RFR: 8309502: RISC-V: String.indexOf intrinsic may produce misaligned memory loads [v2]

[Fei Yang]

On Wed, 7 Jun 2023 18:09:51 GMT, Vladimir Kempik <vkempik at openjdk.org> wrote:

>> Vladimir Kempik has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   make DO2 read by one character from memory per loop
>
> Numbers on DO4 ( comparing 4 characters at once) ( substring has to be final String of 4 characters)
> DO4:
> 
> hifive
> Benchmark                                 Mode  Cnt     Score    Error  Units
> before
> StringIndexOf.advancedWithShortSub4Chars  avgt   25  69514.891 ± 128.730  ns/op
> after
> StringIndexOf.advancedWithShortSub4Chars  avgt   25  2481.448 ± 13.481  ns/op
> 
> thead
> 
> Benchmark                                 Mode  Cnt     Score    Error  Units
> before
> StringIndexOf.advancedWithShortSub4Chars  avgt   25  753.125 ? 2.859  ns/op
> after
> StringIndexOf.advancedWithShortSub4Chars  avgt   25  741.031 ? 9.075  ns/op

> @VladimirKempik : Thanks for the update. Would you mind one more tweak? Since `needle_chr_shift` and `haystack_chr_shift` could be 0 for the L case, I think we should guard the shift instructions at https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp#L634, https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp#L637, https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp#L679, https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp#L700, https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp#L724, https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp#L775 with conditions `if (needle_chr_shift)` or `if (haystack_chr_shift)`.

Ah, this won't save us any instructions. Let's change this code snippet: https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp#LL754-L757
 into a single line: `slli(tmp3, result_tmp, haystack_chr_shift);`

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PR Comment: https://git.openjdk.org/jdk/pull/14320#issuecomment-1581808899