RFR: 8331114: Further improve performance of MethodTypeDesc::descriptorString [v2]

[Claes Redestad]

On Thu, 25 Apr 2024 13:34:50 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> When analyzing (startup) performance of the Classfile API I found this opportunity to further improve `MethodTypeDescImpl::descriptorString`.
>> 
>> Performance improves across the board in existing microbenchmarks:
>> 
>> Name                                                                 (descString) Cnt   Base   Error    Test   Error  Unit  Change
>> MethodTypeDescFactories.descriptorString  (Ljava/lang/Object;Ljava/lang/String;)I   6 55,179 ± 2,027  32,920 ± 1,189 ns/op   1,68x (p = 0,000*)
>> MethodTypeDescFactories.descriptorString                                      ()V   6 17,689 ± 1,871  11,060 ± 0,331 ns/op   1,60x (p = 0,000*)
>> MethodTypeDescFactories.descriptorString ([IJLjava/lang/String;Z)Ljava/util/List;   6 86,627 ± 1,646  41,035 ± 0,636 ns/op   2,11x (p = 0,000*)
>> MethodTypeDescFactories.descriptorString                    ()[Ljava/lang/String;   6 18,305 ± 1,974  13,110 ± 0,089 ns/op   1,40x (p = 0,000*)
>>   * = significant
>> 
>> 
>> The improvement is even more pronounced when running with `-Xint`, which is relevant for reducing startup overheads of early ClassFile API use:
>> 
>> Name                                                                 (descString) Cnt     Base      Error      Test     Error  Unit  Change
>> MethodTypeDescFactories.descriptorString  (Ljava/lang/Object;Ljava/lang/String;)I   6 5122,061 ±   81,335  2626,481 ± 101,466 ns/op   1,95x (p = 0,000*)
>> MethodTypeDescFactories.descriptorString                                      ()V   6 3481,316 ±  258,904  1489,267 ±  15,506 ns/op   2,34x (p = 0,000*)
>> MethodTypeDescFactories.descriptorString ([IJLjava/lang/String;Z)Ljava/util/List;   6 7741,081 ± 1628,244  3281,778 ±  41,892 ns/op   2,36x (p = 0,000*)
>> MethodTypeDescFactories.descriptorString                    ()[Ljava/lang/String;   6 3677,803 ±   63,432  1495,291 ±   8,995 ns/op   2,46x (p = 0,000*)
>>   * = significant
>>   ```
>>   
>>  I also applied similar approach to `MethodTypeDesc::displayDescriptor`: while not performance sensitive I think these are so inter-related that it makes sense to implement them in a similar fashion.
>
> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
> 
>   comma-separated

Correction: I ran the wrong benchmark for that long descriptor test! 🤦🏽 On the `descriptorString` benchmark there _is_ a regression with my patch on longer descriptors, -30% on that example. Since the strings are all readily available pre-calculating the size is cheap enough to make sense. This makes the patch more or less neutral on large descriptors, extends the gain on few-arg methods, but reduces the win on cases where there are no args (due an extra branch et.c.). On balance this seems like an improvement:


Name                                                                 (descString) Cnt    Base   Error    Test    Error  Unit  Change
MethodTypeDescFactories.descriptorString  (Ljava/lang/Object;Ljava/lang/String;)I   6  43,180 ± 0,575   30,547 ± 0,387 ns/op   1,41x (p = 0,000*)
MethodTypeDescFactories.descriptorString                                      ()V   6  20,717 ± 0,128   17,233 ± 0,242 ns/op   1,20x (p = 0,000*)
MethodTypeDescFactories.descriptorString ([IJLjava/lang/String;Z)Ljava/util/List;   6  89,834 ± 1,951   40,149 ± 0,326 ns/op   2,24x (p = 0,000*)
MethodTypeDescFactories.descriptorString                    ()[Ljava/lang/String;   6  21,438 ± 0,490   15,937 ± 0,263 ns/op   1,35x (p = 0,000*)
MethodTypeDescFactories.descriptorString                                 (..IIJ)V   6  52,429 ± 0,117   49,396 ± 0,153 ns/op   1,06x (p = 0,000*)
MethodTypeDescFactories.descriptorString                 (.....................).   6 178,205 ± 0,518  176,856 ± 5,577 ns/op   1,01x (p = 0,158 )
  * = significant


`.` is illegal in descriptor strings I used this as wildcard to be replaced with the descriptor string for the benchmark class as a ways to produce very large descriptors.

Since this strays a bit from what's doable in `MethodTypeDesc::displayDescriptor` I'm reverting those changes and focus this PR on the relevant piece of machinery.

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PR Comment: https://git.openjdk.org/jdk/pull/18945#issuecomment-2078267325