RFR: 8310268: RISC-V: misaligned memory access in String.Compare intrinsic [v7]

[Fei Yang]

On Tue, 25 Jul 2023 08:07:09 GMT, Vladimir Kempik <vkempik at openjdk.org> wrote:

>> Please review this fix. it eliminates misaligned loads in String.compare on risc-v
>> 
>> for small compares ( <= 72 bytes), the instrinsic in c2_MacroAssembler.cpp is used,
>> it reads ( in case of UU/LL) 8 bytes per loop, and at then end, it reads tail - misaligned load of last 8 bytes from the string.
>> 
>> so if string length is not 8x bytes long then last load is misaligned, also it performs read/compare of some data which already was processed.
>> 
>> I have changed that to compare only last length%8 bytes using SHORT_STRING part of intrinsic for UL/LU. But for UU/LL I have made an optimised version.
>> 
>> Thanks to optimisations for conditional branching at line [947](https://github.com/openjdk/jdk/pull/14534/files#diff-35eb1d2f1e2f0514dd46bd7fbad49ff2c87703d5a3041a6433956df00a3fe6e6R947) I’ve got no perf drop on thead ( with +AvoidUnalignedAccesses) which supports misaligned access.
>> 
>> Improvements to inflate_XX() methods gives 3-5% improvements for UL/LU cases on thead, almost no perf change on hifive.
>> 
>> for large strings, the instrinsics from stubGenerator.cpp is used
>> for UU/LL - generate_compare_long_string_same_encoding, I have just replaced misaligned ld with load_long_misaligned. Since this tail reading is not on hot path, this give some small penalty for thead when -XX:+AvoidUnalignedAccesses.
>> 
>> large LU/UL comparision is done in compare_long_string_different_encoding in sutbGenerator.cpp:
>> These changes are partially based on feilongjiang's patch, but I have changed tail reading to prevent reading past the end of string. I have observed no perf difference between feilongjiang's and my version.
>> 
>> This also enables regression test for string.Compare which previously was aarch64-only
>> 
>> Testing: tier1 and tier2 clean on hifive.
>> 
>> JMH testing, hifive:
>> before:
>> 
>> Benchmark                                   (delta)  (size)  Mode  Cnt     Score    Error  Units
>> StringCompareToDifferentLength.compareToLL        2      24  avgt    9     6.474 ±  1.475  ms/op
>> StringCompareToDifferentLength.compareToLL        2      36  avgt    9   125.823 ±  1.947  ms/op
>> StringCompareToDifferentLength.compareToLL        2      72  avgt    9    10.512 ±  0.236  ms/op
>> StringCompareToDifferentLength.compareToLL        2     128  avgt    9    13.032 ±  0.821  ms/op
>> StringCompareToDifferentLength.compareToLL        2     256  avgt    9    18.983 ±  0.318  ms/op
>> StringCompareToDifferentLength.compareToLL        2     512  avgt    9    29.925 ± ...
>
> Vladimir Kempik has updated the pull request incrementally with one additional commit since the last revision:
> 
>   reduce registers usage and nits

Marked as reviewed by fyang (Reviewer).

src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp line 917:

> 915:     }
> 916:     addi(cnt2, cnt2, isUL ? 4 : 8);
> 917:     bne(tmp1, tmp2, DIFFERENCE);

You might want to place this `bne` before the preceding `addi(cnt2, cnt2, isUL ? 4 : 8);`

src/hotspot/cpu/riscv/stubGenerator_riscv.cpp line 2396:

> 2394:       __ bind(LOAD_LAST);   // cnt2 = 1..7 characters left
> 2395: 
> 2396:       __ addi(cnt2, cnt2, -wordSize); //cnt2 is now an offset in strL which points to last 8 bytes

Indentation: // cnt2 is now an offset in strL which points to last 8 bytes

-------------

PR Review: https://git.openjdk.org/jdk/pull/14534#pullrequestreview-1546775080
PR Review Comment: https://git.openjdk.org/jdk/pull/14534#discussion_r1274340284
PR Review Comment: https://git.openjdk.org/jdk/pull/14534#discussion_r1274340740