RFR: 8293170: Improve encoding of the debuginfo nmethod section [v2]

[Boris Ulasevich]

On Thu, 6 Jul 2023 22:32:41 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> Boris Ulasevich has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   addressing review comments: super call, clarifying, and adding tests
>
> src/hotspot/share/code/compressedStream.hpp line 118:
> 
>> 116:   bool handle_zero(juint value) {
>> 117:     if (value == 0) {
>> 118:       _zero_count = (_zero_count == 0xFF) ? 0 : _zero_count;
> 
> The case of `_zero_count` overflow is not clear. Apparently, I'm missing something here.
> Current code is just clearing the previously counted `_zero_count`.
> I'd expect some action like storing the current number of zeros or advancing the `_position`.
> Do you have a test for this?

Good question. It works like this. On each write(), values are stored in the array, either normally or as a chain optimization (two bytes: zero byte and a number of zeros). When we get another zero value, the chain is updated. When the chain is full, we exit this function and the value is written by UNSIGNED5::write_uint_grow().

To make things clear, I have updated this code:

-      _zero_count = (_zero_count == 0xFF) ? 0 : _zero_count;
+      if (_zero_count == 0xFF) { // biggest zero chain length is 255
+        _zero_count = 1;
+        // for now, write it as an ordinary value (UNSINGED5 encodes zero int as a single byte)
+        // the new zero sequence is started if there are more than two zero values in a raw
+        return false;
+      }


Plus, I have added gtests to check this.

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PR Review Comment: https://git.openjdk.org/jdk/pull/12387#discussion_r1270543845