RFR: 8293170: Improve encoding of the debuginfo nmethod section [v5]

[Boris Ulasevich]

On Wed, 19 Oct 2022 19:38:45 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

>> Boris Ulasevich has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   cleanup and rename
>
> src/hotspot/share/code/compressedStream.cpp line 182:
> 
>> 180:     return;
>> 181:   }
>> 182:   int bit0 = 0x80; // first byte upper bit is set to indicate a value is not zero
> 
> This is untraditional and confusing. In a byte we are preparing it is bit 7. The same comment is about `bit1`.

Ok

> src/hotspot/share/code/compressedStream.cpp line 190:
> 
>> 188:     write_byte_impl(bit1 | (next & 0x7f));
>> 189:     next >>= 7;
>> 190:   }
> 
> Could you please document the encoding/decoding schema in comments to the class? Could you also please include examples?
> Why is it chosen to split 32 bits into 6, 7, 7, 7 and 5?

The first bit is occupied by a flag holding a non-zero state, additionally each byte has a last byte flag, leaving only 6/7/7/7/7 bits for the payload.

I have added an examples table:

  // integer value encoded as a sequence of 1 to 5 bytes
  // - the most frequent case (0 < x < 64) is encoded in one byte
  // - the payload of the first byte is 6 bits, the payload of the following bytes is 7 bits
  // - the most significant bit in the first byte is occupied by a zero flag
  // - each byte has a bit indicating whether it is the last byte in the sequence
  //
  //       value | byte0    | byte1    | byte2    | byte3    | byte4
  //  -----------+----------+----------+----------+----------+----------
  //           0 | 0        |          |          |          |
  //           1 | 10000001 |          |          |          |
  //           2 | 10000010 |          |          |          |
  //          63 | 10111111 |          |          |          |
  //          64 | 11000000 | 00000001 |          |          |
  //          65 | 11000001 | 00000001 |          |          |
  //        8191 | 11111111 | 01111111 |          |          |
  //        8192 | 11000000 | 10000000 | 00000001 |          |
  //        8193 | 11000001 | 10000000 | 00000001 |          |
  //     1048575 | 11111111 | 11111111 | 01111111 |          |
  //     1048576 | 11000000 | 10000000 | 10000000 | 00000001 |
  //  0xFFFFFFFF | 11111111 | 11111111 | 11111111 | 11111111 | 00011111
  //

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PR: https://git.openjdk.org/jdk/pull/10025