RFR: 8291550: RISC-V: jdk uses misaligned memory access when AvoidUnalignedAccess enabled [v9]

[Vladimir Kempik]

On Thu, 11 May 2023 13:25:12 GMT, Vladimir Kempik <vkempik at openjdk.org> wrote:

>> On an fpga I can see these numbers with perfnorm profiler:
>> 
>> Just this PR:
>> 
>> Secondary result "org.openjdk.bench.java.lang.StringEquals.equal:IPC":
>>   1.245 ±(99.9%) 0.037 insns/clk [Average]
>>   (min, avg, max) = (1.234, 1.245, 1.258), stdev = 0.010
>>   CI (99.9%): [1.209, 1.282] (assumes normal distribution)
>> 
>> This PR + moving xor:
>> 
>> Secondary result "org.openjdk.bench.java.lang.StringEquals.equal:IPC":
>>   1.239 ±(99.9%) 0.050 insns/clk [Average]
>>   (min, avg, max) = (1.224, 1.239, 1.253), stdev = 0.013
>>   CI (99.9%): [1.190, 1.289] (assumes normal distribution)
>
> You are right, if the object is located at the end of an allocated memory region ( and nothing past it) then this might produce sigsegv.
> 
> I'll ty to modify old version of string_equals to use TAIL logic for the tail of long strings, lwu+lhu+lbu is slower than ld but still faster than falling to misaligned access emulator.

new version results in jmh:


Benchmark                      Mode  Cnt   Score   Error  Units
StringEquals.almostEqual       avgt   25  29.459 ± 0.173  ns/op
StringEquals.almostEqualUTF16  avgt   25  29.833 ± 0.596  ns/op
StringEquals.different         avgt   25  19.896 ± 1.024  ns/op
StringEquals.differentCoders   avgt   25  14.986 ± 1.748  ns/op
StringEquals.equal             avgt   25  31.174 ± 0.226  ns/op
StringEquals.equalsUTF16       avgt   25  32.718 ± 0.982  ns/op


should not read past an object boundaries anymore

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PR Review Comment: https://git.openjdk.org/jdk/pull/13645#discussion_r1191381460