RFR: 8358533: Improve performance of java.io.Reader.readAllLines [v8]

[Xueming Shen]

On Fri, 27 Jun 2025 19:41:01 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

>> Replaces the implementation `readAllCharsAsString().lines().toList()` with reading into a temporary `char` array which is then processed to detect line terminators and copy non-terminating characters into strings which are added to the list.
>
> Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8358533: Make no line terminator case clearer

seems i was wrong about "consolidate two nested parsing loops" :-)  the throughput appears to back to normal with 'find term while loop' fast path .

        int start = 0;
        int pos = 0;
        int limit = 0;
        boolean skipLF = false;
        int n;
        while ((n = read(cb, pos, cb.length - pos)) != -1) {
            limit = pos + n;
            while (pos < limit) {
                if (skipLF) {
                    if (cb[pos] == '\n') {
                        pos++;
                        start++;
                    }
                    skipLF = false;
                }
                while (pos < limit) {
                    char c = cb[pos++];
                    if (c == '\n' || c == '\r') {
                        lines.add(new String(cb, start, pos - 1 - start));
                        skipLF = (c == '\r');
                        start = pos;
                        break;
                    }
                }
                if (pos == limit) {
                    int len = limit - start;
                    if (len >= cb.length) {
                        // allocate larger buffer and copy chars to beginning
                        char[] tmp = new char[2 * cb.length];
                        System.arraycopy(cb, start, tmp, 0, len);
                        cb = tmp;
                    } else if (start != 0 && len != 0) {
                        // move fragment to beginning of buffer
                        System.arraycopy(cb, start, cb, 0, len);
                    }
                    pos = limit = len;
                    start = 0;
                    break;
                }
            }
        }
        // add a string if EOS terminates the last line
        if (limit > start)
            lines.add(new String(cb, start, limit - start));

        return Collections.unmodifiableList(lines);
    }

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PR Comment: https://git.openjdk.org/jdk/pull/25863#issuecomment-3020286174