RFR: 8334554: RISC-V: verify & fix perf of string comparison

[Fei Yang]

On Tue, 25 Jun 2024 10:03:50 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> Sounds reasonable. Further, I wonder if we should consider m1 for these <=36 cases.
>
> By https://github.com/openjdk/jdk/pull/18382#discussion_r1645356197, do you mean current lastest implementation of it still brings negative impact on performance after https://github.com/openjdk/jdk/pull/18382/commits/1d4320b205c173256eb6a50cedaa0ccd9b4122d8?

I just tried m1 like following change, it only brings negative impact on banana-pi. So ignore my previous comment.

diff --git a/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp b/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp
index 9ce197a44bf..24ebc1fbe54 100644
--- a/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp
+++ b/src/hotspot/cpu/riscv/c2_MacroAssembler_riscv.cpp
@@ -2331,7 +2331,7 @@ void C2_MacroAssembler::element_compare(Register a1, Register a2, Register resul
   Assembler::SEW sew = islatin ? Assembler::e8 : Assembler::e16;

   bind(loop);
-  vsetvli(tmp1, cnt, sew, Assembler::m2);
+  vsetvli(tmp1, cnt, sew, Assembler::m1);
   vlex_v(vr1, a1, sew);
   vlex_v(vr2, a2, sew);
   vmsne_vv(vrs, vr1, vr2);

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PR Review Comment: https://git.openjdk.org/jdk/pull/19825#discussion_r1652655936