the method call overhead/non-constant stride is not a win for shorter strings or heavily non-ascii inputs.
Suggestion:
for (int i = positives; i < dst.length; i = StringCoding.countPositives(dst, i + 1, dst.length - i);) {
if (dst[i] < 0) {
dst[i] = '?';
}
-------------
PR: https://git.openjdk.org/jdk/pull/12640
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